In this article we will discuss about Nephridium of Annelida:- 1. Definition of Nephridium 2. Research Work on Excretory System 3. Classification of Nephridia 4. Physiology 5. Functions.
Definition of Nephridium:
An excretory tubule which opens to the exterior through the nephridiopore and the inner end of the tubule is blind (associated with terminal cells or solenocytes) in the protonephridium or opens in the coelom through the ciliated funnel or called nephrostome in metanephridium.
Research Work on Excretory System:
The excretory system in Annelida has had a long history. Many zoologists namely Gegenbaur (1833), Stephenson (1930), Goodrich (1946), K. N. Bahl (1934, ’42, ’45, ’46, and ‘47) and Ramsay (1947) worked on nephridia of different species in Annelida. K. N. Bahl worked on nephridia of Pheretima posthuma and Ramsay worked on Lumbricus.
Structure of a Typical Nephridium:
(i) A typical nephridium (Fig. 17.58) consists of a nephrostome or a ciliated funnel which hangs into the coelom and leads to the nephridial duct.
(ii) The nephridial duct or body of the nephridium may be long, short, convoluted or modified otherwise.
(iii) The duct is ciliated internally, situated transversely and is accompanied by blood vessels.
(iv) The nephridial duct opens to the exterior by an opening, called nephridiopore.
In general, the excretory system consists of paired lobes, called nephridia, which are metamerically arranged and the inner aperture of the nephridium lies in the coelom, and the outer aperture is situated in the integument.
Each nephridium develops from a single cell, called nephroblast. The nephridia are ectodermal in origin.
Classification of Nephridium:
Prof. K. N. Bahl classified nephridia which appears to be most plausible and accepted all over the world.
In following up the developmental stages of nephridium of Annelida two types of nephridial systems are encountered (Fig. 17.58). They are (A) Provisional or embryonic nephridia and (B) the Permanent nephridia.
(A) Provisional or embryonic nephridia:
Embryonic nephridia are temporary structures and disappear as soon as permanent nephridia start developing.
It is divided into following:
1. Embryonic head nephridia:
(i) Paired in larva and embryos.
(ii) Their ends lie in the embryonic head cavity.
(iii) Solenocytes at the end of the tube occur.
(iv) They are found in many polychaetes and oligochaetes.
(v) They are branched occasionally, e.g., Echiurus, Polygordius.
2. Embryonic trunk nephridia:
(i) Strictly segmentally arranged.
(ii) Occur one pair in each segment.
(iii) Funnel opening into trunk region.
Embryonic trunk nephridia:
Embryonic trunk nephridia may persist in those forms where permanent nephridia do not develop. Five such pairs of nephridia persist in Nereis. In most oligochaetes permanent nephridia are absent in some of the anterior segments. The same is true for many polychaetes and hirudinea.
It is probable that provisional nephridia were present in the larval stage and subsequently permanent nephridial development never occurred there. Thus the absence of permanent nephridia in the anterior segments may be explained.
Structurally embryonic nephridium is similar to those of permanent nephridium. But in Glycera and Phyllodoce the inner aperture and in Hirudinea both inner and outer apertures are absent.
(B) Permanent nephridia:
Characters same as typical nephridium:
(i) Ciliated nephrostome opening into coelom.
(ii) Long internally coiled duct opens externally by nephridiopore.
(iii) Nephrostome and nephridiopore may occur in the same segment or the former a segment forward.
Depending upon the size and number present in a segment the nephridia are divided into 4 types:
(a) Meganephridia or Holonephridia.
(b) Micronephridia or Meronephridia
(d) Tufted nephridia.
(a) Meganephridia or Holonephridia:
These are large in size and one pair in each segment.
(b) Micronephridia or Meronephridia:
These are small and numerous in each segment. It is believed that the micro-nephridia are nothing but broken or disintegrated meganephridia.
These are formed by the modification of salivary glands in buccal and pharyngeal region in the form of clusters, found in oligochaeta, aid in digestion.
