Phylum Annelida: Habitat, Features and Classification

In this article we will discuss about Phylum Annelida:- 1. Taxonomic Retrospect of Phylum Annelida 2. Definition and Fossil Record of Phylum Annelida 3. Habit and Habitat 4. Generalised Characters 5. Specialised Characters 6. Features 7. Trend in Classification 8. Phylogenetic Relationship 9. Locomotion 10. Circulatory System 11. Respiratory System 12. Reproductive System 13. Nervous System 14. Regeneration.

Contents:

1. Taxonomic Retrospect of Phylum Annelida
2. Definition and Fossil Record of Phylum Annelida
3. Habit and Habitat of Phylum Annelida
4. Generalised Characters of Phylum Annelida
5. Specialised Characters of Phylum Annelida
6. Features of Phylum Annelida
7. Trend in Classification of Phylum Annelida
8. Phylogenetic Relationship of Phylum Annelida
9. Locomotion of Phylum Annelida
10. Circulatory System of Phylum Annelida
11. Respiratory System of Phylum Annelida
12. Reproductive System of Phylum Annelida
13. Nervous System of Phylum Annelida
14. Regeneration in Phylum Annelida

1. Taxonomic Retrospect of Phylum Annelida:

1. The soft bodied animals including para­sitic arachnids and crustaceans were grouped under a phylum vermes by Linnaeus (1768).

2. Cuvier first separated it into an independ­ent segmented higher group. He united the annelids with the arthropods under the taxon Articulata.

3. Lamarck first coined the term ‘Annelida’ in 1802.

4. Grube (1851) used the taxa-Polychaeta and Oligochaeta respectively.

2. Definition and Fossil Record of Phylum Annelida:

Bilaterally symmetrical, elongated, metamerically segmented eucoelomates and soft bodies covered with thin cuticle containing segmental chitinous setae.

Fossil Record:

The whole body fossil record of annelids is scanty but well represented of semi-polychaetes have recorded from Middle Cambrian in Canada.

3. Habit and Habitat of Phylum Annelida:

Most polychaetes are marine. Larger forms like Aphrodite, Glycera live in mud while slender and smaller forms live in water spaces between sand grains. Many forms like Polygordius, Protodrilus secrete sticky sub­stances and remain attached on sand.

In some forms the setae are used in attachment to the sand. Nereis vitabunda of Sumatra is a terrestrial form and lives in soil channels. Most polychaetes on being transferred to freshwater die, while a few species like N. diversicolor, Troglochaetus can withstand and live in freshwater.

Oligochaetes are freshwater animals though a few of them like Helodrilus are marine. Most oligochaetes are terrestrial and live on mud-shores, in forests, meadow and cultivated lands. In Borneo, a group of earth­worms under the genus Pheretima, climb trees and live on barks and leaves of trees.

Seventy-five per cent of the members of the class Hirudinea live in freshwater and are very common in marshes and ponds. South American soil leeches (Erpobdellidae) live in moist soil, cow dung and rotten wood and act as predators of worms and insects.

Terrestrial leeches usually live in moist vege­tation of the tropics and feed on the blood of Vertebrates. Piscicola geometra is a brackish water form and remains attached to bottom- dwelling fishes. Branchellion is found attached to skates and rays.

Echiurids are semisessile and marine ani­mals many forms live in burrows, and cavi­ties made by other animals or on empty shells of molluscs.

Annelids vary widely regarding their sizes. Most polychaetes are minute to moderate in size but Neanthes brandti of Californian coast attains a size of about 1.5 metres.

Oligochaetes like Chaetogaster, Aeolosoma are a few cm in length but a few attain giant size. Rhinodrilus fafneri of Ecuador and Australian Megascolides australis grow over two metres in length and 25 mm in diameter. The average length of leeches is between 10 and 200 mm.

4. Generalised Characters of Phylum Annelida:

The annelida as the name implies (annulus = ring) are segmented worms and the segments in many cases are externally recog­nised as ring-like constrictions. In many forms a distinct head consisting of a pre-oral ‘prostomium’ and a post-oral ‘peristomium’ is present.

In most of the forms anterior, posterior, dorsal and ventral surfaces are well-recognised. The food canal is a straight tube with demarked regions and extends from anterior mouth to terminal anus. In many forms the perivisceral cavity or coe­lom is well-developed.

The coelom is lined with mesoderm and communicates to the exterior through paired nephridia. The body wall is muscular having circular and longi­tudinal layers of muscles and many have setae embedded in skin. Typical members have thin and non-chitinous cuticle. The blood-vascular system is of closed type.

The organs of excretion are metamerically ar­ranged tubular nephridia or tegmental or­gans which are closed internally or lead from coelom to exterior. A series of paired ducts called coelomoducts, either united with or distinct from the nephridia may be present to carry the reproductive elements outside.

The central nervous system consists of a pair of pre-oral ganglia connected by commissure with a pair of ventral cords which is ex­panded at each segment to form ganglion. Development may be direct or indirect. The larva is called trochophore and undergoes metamorphosis.

5. Specialised Characters of Phylum Annelida:

In Annelids the triploblastic condition has attained perfection. The body cavity is a true coelom which lies between the body wall and the tubular food canal. The body plan may be best described as a tube within a tube. This structural plan is maintained in higher forms. Definite segmentation is encountered in annelids. The body is composed of numerous distinct segments arranged in a linear series.

Bilateral symmetry is well represented in annelida and this is an evolutionary advance­ment over more primitive radial symmetry. Nervous system of annelids is more organ­ised and consists of a pair of dorsally-placed cerebral ganglia and ventral nerve cord. The nerve cells are distributed in the nerve cord and ganglia.

