The term systematics is derived from Latinized Greek word-systema-as applied to the systems of classification developed by early naturalists, notably Linnaeus.
According to Simpson, Systematics is the scientific study of kinds and diversity of organisms and of any and all relationships among them. Systematics includes taxonomy, identification, classification and nomenclature and all other aspects of dealing with different kinds of organisms and data accumulated about them is also included in systematics.
Origin and Development of Systematics:
The origin and development of systematics and human civilization started simultaneously, i.e., civilization of human being is the systematics or in other words we say that in our daily life those people who maintain their houses systematically that means they are civilized. As the knowledge developed, they started naming the plants and animals according to their own choice.
The clue for the earliest classification comes from Vedas and Upanishads (1500 BC to 600 BC). In Vedas and Upanishads, several technical terms are used in the description of plants and plant parts, both morphologically and anatomically.
Plants of medicinal importance were collected and studied. Charaka and Susruta, the two eminent ancient Indian scholars and ayurvedic physicians, contributed a lot to our knowledge of diversity and utility of organisms.
One of the remotest works dealing with plant-life in a scientific manner is the Vrikshayurveda (science of plants and plant-life) compiled by Para Sara, even before the beginning of Christian era, which formed the basis of botanical teaching and medical studies in ancient India.
Several early Greek scholars notably Hippocrates (460-377 B.C) and Democritus (465-370 B.C.) made observations on animals but their classification was not useful. Later on Aristotle (384-322 B.C.) also studied the living organisms, viz., plants and animals and gave statement on classification that “animals may be characterised according to their way of living, their action, their habit and their bodily parts”.
He classified the major groups of animals as birds, fishes, insects and whales. Aristotle is called the “Father of biological taxonomy.” Theophrastus (370- 385 B.C.) who was the student of Plato and then of Aristotle, is known as the “Father of Botany”, classified all plants on the basis of form and texture and divided them into trees, shrubs, under-shrubs and herbs.
His classification was strictly artificial and in his “Historia Plantarum” he has classified and described 480 plants.
Albertus Magnus (1193-1280) recognised the differences with the help of crude lenses in between the monocotyledons and dicotyledons and recognised the classification of Theophrastus. Otto Brunfels (1464— 1554), a German, first recognised the Perfecti and Imperfect group of plants based on the presence or absence of flowers respectively.
Jerome Bock (1498-1554), another German, classified the plants into trees, shrubs and herbs. Andrea Cesalpino (1519-1603) taxonomically classified plants on the basis of habit and subdivided them on the characters of fruits and seeds.
Gaspard Bauhin (1560-1624) classified the plants on the basis of texture and form. He was first to find out the binomial nomenclature although it is usually credited to Linnaeus. In the history of systematics, John Ray (1627-1705) has done a great job. Before him the classification was baseless and there was not any strong scientific background. John Ray travelled widely in Europe with Francis Willougby (1635-1672).
They not only observed and collected plants and animals, but also planned to classify them. Unfortunately, Willougby died prematurely and later John Ray published his Historia Generalis Plantarum in three volumes between 1668 and 1704. In this book, John Ray described accurately and in meticulous detail and catalogued over 18,000 plants.
He was the first person who recognised the difference between genus and species and through valuation of both similarities and dissimilarities in animals arrived at a more natural higher classification than of the former persons.
After John Ray, the most remarkable man in the field of taxonomy was Carolus Linnaeus (1707- 1778) who is often referred to as the “Father of Taxonomy” for his outstanding contribution in this field. Linnaeus visited many European countries and made careful observations on the plants and animals.
He published his scheme of classification in his famous book Systema Naturae in 1753. The 10th Edition of this book was published in 1758 and the 1st January of that year marks the beginning of the consistent application of what is known as the “binomial system of nomenclature”.
This system which introduces the principles of naming an organism by two words, was first proposed by Linnaeus and is universally followed today. This work of Linnaeus became the foundation of systematics. He also laid down the clear delimitations for species.
Jean Baptiste Monet de Lamarck (1744-1829) wrote Flore Francoise in 1778 in which he laid down the principles of his concept of natural classification. Charles Darwin (1809-1882) a great evolutionist, on the basis of his extensive studies and with the help of convincing evidences gathered on the Voyage of the Beagle, explained the origin of species through natural selection.
