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Ever since man became interested in the study of animals, more than a million species of living organisms have been discovered and described, and the process still continues.
The vast number of animal species that have been identified and described, exhibit a great deal of variation in their form, habit and habitat, mode of life as well as their distribution.
For a systematic study of animals to be made feasible, it is of paramount importance that they be categorized into discrete groups based on the specific characteristics they exhibit.
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The classification of animal kingdom presents a definite, highly organized and graded order of diversity. It helps to reveal the natural relationships between the various types of animal forms. It also provides academic convenience for the study of a given type and basic knowledge of the whole group/ groups to which they belong.
The famous Greek philosopher, Aristotle (384-322 B.C.), in his book Historia Animalium was the first to classify the then known animal forms on the basis of natural resemblances and dissimilarities. He classified them into two main groups: Anima and Enaima.
All forms without red blood, such as sponges, cnidaria, mollusca, arthropoda etc. (all invertebrates) were placed in Anima and forms having red blood, viz, the present day vertebrates were placed in Enaima. The Enaima were further classified into into vivipara – animals that give birth to children and vivipera – animals that lay eggs.
Since then many efforts were made to classify the animal forms on the basis of various observable characters. However, John Ray (1686) classified the animal species based on fundamental structural resemblances, which earned much appreciation. Carolus Linnaeus (1707- 1778) in his publication ‘Systema Naturae’ adopted a new approach to classify the then known species which formed the basis of modem classification. In this edition, Linnaeus also devised binomial nomenclature system for naming the species. Later, Lamarck (1801), Cuvier (1829). Lankester (1872) made significant contributions to the science of taxonomy.
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The Animals Kingdoms is broadly divided into two Subkingdoms:
(1) Protozoa (unicellular):
The species under this group are unicellular. Example -Amoeba.
(2) Metazoa (multicellular):
It includes all animals except proyozoans. In these, the body, is multicellular and there is a great diversity among the life forms of metazoa.
The subkingdom Metazoa is divided into three infrakingdoms or branches:
(i) Mesozoa:
This group includes certain minute worm-like parasites of marine invertebrate animals. These are without tissues; the whole body simply consists of an external layer of ciliated digestive cells, surrounding one or more reproductive cells. These are classified under a single phylum Mesozoa. Example- Dicyema.
(ii) Parazoa:
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It includes aquatic animals known as sponges. These have no tissues and possess a “cellular grade” of body organization. Digestive cells internally situated and flagellated. No obvious mouth or digestive cavity. Classified under a single phylum, “Porifera”. Example- Sycon.
(iii) Enteroza:
It includes those metazoan which have a “tissue”, or “organ grade” of body organization. These possess obvious mouth and digestive cavity.
Enteroza is divided into two grades- Radiata and Bilateria.
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(A) Radiata:
Enterozoans with a radial symmetry, i.e., the body is divisible lengthwise into identical halves in several planes passing through the central axis. These are further characterized by a “tissue grade” of body organization without distinct organs. The body has a single gastro-vascular cavity, combining the functions of digestive and body cavites.
These are classified under two phyla:
(i) Cnidaria or Coelenterata:
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Radiata possessing nematocyst- containing stinging cells. Mouth is surrounded by soft tentacles. Examples- Aurelia.
(ii) Ctenophora:
Radiata without stinging cells and tentacles, but with 8 external rows of ciliated comb-plates for locomotion. Example- Beroe.
(B) Bilateria:
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These are enterozoans with a bilateral symmetry, i.e., the body is divisible into two identical halves (right and left) only in one plane passing through the median longitudinal axis.
The bilateria are classified under three sections:
(a) Acoeiomata:
The species under this group are without body cavity. All space between body wall and internal organs is filled with a loose mesodermal tissue called parenchyma.
These are classified under two phyla:
(i) Platyhelminthes:
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This includes worms having a flat body. Mostly external or internal parasites of other animals. Examples- Taenia.
(ii) Nemertina:
These are mostly aquatic, some species have been found to be terrestrial. Body is slender, soft, elastic and ciliated. Mouth is distinct. The anterior has a long proboscis. Example- Stichostemma.
(b) Pseudocoelomata:
The young ones are acoelomate, but adults have a false body cavity formed by partial breakdown of the mesodermal packing.
These are classified into three phyla:
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(i) Entoprocta:
Sedentary, stalked, aquatic animals. Digestive tract is U-shaped with mouth and anus close together at anterior end, and surrounded by a circlet of tentacles. Examples- Pedicellira, Loxosoma.
(ii) Aschelminthes:
These are slender, worm-like animals without ciliated tentacles. Digestive tract is straight with mouth and anus at opposite ends of the body. Exampes – Ascaris.