The tendency of ‘Head’ forma­tion is distinct. Head is usually associated with sense organs and as the sense organs help the animal to keep in touch with the sur­roundings, they are usually housed at that end of the body which remains directed forward during locomotion.

Organs and organ sys­tems performing vital functions as nutrition, excretion and reproduction are highly devel­oped in artnelids. Thus association of differen­tiated tissue groups has occurred in them.

6. Characteristic Features of Phylum Annelida:

1. Triploblastic animals with bilateral symmetry.

2. Body soft, vermiform and more or less elongated.

3. Body metamerically segmented and covered by a thin cuticle which pro­tects the body.

4. Head comprised of prostomium and peristomium. Prostomium contains head and sensory appendages.

5. Locomotory organs of Phylum Annelida are epidermal chitinous bristles, called setae or chaetae (lost in leeches and in a few groups of polychaetes).

6. Body cavity of Phylum Annelida is a true coelom which lies between the two layers of mesoderm.

7. Digestive tract straight, tubular run­ning from the anterior mouth to the posterior anus.

8. Digestion is completely extracellular in Phylum Annelida.

9. Gas exchange performed either by general body surface, provided that it is kept moist, or by gills in some tube- dwellers (e.g., Arenicola, Cirratulus, etc.).

10. Closed blood vascular system with dorsal and ventral longitudinal ves­sels connected by smaller vessels. The dorsal vessel acts as pumping vessel. The closed, circulatory system is re­duced or absent in leeches.

11. Respiratory pigments are red haemo­globins or green chlorocruorins. Both pigments are found in blood plasma, not in the R.B.C. as found in verte­brates. Haemerythrin is also present in some polychaetes.

12. Nervous system represented by cerebral ganglia (supra-pharyngeal ganglia) and double ventral nerve cord with segmentally arranged ganglia and lateral nerves.

13. Excretory system are nephridia (protonephridia) in some, and segmentally coiled tubes open at both ends, called metanephrfdia.

14. Animals often provided with coelomoducts which are channels for the outward passage of reproductive elements.

15. Gonads develop from coelomic epi­thelium.

16. Sexes united or separate (gonochoristic), e.g., polychaetes.

17. Development direct (e.g., Oligochaeta or Hirudinea) or indirect (e.g., Polychaeta, Archiannelida).

18. Cleavage spiral.

19. Larval stage when present is a trochophore.

Habitat:

They live in marine, freshwater and terrestrial environments.

7. Trend in Classification of Phylum Annelida:

The classification of Phylum Annelida is still un­stable. No unanimous scheme of complete classification is available in the literature.

Before 1950 the class Annelida has been divided into 3 classes:

(i) Chaetopoda (the marine Polychaeta, terrestrial and fresh wa­ter Oligochaeta),

(ii) Hirudinea (leeches) and

(iii) Archiannelida.

This scheme was used by Parker and Haswell in 1940 (VI^th ed. revised by Otto Lowenstein).

But in the VII^th edition in 1972, Parker and Haswell (the VII ed. was edited by Marshall and Williams) accepted Knox’s scheme in which the Annelida has been divided into 4 classes such as:

(i) Polychaeta

(ii) Archiannelida

(iii) Oligochaeta and

(iv) Hirudinea.

Barnes (1987) divided Annelida into 3 classes:

(i) Polychaeta

(ii) Oligochaeta and

(iii) Hirudinea.

He included archiannelids in polychaeta. At present the class Chaetopoda is not used. Myzostomaria which once rec­ognized as a separate class now treated as a polychaete family. Recently many authors use Fauchald (1997) scheme in case of polychaete classification. In this classifica­tion no Linnaean categories of above family such as superfamily, order, and subclass are used.

Again, Westheide (1997) divided Polychaeta into 25 orders which include all the de­scribed species.

Pechenik (2000) classified Annelida into 4 classes:

(i) Polychaeta

(ii) Clitellata

(iii) Pogonophora and

(iv) Echiura.

He includes oligochaetes (Oligochaeta) and leeches (Hirudinea) under the class Clitellata and places all the species into two sub­classes:

(i) Oligochaeta (earthworms) and

(ii) Hirudinea (leeches).

Brusca and Brusca (2003) reported that the recent phylogenetic reasearch indicates the unification of two classes, Oligochaeta and Hirudinea into a single class Clitellata.

The scheme of classification has adopted in this book (4th ed.) from “Invertebrate Zoology” written by Ruppert and Barnes (1994).

Classification with Characters:

Class 1. Polychaeta (Gk. poly – many + chaeta = setae):

Characters:

1. Body elongated, segmented with iden­tical, cylindrical body segments.

2. Anterior end modified into a head.

3. A distrinct head bears sensory ap­pendages, such as eyes, antennae, cirri and palps.

4. Numerous setae on the trunk seg­ments, hence called polychaeta.

5. Each body segment bears a pair of fleshy, lateral, paddle-like outgrowths, called parapodia, bearing numerous long setae in setigerous sacs. The parapodia act as locomotory and res­piratory organs.

6. Parapodia basically biramous and supported by acicula.

7. Clitellum absent.

8. Alimentary canal is provided with an eversible buccal region and protrusible pharynx.

9. Highly vascularised gills are present in most large-sized polychaetes used for gas exchange.

10. Brain complex is same than oligochaeta and divided into 3 regions:

(i) Fore- brain

(ii) Mid-brain and

(iii) Hind-brain.