The evolution theories of Lamarck and Darwin not only greatly influenced Linnaean classical systematics but also totally rejected the pre-Lamarckian idea of the fixity of species. Ernst Haeckel (1866) introduced the method of representing phylogeny by means of trees or branching diagrams.
Sir Julian Huxley (1940) introduced the new term “New Systematics” which incorporates the results of recent studies in various branches of life sciences in systematics which modify some of the older ideas of classical systematics. Today the definition of species is based on population because the development of population genetics which in turn influenced the further development of population systematics.
Utility of Systematics (Taxonomy):
No scientific ecological survey can be carried out without the most painstaking identification of all the species of ecological significance. A similar dependence on systematics is true for other areas of science. Even the experimental biologists have learned to appreciate the necessity for sound taxonomy. The systematists can fill many gaps in our knowledge.
There are more than one million species of animals and more than half million species of plants described in the biosphere. In addition, there must be a large number of plant and animal species yet to be described. Without the knowledge of systematics the discovery of new species of plants and animals is not possible and we cannot know much about the organisms of our surroundings.
Therefore, there is a great necessity or utility of systematics.
Systems of Classification:
There are three different systems of classifications which have been proposed so far by different taxonomists.
1. Artificial System of Classification:
This type of classification is based upon characters of convenience without relation to phylogenetic significance; classification based upon characters erroneously presented to indicate phylogenetic relationship; also classification based on a single arbitrarily chosen criterion, instead of an evolution of the totality of characters known as artificial classification.
This system of classification was adopted by Pliny in the first century A.D. for animals on the basis of habitats, e.g., land, air and water.
Accordingly animals were classified into two categories on the basis of their flying ability:
(i) Animals that can fly and
(ii) Animals that cannot fly.
In the first group unrelated animals like butterflies, birds and bats were placed together.
The classification of plants on the basis of habit into:
(ii) Under shrubs,
(iii) Shrubs and
(iv) Trees is also an artificial one.
The criteria used in this classification, although very simple and easy to follow are arbitrary and do not reflect any natural relationship existing among the organisms. Moreover, it leads to heterogeneous assemblage of unrelated organisms under one heading and does not do justice to the totality of characteristics of an organism.
The system adopted by Linnaeus was also artificial in which the plants were classified on the basis of number and arrangement of stamens and carpels. Closely related species of organisms are kept far apart in this system of classification.
2. Natural System of Classification:
The natural classification may be defined as “Classification based on characters which indicate natural relationships”. The organisms of a natural systematic category agree with one another in so many characters because they are descendants of one common ancestor. The natural system of classification is based on similarity.
Zoologists and botanists differ in their interpretation of the implication of this system of classification. According to zoologists, the natural system of classification includes the phylogenetic and evolutionary trends which are evident in the word “natural”.
Botanists hold the opinion that the natural system does not necessarily include phylogenetic trends of plants. Here they proposed the “phylogenetic system” of classification separate from the “natural system” to include the evolutionary trends in plants. The “natural system” of classification of the plant kingdom was proposed by George Bentham (1800-1844) and Joseph Dalton Hooker (1817-1911).
3. Phylogenetic System of Classification:
The phylogenetic system is based on the evolutionary and genetic relationship of the organisms. It enables us to find out the ancestors or derivatives of any taxon. Our present-day knowledge is insufficient to construct a perfect phylogenetic classification and all the present phylogenetic systems are formed by the combination of natural and phylogenetic evidences.
This system is adopted by Adolph Engler (1844-1930) and Karl A.E. Prantl and John Hutchinson (1884-1974) in classifying the plants.
Nomenclature is defined as the system of naming of plants, animals and other objects or groups of plants, animals and other objects. Scientific names are the language of taxonomists. When a taxonomist identifies and describes the natural group of animals, he gives appropriate scientific names to the groups.
Common names do not serve the purpose because a particular animal is known by different names in different parts of the world. For example, the bird that we know as gauraiya in India and Pakistan is known by different names in other countries, house sparrow in England; Pardal in Spain; Musch in Holland; Suzune in Japan and so on.
Moreover the common name may be used for different kinds of animals. For example, the name kenchua is used both for the earthworm and Ascaris. On the other hand, a scientific name is universally used for a particular species or particular group of animals. For example, gauraiya or house sparrow is termed Passer domesticus by zoologists throughout the world.