(d) Tufted nephridia:
These are derived from micro or macro-nephridia, incompletely branched and are grouped together. These are usually found in one or several of pre-clitellar segments of many earthworms.
Bahl (1942) states that they represent an intermediate stage between a holonephridium and a group of completely separated meronephridia.
Again, the nephridia may be closed or open according to the presence of nephrostome:
(i) Open type:
When the nephridium possesses a funnel.
(ii) Closed type:
Lacking a funnel in the nephridium.
The nephridia may be exonephric type or enteronephric type according to their opening to the exterior, found in pheretima:
(a) Exo-nephric type:
Having exterior opening, e.g., Integumentary nephridia.
(b) Enteronephric type:
Open into the enteric canal, e.g., Septal nephridia and pharyngeal nephridia.
All the nephridia in Pheretima are of micronephric type. The nephridia of Lumbricus, Chaetogaster and Nereis are meganephric. It is believed that the micronephridia are nothing but broken or disintegrated meganephridia.
In Megascolecidae both micronephridia and meganephridia are present even in the same segment. In Serpula meganephridia are present in the anterior segments while micronephridia occur in the posterior segments.
Some polychaetes possess Protonephridia in which inner end of each nephridium terminates in flame cells and there is no nephrostome, as seen in flatworms and usually other type of nephridia, called metanephridia in which inner end of the nephridium has an open funnel or nephrostome.
Permanent Nephridia in different Classes:
In most polychaetes metanephridia are present.
A typical metanephridium consists of the following:
(a) An inner ciliated aperture opening into the body cavity or coelom and is called nephrostome.
(b) A canal or coiled tube connected to the nephrostome. The canal is dilated internally and sometimes its internal wall is glandular.
(c) A terminal end which usually terminates in a laterally placed aperture, called nephridiopore.
In Errantia each segment has a pair of nephridia. Arenicola is provided only with six pairs. In Capitellidae there may be one to six pairs of permanent nephridia in each of the trunk segments. In Terebellidae there are one to three pairs of nephridia in the thorax.
Sabellidae and Serpulidae have one pair in the thorax. But in all these families numerous nephridia occur in the posterior segments. Numerous nephridia are housed in Earthworm. The wide funnels and short ducts of these nephridia suggest that they serve as gonoducts in some forms.
In many polychaetes like Phyllgdoce segmentally arranged ciliated funnels, called coelomoducts, are present (Fig. 17.59). These ducts rarely open to the outside and often coalesce partially or completely with the nephridia and thus the function of excretory and reproductive ducts combine in one set of segmental organ.
Some families like Phyllodocidae and Glyceridae have protonephridia in place of metanephridia. In protonephridia (Fig. 17.60), the ciliated coelomic aperture (nephrostome) is absent.
The tubes thus open blindly in the coelom and are branched. Separate or groups of specially modified cells, called solenocytes, remain attached to the blind end of the tubes. Each solenocyte is a round cell with a slender tubular projection which anchors on the blind tube.
Electron micrographs show that the long tube of protonephridium consists of a membrane with more than 15 longitudinal ridges or rods and internally carries an unusually long flagellum to drive the internally accumulated fluid (Fig. 17.60).
In oligochaetes metanephridia are usually present in all segments excepting a few anterior segments. A metanephridium differs from a protonephridium in having a ciliated funnel or nephrostome. In aquatic forms reproductive segments lack nephridia.
As a rule there is one pair of nephridia in each segment but in Brachidrilus, there are two pairs, in Trinephros, there are three pairs, and there are four pairs in Acanthoarilus in each segment. In tropical Megascolecidae, the nephridial primordia in each segment splits and as a result numerous nephridia occur in each segment. These nephridia are called diffused or plectonephric nephridia.