11. Protonephridia present in a number of families. Segmental metanephridial systems in most cases.

12. Sexes separate (gonochoristic) in most.

13. Epitoky, a reproductive phenomenon seen in some polychaetes (e.g., nereids, syllids and eunicids).

14. Fertilization external.

15. A trochophore larval stage in the life cycle.

16. Exclusively marine, and mostly car­nivorous but some are herbivorous.

Scheme of Classification:

Distribution:

Fauvel (1953) reported that many of the polychaetes are really cosmopolitan and most of the species are common in the Indo-Pacific coasts. Many species have a worldwide dis­tribution and the inter-tropical are the same in all the oceans. Thus the distribution of polychaeta is mainly limited by temperature.

Trend in polychaete classification:

The class polychaeta is not divided into subclasses or orders.

Some authors, such as Knox (1972) divided into subclass rank:

(i) Subclass Errantia and

(ii) Subclass Sedentaria.

But this subdivision is an artifi­cial, not a natural one. So most of the re­searchers divide polychaeta into two groups.

(A) Group I. Polychaeta errantia (Wan­dering polychaetes) [L. mantis = wandering]:

Characters:

(i) Numerous and usually similar segments.

(ii) Well developed parapodia with acicula and setae.

(iii) Head with a distinct prostomium bearing sensory structures and peristomium with mouth.

(iv) Pharynx usually protrusible with chitinous jaws and teeth.

(v) Branchiae, when present, not found in the anterior end.

Habitat:

Pelagic, crawling, burrowing and tube-dwelling forms.

It includes 20 families. Some of the fami­lies with examples are:

Examples:

Family Aphroditidae (e.g., Aphrodita = Aphrodite)—sea mouse.

Family Phyllodocidae (e.g., Phyllodoce).

Family Polynoidae (e.g., Polynoe)—scale worms.

Family Syllidae (e.g., Syllis, Autolytus).

Family Nereididae (e.g., Dendronereis, Nereis, Platynereis)—ragworms.

Family Glyceridae (e.g., Glycera)— tongue worm.

Family Eunicidae (e.g., Eunice, Marphysa, Palola)

Family Myzostomidae (e.g., Myzostonw— commensals and parasites of echinoderms essentially on crinoids).

(B) Group II. Polychaeta Sedentaria (Sedentary polychaetes) [L. sedentarius = sitting]:

Features:

(i) Body of two or more regions with dissimilar segments and parapodia.

(ii) Reduced parapodia without acicula.

(iii) Head poorly developed and provided with palps and tentacles,

(iv) Prostomium without sensory ap­pendages.

(v) Pharynx usually non-protrusible, and without jaws and teeth.

(vi) Branchiae, when present—usually present in the anterior end.

Habitat:

Sedentary, burrowing and tubicolous worms, feed in detritus or plank­ton.

Examples:

Family Chaetopteridae (e.g., Chaetopterus, Mesochaetopterus, Siphonochaetop- terus)—paddle worms.

Family Arenicolidae (e.g., Arenicola)— lugworms.

Family Terebellidae (e.g., Terebella, Amphitrite, Polymnia).

Family Sabellidae (e.g., Sabella, Myxicola, Fabricia, Potamilla)—fanworms.

Family Serpulidae (e.g., Serpula, Spirorbis, Pomatoceros, Spirobranchus)— fanworms.

Archiannelida (Gk. archi – first):

Features:

(i) Heterogenous minor group.

(ii) Body small, simple, elongated and vermiform.

(iii) Simplified structure.

(iv) External segmentations indistinct but internal segmentation present.

(v) Setae and parapodia usually absent.

(vi) Head bears sense organs,

(vii) Unisexual or hermaphrodite.

(viii) Blood vascular system simple or lack­ing.

(ix) Larva trochophore.

(x) Marine, brackish or freshwater spe­cies.

Examples:

Nerilla, Troglochaetus, Poly­gordius, Protodrilus, Dinophilus, Trilobodrilus.

Remarks:

Previously some zoologists treat ‘Archiannelida’ as an appendix to the class polychaeta but Knox (1972) has treated it as a sepa­rate class. Barnes (1980, ’87) re­ported that Archiannelida is a group which includes most of the degenerate polychaetes.

Most of the recent authors are of opinion that the archiannelids are the specialized aberrant members of the annelids and represent as families of the polychaeta. The status given to the archiannelids as the class is an artificial one.

Class 2. Oligochaeta (Gk. Oligos = few + L. chaetae = bristles):

Characters:

1. Streamlined body with well-developed segmentation and a simple prostomium without sensory ap­pendages, such as eyes, and tenta­cles.

2. Head indistinct.

3. Clitellum present.

4. Parapodia and cirri absent.

5. Setae less distributed along the body.

6. Usually no respiratory organs except a few species (e.g., Dero, Branchiura, etc.) which possess true gills. Gas exchange takes place by diffusion through the moist body wall.

7. Excretory system metanephridial type.

8. Brain simple type with ventral nerve cords.

9. They are hermaphrodites.

10. Fertilization (cross-fertilization) oc­curs externally.

11. Development direct and takes place within cocoon secreted by the clitellum.

12. Asexual reproduction usually com­mon in freshwater species and in­volves by the transverse division of the adult body.

13. No larval stage in the life cycle.

Habitat:

Most species are found in fresh­water or terrestrial habitats, a few species are marine.

It includes 3 orders:

Order 1. Lumbriculida:

(i) 4 pairs setae in each segment.

(ii) Clitellum consists of a single celled layer.

(iii) Male and female gonopores in the clitellum.

(iv) Male pores anterior to female pores.

(v) Inhabitants of freshwater.

Examples:

Lumbriculus, Rhynchelmis, Styloscolex.