To ensure that one scientific name stands for one particular kind of animal everywhere and is the only name for that organism, the taxonomist must see the following:
(1) The name chosen for an animal has not been already given to some other animal or plant.
(2) The animals and plants have been described in such detail that another taxonomist can determine from the description exactly the kind of animal to which the name has been given.
(3) The animal or plant has been duly placed in the system of classification establishing its relationships.
The history of binomial system of nomenclature is very long. Two centuries before Christ, Cato used two names for plants in his De Re Rustica. But he had no knowledge that genera were usually composed of several species. Later, two ideas developed with the evolution of the idea of nomenclature.
One was to translate the descriptive Greek nouns used for genera into Latin. As a result of this translation into Latin, the generic name consists of two words.
These were called binary generic names. The other tendency was to use descriptive phrases for specific names. These tendencies in conjunction gave rise to a polynomial system of nomenclature. According to this system the name of a plant was composed of several words in a series which bore a brief description of the plant.
For example, Bentham used the name Caryophyllum saxatilis, Folis gramineus umbellatis corymbis to represent the Caryophyllum which grew on rocks, with grass-like leaves and flowers in umbellate corymbs. This was the system of cumbersome.
In the middle of the sixteenth century, a number of binary generic names were changed by Brunfels to single ones. Dodonaeus and Gaspard Bauhin later followed in general the binomial system but it is usually credited to Linnaeus who used it more than hundred years later in his Species Plantarum. According to this binomial nomenclature, long names were cut short so that they could be used with greater convenience.
This system postulates that every individual of plant and animal kingdom consists of only two words in Latin; the first word designating the genus and the second, the additional epithet, that signifies the particular species with that genus. It is also known as two naming system or binary system. For example, the genus of modern horse is Equus.
Among its species are Equus caballus and Equus asinus. The word caballus and asinus standing above have no meaning in taxonomy; they are not names of species or anything else. Only when they are part of a binomial combination, they are meaningful taxonomically and then it is the combination that is the name of the species.
Often, specific names of animals and plants are given in honour of some persons. If the person honoured is a man the specific name ends in “i”. For example, the earthworm, Lumbricus friendi is named after Rev. H. Friend. If the person honoured is a woman, the specific name ends “ae”. Sometimes, the specific name indicates a locality (e.g., indica for Indian) or colour (e.g., niger for black).
In scientific literature, it is a general practice to write a specific name followed by the name of the person who first described the species and the year when he did so. For example, the scientific name of man is written as Homo sapiens Linnaeus 1758. If the species, after its publication, is transferred to any other genus or the generic name is changed the first author’s name is written in brackets (parenthesis).
For example, Panthera leo (Linnaeus) means that species leo was originally assigned by Linnaeus to some other genus (Felis).
This system of nomenclature is employed to name the subspecies. In classification the subspecies is a category below the species. The subspecies name is also a Latin or Latinised word and follows the name of the species to which it belongs. For example, the specific name of the house crow, which occurs throughout India, Pakistan, Myanmar and Sri Lanka is Corvus splendens.
The house crows of India and Pakistan, Myanmar and Sri Lanka differ with each other in minute morphological features and are, thus, separated as distinct subspecies. The Indian and Pakistani house crow has been assigned the sub-specific name Corvus splendens splendens, the Myanmarian house crow, Corvus splendens insolens and the Sri Lankan house crow, Corvus splendens protegatus.
The full scientific name of subspecies is, therefore, a trinomial name consisting of three names: the names of genus, the species and subspecies itself.
Rules of Nomenclature:
In 1898, the International Congress of Zoology organised an International Commission on Zoological Nomenclature to formulate a set of rules, which would be binding for all taxonomical publications. The aim of International Code of Nomenclature is to make the stability in naming the taxa, avoiding the use of names which may cause error, ambiguity or confusion.
The standardisation and legislation of nomenclatural practices are usually made at International Botanical and Zoological Congresses. This is done in order to put the nomenclature of the past in order and to provide guidelines for that of the future.
A few commonly followed rules and recommendations which may be considered as the essentials of a code of nomenclature are given below:
1. The system of nomenclature adopted is the binomial system to indicate the specific name and trinomial for sub-specific name.
2. The name of the genus is a single word in a nominative singular and must begin with a capital letter. The name of the species may be a single or compound word and must begin with a small letter.
3. The name of the author, who first publishes the name when describing it, should follow the species name and should rarely be abbreviated and is printed in roman type.