Beddard (1895) erroneously stated that these nephridia are connected with one another so as to form a network. But according to Bahl (1919) each nephridium is a separate and discrete structure and there is no network of any kind. In the tropical Phretima posthuma many nephridia open into the pharynx (Peptonephric) and in the alimentary canal (Enteronephric). This is a device for the reabsorption of water.
In Hirudinaria the permanent nephridium is lacking in many anterior and posterior segments. The metanephridium consists of a ciliated nephrostome or funnel that leads into an ampulla filled with amoebocytes and closed off against a nephridial duct.
Besides the nephrostome all parts of the nephridium are formed by a close set of gland cell traversed by intracellular spaces or ducts. The nephrostome may start from the coelomic spaces, from ventral median channel (Glossiphonia), from contractile spherical enlargement or ampullae (Haemopis) or from blood sinuses in which the testes lie (Hirudo).
In Pontobdella distinct nephridium is absent and its place is taken up by a complex network situated on the ventral side of the body.
The anal tubes in Echiuroidea are considered as excretory structures. The nephridia act as osmoregulatory organs specially in freshwater forms.
Each mesodermal pouch in ancestral coelomates was provided with a pair of ducts, called coelomoducts (gonoducts), which served as a passage for the exit of gametes and a single nephridial tubule, for the removal of nitrogenous wastes.
These primitive nephridia resembled the platy- helminthic type of excretory organs. That is, they consisted of ectodermal tubules projecting into the coelom and ending in specialised cells, called solenocytes.
In many polychaetes the association between the coelomoduct (gonoduct) and nephridium makes an interesting study. Instead of remaining separate they show total or partial fusion and form a dual segmental organ, called nephromyxia. As the nephridium is ectodermal in origin and the coelomoduct is mesodermal in origin a nephromixium is formed by the participation of both ectoderm and mesoderm.
The nephromixium performs two functions. In one hand, it serves the function of excretion and on the other hand, it also serves as a passage for the exit of gametes. In some cases they share the same external opening but when the association between them becomes very close they often share the same duct.
The combinations of the coelomoduct and the nephridium are of the following types:
In this case the coelomoduct becomes united to a protonephridium. Both reproductive and excretory products are conveyed to the exterior by it. Protonephromyxia condition occurs in Phyllodoce.
In this case the coelomoduct becomes united to a metanephridium as in Hesione.
In this case complete fusion between the coelomoduct and the nephridium results in the formation of a simple funnel-like organ only. Mixonephridia condition is most prominent in Arenicola.
(F) Ciliated Organs:
The coelomoducts alone become very much reduced in some case and give rise to ciliated organs which do not open to outside. In Nereis such ciliated organs are found and they remain attached to the dorsolateral longitudinal muscles.
Physiology of Nephridium:
In most annelids, the blood vascular system and coelom (if present) are involved in the excretion of waste products. The polychaetes in which the blood-vascular system is absent or reduced contain protonephridia. The remaining groups of polychaetes and others possess blood-vascular system and metanephridia.
In proto-nephridia, the ultrafiltration of the coelomic fluid takes place with the help of terminal cells (e.g. solenocytes) and the filtrate fluid passes down through the protonephridial tubule. Along the protonephridial tubule some sub-substances such as salts and amino acids are reabsorbed and the chief excretory product ammonia is excreted through the nephridiopore.
The mouth of metanephridium contains open ciliated funnel or nephrostome through which coelomic fluid is drawn by the action of cilia of funnel and the fluid when passing through the metanephridium tubule, some substances like salts, aminoacids are resorbed and the nitrogenous waste products like ammonia (20%), amino acids and urea (40%) are excreted and the urea level varies in different groups of annelids in which environment they live.
Functions of Nephridium:
(i) It eliminates the liquid nitrogenous waste products from the body to the exterior.
(ii) It eliminates the basic and non-volatile acid radicals from the body.
(iii) It maintains the water balance of the body.
(iv) It regulates the osmotic relation between the blood and tissue.
(v) In some cases they act as gonoducts (coelomoducts) by conveying reproductive units.