Order 2. Tubificida:

(i) Setae present in bundles with two or more setae.

(ii) Clitellum consists of a single-celled layer.

(iii) Male and female gonopores in the clitellum.

(iv) Male gonopore in front of the female gonopore.

(v) One pair of testes followed by a pair of ovaries.

(vi) Mostly aquatic.

Examples:

Tubifex, Branchiura, Nais, Dero, Chaetogaster.

Order 3. Haplotaxida:

(i) Setae simple or forked, and may be 4 or 8 or sometimes multiplied in a ring in each segment.

(ii) Clitellum composed of two or more layers of cells.

(iii) Female gonopores always on the 14th segment and male gonopore a few segments behind them.

(iv) One pair of testes or ovaries or both often absent.

(v) Aquatic or semi-terrestrial.

Examples:

Alluroides, Drawidia, Lumbri- cus, Megascolides, Megascolex, Pheretima, Eudrilus.

Class 3. Hirudinea (L Hirudo = a leech):

Characters:

1. Body consists of definite and limited number of segments.

2. Trunk consists of 21 segments with preclitellar region, clitellum and post clitellar region.

3. Clitellum includes 3 segments and never conspicuous except reproduc­tive period.

4. Segments are marked externally by secondary rings or annuli.

5. Usually with a small suctorial ante­rior sucker and a large powerful posterior sucker.

6. Parapodia and head appendages absent.

7. Coelom generally reduced by the presence of connective tissue, called botryoidal tissue, and muscles.

8. Both sinuses and muscular blood vessels present.

9. Excretory organs include segmentally arranged 10 to 17 pairs of metanephria.

10. Asexual reproduction absent.

11. Hermaphrodite.

12. Gonads and gonoducts restricted to anterior few segments.

13. Fertilization internal.

14. Development direct and takes place within cocoons secreted by clitellum.

Habit and Habitat:

They are either free living or permanently or intermittently ectoparasites. In freshwater large number of species prey on invertebrates rather than blood sucking species but most marine spe­cies are ectoparasites on fishes.

Remarks:

Due to similarities in several points between Polychaeta and Hirudinea, such as:

(i) Absence of parapodia and sensory ce­phalic appendages,

(ii) Presence of clitellum,

(iii) Both groups are hermaphrodites and

(iv) De­velopment within cocoons, many authors suggest that they have evolved from a common ancestor and to be mono- phyletic.

Hence, the two groups are generally considered as sub­class within the class Clitellata.

It contains 3 orders:

Order 1. Acanthobdellida:

(i) Short proboscis.

(ii) Anterior sucker absent but posterior sucker well developed.

(iii) Setae present in the anterior five segments (an exceptional case) and a compartmented coelom.

(iv) Parasite on salmonid fish.

A primitive order which forms a connect­ing link between Oligochaeta and Hirudinea. This order contains a single genus with a single species.

Example:

Acanthobdella.

Order 2. Rhynchobdellida:

(i) Anterior part of the body can be protruded or retracted as proboscis.

(ii) Anterior sucker present.

(iii) Jaw and setae absent.

(iv) Each typical body segment contains 3, 6 or 12 annuli.

(v) Colourless blood.

(vi) Coelom reduced to sinuses without botryoidal tissues.

(vii) No penis.

(viii) All are aquatic, ectoparasites of both invertebrates and vertebrates in fresh- Water and marine habitats.

Examples:

Glossiphonia (parasite on aquatic snails); Piscicola, Pontobdella (parasite on fish or other aquatic vertebrates); Ozobranchus (parasite on tur­tles and crocodiles).

Order 3. Arhynchobdellida:

(i) Non-eversible pharynx present.

(ii) Anterior sucker with 3 pairs of jaws (e.g., Hirudinidae) or jaws absent (e.g., Erpobdellidae).

(iii) Each typical body segment contains 5 annuli.

(iv) Blood red coloured.

(v) Botryoidal tissue present.

(vi) Fertilization by the insertion of penis.

(vii) Aquatic and terrestrial leeches.

Examples:

Hirudo (the typical—leech which is parasite on verte­brates), Aulostoma (horse leech, free-living and carnivo­rous), Hirudinaria (H. granularia, cattle leech), Haemadipsa, Phytobdella (terres­trial leeches), Erpobdella (worm leeches), Dina.

8. Phylogenetic Relationship of Phylum Annelida:

The archiannelids were regarded to be the most primitive members amongst the annelids. But recently the archiannelids are regarded to be the aberrant members of other annelidan families. This idea has made the phylogenetic concept complicated.

The members of polychaeta are regarded to be ancestral forms of all annelids because they possess the undernoted features:

(i) Marine and dioecious,

(ii) Serially repeated unspecialised reproductive units,

(iii) Nu­merous minute eggs,

(iv) External fertiliza­tion,

(v) Pelagic larval stage, and

(vi) Primitive organisation of the nephridia.

But Clark (1969) considered the oligochaetes to be most primitives because they resemble the proto-annelids in many features, especially in the segmental organization of the muscu­lature and the compartmentalization of coe­lom by inter-segmental transverse septa and dorsal and ventral mesenteries.

The specialisations encountered in the reproduc­tive and excretory systems in oligochaetes are directly related to their terrestrial and freshwater mode of living. The locomotory behaviour of the oligochaetes resembles closely that which might be expected of the proto-annelidan form.

According to David Nichols (1971)-the archaic annelids have evolved from Turbellaria-Nemertean clade. Ruppert and Barnes (1994) have suggested that the members of Oligochaeta which are the inhabitants of freshwater or terrestrial as well as marine environs may have stemmed from some early polychaetes but likely the oligochaetes evolved independently from some ancestral annelids.