4. The scientific names of animals and plants must be different.
5. The names must be in Latin or Latinized form and are usually printed in italic type.
6. Within the animal and plant kingdom, no two genera can have the same name, and within a genus, no two species can have the same name.
7. The generic or specific name first published is the only one recognised. All duplicate names are synonyms.
8. When the name of the genus is not the one under which a species is placed by the original author, or if the generic name is changed the original author’s name is written in parenthesis.
9. The formation of family and subfamily names follow rules which are different in the Zoological and Botanical Codes.
10. A name must retain its original spelling, obvious errors and misprints may be corrected; diacritic marks are dropped.
11. A name may be based on any part of an animal or a plant, or on any stage of an organism’s life history.
To make new names the following suggestions are followed:
1. A name should be in Latin or easily converted into Latin form.
2. A name should not contain less than three and more than twelve letters.
3. A name should be easy to pronounce.
4. The name given should preferably describe some characteristics of the organism.
5. A name should not be derived from two languages.
6. A name should not be frivolous.
Familiarity with Taxa:
According to Darwin (1850), “All organic beings are found to resemble each other in descending degree, so that they can be classed in groups under groups”. All major groups of animals can individually be subdivided into smaller and smaller subgroups.
Within the vertebrates we can distinguish subgroups such as birds and mammals; within the mammals, carnivores and rodents; within the carnivores, those that are dog-like, those that are cat-like and so forth. If one wants to construct a classification of these species, this classification is not arbitrary.
The task of classification then is the delimitation of these groups and their arrangement in an orderly sequence, i.e., hierarchy.
Since the number of animal and plant species is very large, it is not possible to either know them individually by their names or to refer them in the literature. This necessitated arranging them into categories and taxa of different grades. Then arranging these categories and taxa in an ascending order so that a higher category includes one or more lower categories and higher taxa include one or more lower taxa.
Linnaeus was the first taxonomist to establish a definite hierarchy of taxonomic categories recognised within the animal kingdom. These are classes, order, genus, species and varieties. The varieties, used by Linnaeus as an optional category of various types of intraspecific variants, was eventually discarded or replaced by the species.
These few categories sufficed to cope with small number of animals and plants known at that time.
However, as the number of known species increased and with it our knowledge of the degrees of relationship of these species, the need arose for a more precise indication of the taxonomic position of species and inserting additional ones among them.
Most are formed by combining the original category names with the prefixes super or sub. Thus, there are superorder, super-families and subfamilies, etc. The most frequently used additional new category name is perhaps the term tribe for a category between genus and family.
Vertebrate palaeontologists also used in routine the category cohort between order and class. Some authors used terms for additional subdivisions, such as cladus, legio, and sectio. Some used infraclass below the subclass and infra-order below the suborder.
The generally accepted categories are the following:
Indicated in the parenthesis are the standardised endings for the names of tribes, subfamilies, families and super-families. The systematic hierarchy or Linnaean hierarchy as it is commonly known, with its need for arbitrary ranking has often been attacked as an unscientific system of classification.
Alternate methods, such as numerical scheme have been proposed but have not found favour among taxonomists, primarily for following two reasons:
(i) Assigning definite numerical values to taxa demands a far greater knowledge of the relationships of taxa than can be inferred from available evidences.
(ii) An assignment of such values would freeze the system into a family which would preclude any further improvement.
It is the very subjectivity of the Linnaean hierarchy which gives it the flexibility required by the incompleteness of our knowledge of relationships. It permits the proposal of alternate models of relationships and gives different authors an opportunity to test which particular balance between splitting and lumping permits the presentation of maximum amount of information.
Like any other scientific theory it will forever be provisional.
Taxon and Category:
Taxon (Plural: Taxa.):
The taxa are the groups of animals generally groups of species. The words insects, fishes, birds, mammals in animals; algae, fungi, ferns, mosses, grasses, etc., in plants are the groups of organisms. These are the concrete objects of classification. Any such group of such population is called taxon.
But in ordinary usage only the so called basic categories (genus, family, order, class, phylum, kingdom), are treated as such groups. The super taxa at all levels are treated as groups of the basic taxa (a superclass as a group of classes) and the sub taxa at all levels as a subdivision of the basic taxa (a suborder as a section of the order).
According to Simpson “A taxon is a group of real organisms recognised as a formal unit at any level of a hierarchic classification.”