(iii) Acanthocephala:
These are small and flattened endoparasites of vertebrates. Anterior end has a retractile spiny proboscis. No digestive tract is present. Examples- Echinorhynchus, Acanthocephalus.
(c) Eucoelomata:
This group includes metazoans with true coelomic body cavity lined by mesodermal coelomic epithellium or peritoneum. It is sub- classified into 13 phyla as under:
(i) Bryoza:
These include minute and aquatic colonial forms found as moss- like encrustations or gelations masses on rocks, shells etc. Digestive tract is U-shaped with mouth and anus close together. Mouth is surrounded by cilliated tentacleds. Example – Bugula.
(ii) Phoronida:
The members of this phylum are marine, cylindrical, unsegmented, wormlike, body. Digestive tract U-shaped. Mouth is surrounded by ciliated tentacles and anus by a lophophore. Coelom is divided into six chambers. Example- Phoronis.
(iii) Brachiopoda:
This includes marine species which have sedentary, stalked forms with bivalved slimy shell. Two lophophores with ciliated tentacles. Anus may be absent. Example- Lingula.
(iv) Mollusca:
These organisms are found both in aquatic and terrestrial habitat. That possesses soft and slimy body covered by a skin and a slimy shell. Locomotion takes place by a fleshy foot. They usually respire through gills. Example- snails, mussels.
(v) Annelida:
These are segmented aquatic or terrestrial worms. They have unjointed paired appendages (setae in skin, parapodia, tentacles, palps etc.). Cuticle is nonchitinous. Coelom large. Example – Earthworms, leeches.
(vi) Sipunculoidea:
These are marine worms commonly called as “Peanut worms”. They have cylindrical, unsegmented contractile body. The anterior end is retractile. The mouth is surrounded by short and hollow tentacles and the coelom is large. Example- Sipunculus.
(vii) Echiuroidea:
These are marine. Organisms. In these forms, body is un-segmented. At the anterior end of the body, a long, nonretractile elastic proboscis is present in front of the mouth. Anus is present at the posterior end. Two large ventral setae are present behind the mouth. Example- Echiurus.
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(viii) Arthropoda:
The organisms of this group may be aquatic, terrestrial, or aerial. Their body is segmented and regionated into head, thorax and abdomen. They possess three or more pairs of jointed appendages, chitinous. Cuticle and the coelom is reduced. Examples- Crabs, spiders, insects.
(ix) Chaetognatha:
These are marine worms commonly called “arrow-worms”. The body is small, slender and transparent, and regionated into head, trunk and tail. Mouth is surrounded by bristles or hoods. Fins are present on trunk and Example- Sagitta.
(x) Echinodermata:
These are marine organisms. They have unsegmented body with a secondary pentamerous radial symmetry. Body wall possess external spines and internal calcareous plates. Hollow tube-feet aid in locomotion. Examples – Asterias, Echinus.
(xi) Pogono-phora:
These are marine worms commonly called “Beardworms”. They have elongated cylindrical body in the form of chitinous tubes. One to many ciliate tentacles are also present. These forms do not have digestive tract. Example- Polybrachia.
(xii) Hemichordata:
These are marine worms commonly called “Tonguewoms”. They have elongated, un-segmented, worm-like body having pharyngeal gill clefts, rudimentary notochord and dorsal nerve cord. Example- Balanoglossus.
(xiii) Chordata:
These are aquatic, terrestrial, or aerial organisms. These are characterized by presence of notochord, pharyngeal gill clefts, dorsal hollow nerve cord and postanal tail. Example-fishes, amphibians, reptiles, birds, mammals.
The phyla are the commonly accepted principal groups of taxa in modern classification. As described above, the entire Animal Kingdom has been divided into 23 phyla. Of the metazoans, the phyla Porifera, Cnidaria and Ctenophora are diploblastic, i.e., their body is derived only from two embryonic germ layers (ectoderm and endoderm), while all others are triploblastic in which embryonic mesoderm is also formed.
Amongst the Eucoelomates, Mollusca, Annelida, Sipunculoidea, Echiuroidea and Arthropoda are Schizocoelic, i.e., their coelomic cavity forms by ‘splitting’ of embryonic mesodenn. Chaetognatha, Echinodennata, Pogonophora, Hemichordata and Chordata are, on the other hand, Enterocoelic with their coelom developing from pouches of embryonic gut (archenteron).
The enterocoelic eucoelomates are called Deuterostomes, because the blastopore of their gastrula becomes the anus of adult, while the mouth arises away from blastopore. Conversely, all other metazoa are called Protostomia, because the blastopore of their gastrula becomes the mouth of adult.