The class Hirudinea which includes leeches is believed to have evolved from oligochaete stock.

9. Locomotion of Phylum Annelida:

Locomotion in Annelids is carried by three agents:

(a) Locomotor structures,

(b) Body musculatures and

(c) Hydrostatic skeleton.

In different annelidan species, lo­comotion is not caused by any individual locomotory agent, but it is the resultant outcome of a coordinated effort of all those three agencies.

Locomotor structures:

Parapodia:

Most polychaetes move about by the parapodia. By the action of the segmentally arranged parapodia they pad­dle through water. During paddling two parapodia of a segment always remain in an opposite phase of motion.

Parapodia are hollow segmentally arranged lateral exten­sions of the body, typically divided into dorsal notopodium and ventral neuropo­dium. Each lobe carries a bundle of bristles strengthened by supporting aciculum.

The point of attachment of the parapodia with the body wall acts as a hinge for forward and backward movement. The coelomic cavity extends into parapodia and the hydrostatic pressure is exerted by the coelomic fluid.

Two sets of oblique muscles, originating from the midventral line of the body wall are attached to the parapodia dorsally and ventrally. In addition to these muscles, there are intrinsic protractor and retractor mus­cles that are responsible for protrusion and withdrawal of bristles and the supporting acicula.

The parapodia become variously modi­fied in different polychaetes (Fig. 17.55). The modifications are correlated with different functions. They are well-developed and modified into creeping and swimming types.

In these forms the parapodia are restricted on some anterior-most segments as head crown and are poorly developed or absent in the rest of the body segments. In sand or mud burrowers and tube-dwellers the para­podia are poorly developed or absent espe­cially those of the posterior part of the body.

Setae:

In oligochaetes locomotion is caused by the setae which are implanted directly in the body muscles and are mostly oriented in the central region of the body segments. The setae are of various types (Fig. 17.56). They might be long or short, straight or curved, simple or forked pectinate or plumose type.

In general, the longer plumose or forked setae are the characteristic features of the aquatic swimming species. In bur­rowing species, the setae are short, straight, simple and blunt. The setae are embedded in and are developed from setal sac. The exten­sion and withdrawal of the setae during movement are caused by a pair of setal muscles and the associated circular muscles.

Suckers:

The parapodia and setae are absent in Hirudinea. Anchorage on the sub­stratum during locomotion is caused by two suckers, one is situated at the anterior (An­terior sucker) end and the other is located at the posterior end (Posterior sucker) of the body.

The suckers are formed by the fusion of several body segments and they play their role alternately as adhesive organs. Adhe­sion with the substratum during locomotion is possible for the presence of specialised epidermal sucker glands situated in masses in the anterior and posterior suckers.

A general survey of the locomotor struc­tures will reveal that there is a gradual ten­dency towards the loss of well-formed loco­motor structures in course of evolution. The loss of distinct locomotor structures is com­pensated by the development of body mus­culature.

Body musculature:

Three major sets of muscles, viz. longitu­dinal muscles, circular muscles and ob­lique muscles, help in locomotion. In polychaetes these three types of muscles are less developed. The longitudinal muscles do not form a continuous layer, instead they are broken up into two-paired blocks, one is dorsal and the other is ventral.

The circular muscles are thin and feebly developed. They are oriented outside the longitudinal muscle bands. The oblique muscles are present as two thin strands in each segment and are connected between circular muscles of dor­sal and ventral sides. The function of these muscles is antagonistic in nature and causes waves of contraction along the body.

The oblique muscles and parapodial muscles are interpolated in each segment from the long muscles along its body length, creating retardation of contraction and expansion of body in lengthwise. In oligochaetes, the body wall is composed of a continuous layer of longitudinal muscles on the inner side and circular muscles on the outer side.

These muscles are so arranged that they can exert considerable pressure upon the coelomic fluid which they surround. The muscle fibres of the longitudinal muscles are considerably long extending 2-3 segments. As a conse­quence the body segments are linked into small groups.

Each segment has its own segmental nerves which control the localised muscular contraction under the central nerv­ous system. The longitudinal muscles are divided into seven blocks; each is encircled by a sheath of connective tissue. The oblique muscles, connecting the dorsal and ventral circular muscles, are better developed and help in contraction and extension of local­ised body segment.

The activity of oblique muscle is correlated with that of longitudinal and circular muscles. Besides, the intrinsic muscle fibres in the transverse septa sepa­rating the coelomic cavity help to resist the stress caused due to change of pressure in the coelomic fluid.

The continuity of the coelomic fluid in different body segment is caused through a foramen present in each transverse septum in the ventral nerve cord region. These foramina are provided with sphincter muscle. Greatest development of body musculature is encountered in Hirudinaria. They have well-formed circu­lar, longitudinal and oblique muscles.

Between the outer circular and inner longi­tudinal muscles, a double layer of oblique muscles is present. The oblique muscles co­ordinate the activity of the circular and lon­gitudinal muscles. When the body of a leech becomes long, the oblique muscles reinforce the action of the longitudinal muscles to make it short.

When the body of leech be­comes short, they reinforce the action of the circular muscles, thus causing elongation. At places the oblique muscles are so oriented that they offer rigidity of the body by en­hancing the hydrostatic pressure. This action enables leech to sit upright on its posterior suckers.

The existence of dorsoventral mus­cles in leech is significant, because these muscles make the body of the leech flat and ribbon-like and also increase the efficiency of dorso-ventral undulations of the body dur­ing swimming.