According to Mayr, “A taxon is a taxonomic group of any rank that is sufficiently distinct to be worthy of being assigned to a definite category.”
The group of animals are taxa. Each taxon is placed at some level in hierarchy. A category designates rank or level in a hierarchic classification. It is a class, the members of which are all the taxa assigned a given rank. A category can be higher or lower than some other one, so we may speak of a higher category.
The categories have names, but these are terms and not names in biological nomenclature. They are kingdom, phylum, class and so on. It is an error to state “this animal belongs to category Mammalia”, Mammalia is the name of taxon not of category.
Species is the most important category in the taxonomic hierarchy. It is the basic unit in taxonomy and also in evolution. Its definition has long been one of the major problems of taxonomy. Several definitions and aspects are discussed about the definition of species.
According to Blackweldler, the species can be defined as follows:
(i) One of the groups, the one placed in the category called the species level (a species group).
(ii) The category or level at which the species groups are placed (the species level).
Two main definitions are given for species.
These are as follows:
Biological species are usually defined as groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. This gives theoretical groups which can seldom be distinguished in practice.
Simpson has pointed out that all the definitions of animal species give us biological species. He, therefore, prefers the name genetical species for this and cites also bio-species. (It should be noted that populations do not interbreed, only individual animals and plants interbreed).
Genetical species are groups of interbreeding populations which are reproductively isolated from each other. They are, thus, the same as biological species. For example, in Homo sapiens, the sapiens is a species of Homo.
The taxa placed in the genus category are the genera. These are groups of species brought together by the taxonomists as evidenced by the fact that generic name is a part of each name of each of the included species.
The genus is involved from naming of the first species of whose name it forms the first part. Again it is possible to say that a genus is any group of species included under the one generic name by any taxonomist. This is completely subjective, but it is approximately the working definition which is the basis of most taxonomic work.
The genus cannot properly be described as the next higher level above the species, because it is common and always possible to. use subgenera between the genus and species, and to use also sections or other informal categories.
A pragmatic definition of the genus states, “A genus is a taxonomic category containing a single species or a monophyletic group of species, which is separated from other taxa of the same rank (other genera) by a decided gap.” The genus Felis includes the golden cat (Felis temincki) the fishing cat (Felis viverrina) and the leopard cat (Felis bengalensis).
This is a taxonomic category containing one or more related genera and which is separated from other related families by important and characteristic differences. The family Felidae which includes the lion, the leopard, the tiger and all types of cats belonging to different genera. This family is distinctly separate from the family Canidae which includes dogs and foxes.
The order is the basic category of what has been called the order group which includes also super-orders, the suborders, the infra-orders and taxa at any other levels interpolated between superfamily and infraclass. In many phyla, orders are very well known groups, but in some phyla they are less well known than the classes, whereas classes do have a fairly evident uniformity throughout the animal kingdom.
The orders of vertebrates, for example, are scarcely comparable to families in insects and levels vary in other groups. For example, the order Carnivora includes families Felidae and Canidae.
The class is the basic category of what has been called the class group which included also super classes and infraclasses, as well as any others interpolated among these. In the animal kingdom as a whole, the classes are undoubtedly the best known taxa, even the phyla being subject to more differences of the opinion.
A class is generally a subdivision of a phylum. For example, the order Carnivora includes the lion, the cat, etc., are included in the class Mammalia.
The taxa placed in the phylum category are the phyla, subdivisions of the kingdom. They may be assembled into super-phyla or subdivided into subphyla. The phylum Porifera includes three classes such as Calcarea, Hexactinellida and Demospongia.
This is the highest taxonomic category. All animals are included in the animal kingdom and all plants are included in the plant kingdom.
Species as a Category:
The importance of the term species in all fields of biology is so immense that it deserves special consideration. It has already been stated that individual organisms which have many features in common and are able to breed only amongst themselves are encompassed by the term species.
The definition of species is not restricted only to the taxonomists. Nowadays the other fields of biology also consider the species to a great extent. Cytologists, geneticists, ecologists, biochemists and others have also defined the species.
The definitions given by different workers are as follows:
The species inhabiting different geographical areas.
The species normally occupying the same geographical areas.
“These are ones established by the morphological similarity regardless of other considerations” (Simpson).
Bio-species and genetical species:
A group of inter-breeding populations which are reproductively isolated from other such group.