Hydrostatic skeleton:

In annelids, the coelom appears to play an important role in locomotion. The coelom in annelids, filled with incompressible coe­lomic fluid, acts as hydrostatic skeleton. It is enclosed by the muscles of the body wall. The importance of hydrostatic skeleton in locomotion of annelid has been emphasised by Chapman and Newell.

Transmission of coelomic fluid from one coelomic compart­ment to another during localised muscular contraction depends greatly on the nature and disposition of transverse septa. The body of an annelid is a cylinder, the wall is made up of muscles and the lumen is filled with coelomic fluid.

A cylinder encircled by circu­lar muscles only, when contract towards one end, will cause diminution of the diameter at that end thus resulting in movement of coe­lomic fluid.

The movement of coelomic fluid may cause:

(i) Increase in length of the contracting end of the body (the other end to remain unaltered);

(ii) Increase the diameter of the other end (total length remaining the same),

(iii) increase in the length of the other end (the diameter remaining unchanged) and

(iv) elongation of both ends of the body.

But the existence of longitudinal muscles and their contraction in conjunction with regional hydrostatic or coelomic fluid pres­sure help in restoration of the cylinder to its original state.

Hydrostatic Skeleton in Polychaetes:

The hydrostatic skeleton is less developed in Polychaetes. The coelom is spacious with ill- developed muscular layers and is divided into compartments by transverse septa. The perforations in the septa permit transfer of coelomic fluid from one compartment to another. In this group of annelids the circular muscles are feebly developed and are inter­rupted laterally by the parapodial muscles.

Hydrostatic Skeleton in Oligochaetes:

The body forms a complete cylinder with well-developed circular and longitudinal muscles and uniformly developed transverse septa. The hydrostatic skeleton is well-or­ganised and it actively participates in the various modes of locomotion encountered in oligochaetes.

The transverse septa regulate the changes of pressure and limit them to particular regions of the body. The septa control the pressure of the fluid through the ventral foramen. The increase of pressure in the coelomic fluid of a segment is subs­tantially isolated from adjoining segments.

Hydrostatic Skeleton in Hirudinea:

Due to overdevelopment of body musculature and obliteration of coelom by the thick muscular coat and botryoidal tissue the coe­lomic spaces become greatly reduced. Re­duction of coelom results in the increase of hydrostatic pressure and renders rigidity to the body. As a consequence the hydrostatic skeleton is highest developed in hirudinea amongst the annelids.

Mechanism of Locomotion:

Four types of movement are observed in Polychaetes.

They are:

(i) Slow crawling:

This type of move­ment is caused by the paddle-like action of parapodia.

(ii) Rapid crawling:

This serpentine movement is caused by the co­ordinated action of the longitudinal muscles and the parapodia.

(iii) Peristaltic motion:

This type of movement is best seen in Arenicola. The movement is resulted from the anterior to the posterior passage of swollen transverse bands. Swelling is caused by the local relaxation of the body wall where the turgor of the coelomic fluid plays an important role.

(iv) Swimming:

This type of movement is resulted by the paddling of parapodia accompanied by the hori­zontal serpentine movement. Most of the pelagic and benthic forms swim for the purpose of food-collection and reproduction.

The oligochaetes move by either crawl­ing or by digging. Crawling is effected by the peristalsis of the body by the action of the circular and longitudinal muscles and the coelomic fluid too. The peristaltic motion is initiated by the ventral ganglion in each segment and the rythmic co-ordination de­pends entirely upon the excitation of the ventral ganglion.

In oligochaetes co-ordination between nerve ganglia and muscles is significant. Digging is done by the forward extension of the anterior segments into spaces between the soil particles. The coelomic pres­sure is raised to widen the space and to pull the posterior end.

The crawling of oligochaetes is performed by alternate expansion and contraction of the body musculature. The seta at two different places in the body anchor on the soil, and the portion in between extends forward with setae of that region pulled.

In Lumbricus the contraction of circular muscles causes the withdrawal of setae and consecu­tively, relaxes the longitudinal muscles of all segments or metameres up to its middle starting with the first. Aquatic Aelosoma crawls with the ventral cilia of the head lobe.

Leeches move by crawling on a solid substratum and by swimming. Leeches crawl by exhibiting looping movements. Hirudo first attaches itself on the substratum with the help of posterior sucker and then extend the body by the action of the body muscula­ture and hydrostatic skeleton to the oral disc. It then bends its body like an inverted ‘U’ and drags the posterior sucker just be­hind the region of attachment.

The rate of looping movement depends upon the extrin­sic factors, especially the temperature. Most leeches can swim quite effectively in water by causing dorsoventral serpentine move­ment. Branchiobdella moves with mouth and sucker.

10. Circulatory System of Phylum Annelida:

The circulatory system is closed type and consists of blood vessels through which blood is distributed and collected from different parts of the body. The colour of blood is red due to the presence of haemoglobin which remains dissolved in the plasma. In all oligochaetes haemoglobin is present.

As a result of reduction of blood vessels in many polychaetes like Aphrodita and Phyllodoce, haemoglobin is absent in the blood but haemoglobin-containing cells are found in the coelomic fluid. Sabellids and Serpulids have green chlorocruorin and haemerythrin is present in Magelona.

The dorsal vessels in polychaetes show peristaltic waves and blood flows in it from the posterior to the anterior direction. The ventral vessel cannot contract while lateral vessels pulsate.

A true blood- vascular system with close vessels and their ramification is present in the Acanthobdellida and the Rhynchobdellida. But haemoglobin is usually absent in them. The ground plan of blood vessels inside the body consists of dorsal, ventral and lateral vessels. These vessels are interconnected with each other through segmental loops or ring vessels.

In Oligochaetes, the dorsal vessel, with its valves, functions as heart. The dorsal vessel collects blood from the vascular areas of the intestine and drives it towards the anterior end. From the dorsal vessel arise in each segment a pair of ring vessels which pass to the sub-neural vessel running beneath the ventral nerve cord.

In the anterior end of the body one or more of these ring vessels become muscular, contain valves and pul­sate rhythmically. These modified ring ves­sels are called lateral hearts. The lateral hearts drive blood towards the ventral ves­sels. In the ventral vessel blood flows from anterior to the posterior region.

The ring vessels and the ventral vessel supply blood to the individual organs of the body. In larger forms of oligochaetes additional ves­sels like sub-intestinal or supra-intestinal vessel are often present. In an active lumbricoid the blood pressure is about 10 mm of mercury.

In Hirudinea, the dorsal vessel conveys blood anteriorly and the ventral vessel con­ducts blood towards posterior region. The dorsal and ventral median vessels lie in longitudinal coelomic channels and are con­nected with each other at the end of the body through loops.

Many minute vessels that arise from the vessels anastomose beneath the integument and organs. These anastomo­sing branches remain surrounded by botryoidal tissue in Hirudo.

Presence of glucose, amino-acids and lipids in blood of Pheretima has been demon­strated very recently. This observation indi­cates very conclusively that blood in this animal is a medium which transports nutri­ents to the tissues.

Heart:

What is heart?

The muscular pumping organ whose pulsa­tions drive blood throughout the circulatory system.

Types:

Morphologically there are four types of hearts.

These are:

1. Pulsatile heart:

When some commissural blood vessels are enlarged and achieve the function of contraction and relaxation, it is called pulsatile hearts. In closed blood vascular system of invertebrates the flow of blood relies on body movements and coelomic pressure on blood vessels.

These activities help the blood vessels to contract in peristaltic waves. It is the simplest type of hearts and valves may or may not be present. The vessels lack endothelium. In some annelids these types of hearts are present. In the earthworms there are four pairs of pulsatile hearts containing valves. These hearts are also called Pseudo-hearts. These are not like the true hearts of vertebrates.

2. Tubular heart or Ostiate heart:

The heart is a muscular tube which is an enlarged dorsal vessel with a number of lateral openings provided with valves, called ostia, through which blood enters into the heart from a large surrounding sinus, the pericardium. The chamber of the heart var­ies from one or more (13 chambers in cock­roaches) which are linearly arranged and the closing of the ostia is performed by the contraction of the alary muscles.

Contrac­tion of allary muscles and the muscles of the heart (cardiac muscles) create wave of con­traction which passes from the posterior to the anterior side. The tubular hearts or ostiate hearts are found in insects and in some other groups of arthropods.

3. Booster heart:

A contractile muscular expansion of a blood vessel or lymphatic vessel which drives the blood or lymph toward the veins. This type of heart is found in some crustaceans and cephalopods. The hearts of fish, am­phibians and reptiles are included under lymph hearts and also called booster hearts.

In cephalopods the heart is of two types:

(i) Systemic—which is situated in the tho­racic cavity of the body and receives oxygen­ated blood from the gills and returns it to the tissues,

(ii) Branchial heart—which is situ­ated near the gills and pump the blood through the gills.

The contraction of the branchial hearts, helps to receive the deoxygenated blood from the different parts of the body, boosts the pressure of the blood, by which it reaches to the capillaries through the gills.

4. Chambered heart:

When the heart attains more than one special chamber such as auricle and ventri­cle and by their contraction (systole) and expansion (diastole) the blood circulation throughout body is maintained. The exam­ples are some gastropods, cephalopods and all vertebrates.

The heart of fishes contains one auricle and one ventricle and the heart is called venous heart, because the heart always receives venous (deoxygenated) blood. In amphibians and most reptiles there are two auricles and one ventricle. In croco­diles, birds and mammals, there are two auricles and two ventricles.

On the basis of initiating contraction of heart muscle (pacemaker mechanism) in all animals the hearts are divided into two groups:

1. Myogenic heart:

The hearts where the initiating impulses for contraction arise within the muscles itself (intrinsic). The cardiac muscle of vertebrates is myogenic and capable of generating an action potential and depolarization within the muscle itself, as a result the contraction starts.

Ultimately self-excitation takes place in a specialized muscle fibre group centres (e.g., Sinuauricular, SA node). The example of this type of heart is seen among most molluscs, some insects and vertebrates.

2. Neurogenic heart:

The hearts where the initiating impulses for the contraction of heart muscles originate in neurons (motor nerves) that drive the heart muscles (extrinsic). This type of heart is seen among arthropods (e.g., higher crus­taceans, xiphosurans, lobsters, etc.) and some annelids (e.g., earthworms).

11. Respiratory System of Phylum Annelida:

Usually the entire surface of the skin is used in respiration of annelids, but there exist specialised respiratory structures in many annelids (Fig. 17.57). In many Polychaetes there are highly vascularised gills attached to the notopodium as in Glycera.

Eunice and Nepthys. In Arenicola the gills are red in colour. Some consider the tentacles of Sabellids and Serpulids as respiratory struc­tures. In Galeolaria ((a Serpulid) the branchiae are located anteriorly. They are branched and associated with an operculum.

In Nereis amongst the polychaetes the parapodia and their adjoining areas are highly vascularised, oxygen enters the body through diffusion. It has been estimated that the O₂ capacity is 11.5 cm³ for 100 cm³ of blood in this species. In resting condition the O₂ up­take is slowed down in Nereis.