It is a term applied to pairs or groups of very similar and closely related species. When applied to closely related species (in phylogenetic sense) this expression refers to hypothetical species, these cannot be dealt with in taxonomy but can be useful in speculations on evolution.
A species which has been provided a specific name under the International Rules of Nomenclature.
These are lineages (ancestral descendent sequences of populations) evolving separately from each other and with their own unitary evolutionary roles and tendencies.
Polytypic species are those which consist of two or more subspecies.
Monotypic species consist of a single subspecies.
Outline Classification of Animals:
The animals have been classified in various ways by different authors. The classification which follows is based on the classification proposed by Meglitsch, P.A. (1972). In this classification, only larger groups have been taken into consideration.
This is the largest group of animal classification. It includes the entire animal population (fauna) of the world.
It is divided into two subkingdoms:
Protozoa and Metazoa.
Subkingdom A. Protozoa:
About 50,000 species. Acellular microscopic animals. Solitary or colonial. Specialised cell organelles. Single to many nuclei. Nutrition holozoic, holophytic or saprozoic or parasitic. Freshwater, marine or moist terrestrial.
Protozoa (First animals). Characters as those of the subkingdom.
Subkingdom B. Metazoa:
Multicellular animals. Body comprises many cells, usually arranged in layers or tissues.
It is divided into three branches:
Mesozoa, Parazoa, Eumetazoa.
Branch I. Mesozoa:
Cellular animals having the structure of a stereo-blastula,composed of surface layer of somatic cells and interior reproductive cells.
Mesozoa (Middle animals). About 50 species,Worm-like, small. Symmetry bilateral. An external layer of ciliated digestive cells surrounding one or several reproductive cells. Parasitic in cephalopods and other invertebrates.
Dicyema, Rhopalura, etc.
Branch II. Parazoa:
Animals of cellular grade of organisation with incipient tissue formation. Interior cells of several different kinds. There is no mouth or digestive tract and no organ systems are present. The body is porous with one to many internal cavities lined by choanocytes. Sessile, marine, a few freshwater. Solitary or colonial.
Porifera (Pore-bearers).About 5,000 species. Characters as those of the branch.
Scypha or Sycon, Euplectella, Hyalonema, Euspongia, etc.
Branch III. Eumetazoa:
Animals of tissue or organ-system grade of organisation with mouth and digestive tract (except when lost by parasitic degeneration). Interior cells of several kinds. Body not porous and without cavities lined by choanocytes.
Representative Types of Invertebrate Phyla.
Grade I. Radiata:
Eumetazoa with primary radial symmetry. Tissues are present and organ-systems are incipient. Mesoderm, usually derived from the ectoderm, is present as an incipient tissue, without a high degree of cellular specialisation. The only body space is the digestive cavity, which has a mouth, but no anus.
Phylum 4. Cnidaria (Coelenterata):
About 10,000 species. Symmetry radial, biradial or radio-bilateral. The mouth is encircled by tentacles bearing nematocysts. No rows of ciliated plates. Body cavity as coelenteron. Sessile or free swimming. Solitary or colonial. Marine or freshwater.
Hydra, Obelia, Aurelia, Metridium, Corals, etc.
Phylum 5. Ctenophora (Comb-bearers or Comb-jellies):
About 90 species. Symmetry is biradial. Tentacles when present do not encircle the mouth. No nematocysts. Eight radial rows of ciliated swimming plates. Free-swimming and marine.
Pleurobrachia, Coeloplana, Ctenoplana, etc.
Grade II. Bilateria:
Eumetazoa with bilateral symmetry, or those with embryonic bilateral symmetry later modified into radial symmetry. Organ-system grade of organisation. Mostly with a well-developed mesoderm of endodermal origin. Mostly with body spaces other than the digestive cavity. Mouth and anus generally present.
Bilateria is divided into two divisions:
Protostomia and Deuterostomia.
Division A. Protostomia:
Bilateria in which the mouth arises from the blastopore or from the anterior margin of the blastopore.
Protostomia is subdivided into three subdivisions:
Acoelomata, Pseudocoelomata, and Coelomata.
Subdivision 1. Acoelomata:
No body cavity or coelom. Space between the body wall and digestive tract is filled with mesenchyme. Excretory system of protonephrida with flame bulbs.
Bilateria without a coelom. With mesenchyme between the body wall and digestive tract. Excretory system of protonephridia with flame bulbs. Body un-segmented or consisting of a strobila, with youngest segment toward the head.
Phylum 6. Platyhelminthes (Flatworms):
About 12,700 species. Body dorsoventrally flattened. Anus and circulatory system absent. Free living or parasitic. Terrestrial, freshwater or marine.
Planaria, Fasciola, Taenia, etc.
Phylum 7. Nemertinea or Rhynchocoela- (Ribbon worms):
About 750 species. Body slender, soft, very elastic and covered with cilia. No segmentation. Mouth anterior with a long eversible proboscis. Digestive tract complete with anus. Circulatory system present. Free-living. Mostly marine, few terrestrial and freshwater.
Cerebratulus, Stichostemma, etc.
Subdivision 2. Pseudocoelomata:
Space present between digestive tract and body wall but this space is a pseudocoel (remnant of the blastocoel) and not a coelom. Anus present with or without protonephridia, flame bulbs present or absent.
Phylum 8. Acanthocephala (Spiny-headed worms):
About 500 species. Minute parasitic worms. Proboscis protrusible (eversible) with recurved spines. No digestive tract.
Echinorhynchus, Gigantorhynchus, etc.
Phylum 9. Entoprocta (Moss animals):
About 60 species. Digestive tube U-shaped. Mouth and anus close together lying within a region surrounded by ciliated tentacles. Sessile. Mostly marine, few freshwater. Solitary or colonial.
Pedicellina, Loxosoma, Urnatella, etc.
Super phylum Aschelminthes:
An assemblage of pseudo coelomates. All have anterior mouth, posterior anus and straight digestive tube.
Phylum 10. Rotifera (Wheel animalcule):
About 1,500 species. Microscopic. Anterior end with a ciliated corona. Pharynx with internal jaws known as trophic Protonephridial system with terminal flame bulbs. Mostly freshwater, some marine.
Brachionus, Philodina, Rotatoria, etc.
Phylum 11. Gastrotricha (Hairy stomach worms):
About 150 species. Microscopic. Ventral surface flattened and ciliated. Cuticle with spines, plates or scales and un-segmented. Pharynx tubular devoid of trophi. Freshwater and marine.
Chaetonotus, Macrodasys, etc.
Phylum 12. Kinorhyncha (Jaw-moving worms):
About 60 species. Small. More or less spiny without superficial cilia. Body un-segmented with the anterior end an introvert Marine.
Echinoderes, Pycnophyses, etc.
Phylum 13. Nematoda (Round worms):
About 10,000 species. Body rounded, slender, covered by a continuous cuticle, often tapered at ends. Cilia absent. Epidermis divided into four or more chords. Only longitudinal muscles in the body wall. Long muscular pharynx having triradiate lumen. Free-living or parasitic. Freshwater or marine or in soil.
Ascaris, Rhabditis, Enterobius, Ancylostoma, Wuchereria, etc.
Phylum 14. Nematomorpha (Horse-hair worms):
About 230 species. Body long slender, cylindrical with dorsal and ventral epidermal chords. Gonoducts joining the intestine. Larval stages parasitic and adults free-living.
Gordius, Nectonema, etc.
Subdivision 3. Coelomata:
Animals with a true coelom and usually well-developed ectomesoderm. Excretory organs are protonephridia with or without nephrostome. Anus present.
Un-segmented, coelomate protostomes.
Phylum 15. Priapulida:
About 8 species. Body surface covered with spines and tubercles. Proboscis anterior. Urinogenital pores separate from the digestive tube in both sexes. Protonephridia terminating in solenocytes joining the gonoducts. Marine.
Phylum 16. Sipunculida (Peanut worms):
About 275 species. Body elongated and cylindrical with retractile anterior introvert. Short hollow tentacles around the mouth. No segmentation or setae. Anus dorsal. Marine.
Sipunculus, Phascolosoma, etc.
Phylum 17. Mollusca (Soft-bodied animals):
About 1, 28,000 species. Body soft covered by mantle usually with an anterior head and a ventral muscular foot. Mantle secretes shell. Shell sometimes vestigial and sometimes in several pieces. Coelom reduced. Terrestrial, freshwater and marine.
Chiton, Pila (Snail), Unto (Mussel), Sepia, Loligo (Squid), Nautilus, etc.