Larger forms of Oligochaetes have richly vascularised epidermis. In aquatic forms various types of respiratory organs are seen. Elongated appendages in the anal region of Dero serve as respiratory structures. In Branchiura the last 40 segments are provided with thread-like, blood-filled evaginations. The members of the family Tubificidae use hind gut as respiratory surface.

In Hirudinea the thick network of coe­lomic capillary channels between the epider­mal cells helps in respiration. But many Pisicolidae have special respiratory struc­tures. These are vascularised evaginations having connections with coelomic channels. Similar evaginations in Ozobranchus are lamellated. Hirudo and Haemopsis can respire both on land and in water.

12. Reproductive System of Phylum Annelida:

Asexual:

Amongst the annelids asexual reproduc­tion is encountered in many polychaetes. In forms, like Filigrana and Salmacina, trans­verse fission occurs near the posterior end of the body dividing the animals into two unequal parts. The anterior part of the half regenerates a new pygidium and the poste­rior half regenerates new cephalic regions.

The sexes of the two individuals produced by such fission are always identical. That means metagenesis or alternation of genera­tion is lacking. In some forms, like Syllis hyalina, a constriction occurs somewhere in the middle of the body. The constriction deepens and ultimately two individuals are formed.

The anterior half with the original head behaves like a non-sexual part and regenerates anal region. While the posterior half develops a new head and becomes an independent male or female individual. That means metagenesis or alternation of genera­tion is eminent in this case because sexual worms are being formed out of non-sexual worms.

In Autolytus and Myrianida a zone of proliferation exists between the anterior non­sexual part and posterior sexual part. This zone gives rise to a series of zooids which remain arranged in a linear fashion. The posterior-most zooid in the chain is oldest and most developed.

The sexes of the indi­viduals in the chain are always identical. Syllis ramosa lives a sedentary life inside the canal system of some deep sea sponges. In this type some of the parapodia become transformed into buds which grow laterally and form a colony.

Some branches from the bud develop parapodia, head and gonad and ultimately leave the parent body to form sexual individuals. In Trypanosyllis buds come out from the undersurface of last two segments.

Asexual reproduction is not very com­mon in oligochaetes and Hirudinea.

Sexual:

Most polychaetes are dioecious, but sexual dimorphism is seldom encountered. Well-formed gonads occur only in few polychaetes. The reproductive cells mature in clumps on the walls of the coelom and it is believed that they arise from specially determined cells and not from coelomic epi­thelium.

These clumps form gonads during reproductive season. Gonads may occur in most segments or they may remain limited in some posterior segments. On maturity, the peritonium covering the gonads bursts and sperms or ova, as the case may be, are liberated into the coelomic fluid. Finally the sperms or ova come to the outside of the body through body wall or through ducts.

Often nephridial ducts become transformed temporarily into genital ducts to liberate the reproductive cells outside. In Phyllodoce, geni­tal funnels appear and get connection with the nephridia only when the gonads become mature. Fertilization is external but in some forms copulation may occur as the females of these species are provided with receptacles.

All oligochaetes are hermaphrodite and have well-defined gonads which remain lim­ited to few segments only. Testes are one to four pairs and lie in adjacent segments.

The testes remain encased in special sacs, called seminal vesicles. Male- reproductive cells are detached early from the testes and they are nourished in these vesicles. The sperm ducts bear funnels at their tips and traverse several segments posteriorly, before opening to the outside through the gonopore.

Ovaries are strictly one pair and are lo­cated posterior to the testes bearing seg­ments. Each ovary is followed by an oviduct and the two oviducts unite before opening to the outside through gonopore. Though her­maphrodite, the oligochaetes practise cross- fertilization. Reciprocal copulation enables the worm to pass on sperms of one worm to the other. The sperms thus received remain stored in spermatheca.

Hirudinea, like the oligochaetes, are her­maphrodite. Female reproductive organs consist of several rows of reproductive cells encased in a pair of ovarian tubes. The ovar­ian tubes are short in Gnathobdella, long in Rhyncobdella and looped in Pharyngobdella. The anterior end of the tube is prolonged as oviduct. The two oviducts often fuse to form a vagina which opens to the outside in the 11th segment.

Male reproductive system consists of 4- 17 pairs of metamerically arranged testis sacs. There is a pair of longitudinal ducts, called vas deferens. The vas deferens widens at the tip to form the seminal vesicle. Each seminal vesicle opens into a median atrium. The atrium opens to the outside through the 11th segment. The atrium often remains pro­vided with an intromittent organ, called penis.

13. Nervous System of Phylum Annelida:

The nervous system, at least during de­velopment, exhibits a segmented condition, that is, its segmentation corresponds to the segmentation of the mesoderm. Each seg­ment carries a pair of ganglia on parallel cords and remains connected by commissures and thus give a ladder-like appearance.

As development proceeds, there occurs fusion of ganglia, the nerve cord and commissures. Fusion of anterior segments in many forms results the formation of a sub-pharyngeal ganglion or ganglia. From the apical sense organ of the trochophore arises the supra- oesophageal ganglion or the brain. The supra and sub-pharyngeal ganglia remain connected with each other through the circumoesophageal connectives.

14. Regeneration in Phylum Annelida:

In many polychaetes there is remarkable property of regeneration. In Autolytus pictus any one of the segments between first and seventh can regenerate head and anterior segments after removal. When the segments from 8 to 13 are excised only 4 segments with parapodia and the head will be formed.

When cut behind 14-segment only one seg­ment with setae will develop. Only the head is formed from the posterior end. Harrington (1969, 79) has pointed out that it is easier to appreciate the essentially primitive nature of regeneration which has simply been brought into special prominence under the influence of natural selection.