Phylum 18. Echiurida (Adder-tailed worms):
About 150 species. Body cylindrical with trough-shaped elastic proboscis (non-retractile). Mouth ventral. Trunk with setae. Coelom spacious. Marine.
Echiurus, Urechis, etc.
Segmented coelomate animals. Characterised by segmentation of embryonic stages, even though the adult may have secondarily lost its metameric organisation.
Phylum 19. Annelida (Ringed or Segmented worms):
About 8,700 species. Body elongated and metamerically segmented. Coelom spacious, typically divided into metameric compartments. Terrestrial, freshwater and marine.
Nereis (Clamworm), Pheretima (Earthworm), Hirudinaria (Leech), etc.
Phylum 20. Tardigrada (Water bears or Bear animalcules) Minute:
Body cylindrical and segmented with four pairs of unsegmented legs terminating in claws. Coelom transitory replaced by a haemocoel. No appendages associated with mouth.
Echiniscus, Hypsibius, etc.
Phylum 21. Onychophora (Claw-bearers):
About 73 species. Body worm-like, elongated and un-segmented. Thin un-segmented cuticle covers the body. Many pairs of short un-segmented legs. Head with three pairs of appendages, two of which are associated with mouth. Tracheal system for respiration. Moist terrestrial.
Phylum 22. Pentastomatida (Tongue worms):
About 70 species. Worm-like body with two pairs of claws or short appendages at the sides of the mouth. Respiratory system absent. Parasitic in vertebrates.
Porocephalus, Cephalobaena. etc.
Phylum 23. Arthropoda (Joint-footed animals):
About 9,00,000 species. Body segmented with jointed appendages usually terminating in claws. Exoskeleton chitinous. Coelom greatly reduced and replaced by a haemocoel. Terrestrial, freshwater and marine. Examples : Prawns, Scorpions, Flies, Centipedes, etc.
Division B. Deuterostomia:
Bilateria in which the mouth does not arise from the blastopore or near its anterior margin. Coelom arise from the primitive gut.
Super-phylum Tentaculata (Lophophorates):
Animals with a circular, crescentic or double spirally coiled edge bearing ciliated tentacles and known as a lophophore. Mouth and anus near together. Intestine forming a loop.
Phylum 24. Phoronida:
About 15 species. Solitary worm-like animals. Body enclosed in a chitinous tube. Closed circulatory system. One pair of metanephridia Marine.
Phoronis, Phoronopsis, etc.
Phylum 25. Ectoprocta or Bryozoa (Moss animals):
About 4,000 species. Sessile or colonial animals with a lophophore. Body enclosed in a gelatinous, chitinous or calcareous covering. Anus outside of the lophophore region. Mostly marine, few freshwater.
Bugula, Plumatella, etc.
Phylum 26. Brachiopoda (Lamp shells):
About 260 species. Solitary animals with a lophophore. Body enclosed in a bivalve shell. One or two pairs of metanephridia. Marine.
Lingula, Crania, Hemithyris, etc.
Phylum 27. Echinodermata (Spiny-skinned animals):
About 6,000 species. Animals with secondary radial symmetry. Water vascular system present. Locomotion by tube-feet.
Starfish, Brittle star, Sea urchins, Sea lilies, etc.
Phylum 28. Chaetognatha (Arrow worms):
About 50 species. Small, slender, elongated, transparent animals. Body divided into head, trunk and tail. Hooks or bristles near mouth, paired fins on trunk and a terminal tail fin. Digestive tube complete. Planktonic and marine. Free-living.
Sagitta, Spadella, etc.
Phylum 29. Pogonophora (Beard worms):
About 43 species. Body elongated in a chitinous tube. Anterior end with one to many tentacles. Endoskeleton, gill-slits, and digestive tube absent. Deep water and marine.
Siboglinum, Polybrachia, Spirobrachia, etc.
Phylum 30. Hemichordata (Acorn worms):
About 80 species. Animals permanently bilateral with gill-slits. Endoskeleton poor. Embryo lacking a typical notochord. Marine.
Balanoglossus (Acorn worm), Cephalodiscus, etc.
Phylum 31. Chordata:
About 45,000 species. Animals permanently bilateral. A dorsal tubular nerve cord, a notochord and paired pharyngeal gill-slits present at some stage in life- history. Terrestrial, freshwater, marine.
Ascidians, Amphioxus, Fishes, Frogs and Toads, Lizards and Snakes, Birds and Mammals, etc.