Classification of Animals: Need and Modern Trend

In this article we will discuss about the need and modern trend of classification of animals.

Need for Classification of Animals:

Animals bother neither about their position in the animal kingdom nor they care for any ornamental name. Why then classification and naming are considered to be so impor­tant? Surely it is not to bewilder the begin­ners of Biology with high sounding names. What is then their purpose?

There are millions of animals. It is not possible to study each and every one of them. Thus a scientist ex­amining the pancreatic enzymes in dog can­not go on killing each and every dog in his way to verify his results. The orderly system of grouping helps him to understand the particular mechanism operating in any one group by examining a few representatives.

Moreover, the biologists want to understand the interrelationship among diverse organ­isms. For this reason all the students of com­parative biology, i.e., comparative anatomy, comparative biochemistry, enter into the domain of systematics which is essentially the study of convergence.

The names of animals differ from one language to the other and often a common name covers several types of forms. For ex­ample, if one scientist after observing the behaviour of crane at a particular corner of the globe writes “such and such behaviour was found in cranes”, it does not mean any­thing to others.

There are several types of cranes throughout the world. Anyone who has not seen that particular crane will not be able to repeat the observations and no one will be able to utilise the experience. Thus the use of common name in science simply creates confusion. The use of internationally accepted scientific names is the only prescrip­tion to overcome this difficulty.

Modern Trend of Classification of Animals:

The classificatory scheme of animals can­not be absolute and up-to-date, because every day new species are coming to our knowl­edge and adding fresh complications to the existing scheme. It is really impossible to get a thorough idea of all the varieties of animals present on the earth. Still, it is expected that the students of the subject could know at least the characteristics of major groups.

The animals are usually included in eleven major phyla (sing, phylum), each presenting a well-marked and readily recognisable anatomical character. Inclusion of certain peculiar ani­mals raises doubt and difficulties, because they possess an admixture of diagnostic fea­tures of two or more different phyla.

Such an admixture of characters really makes their systematic position quite uncertain and con­troversial. A brief survey of the animal king­dom is given below. The detailed account of the diagnostic characteristics of each phylum and their division into respective classes, orders, etc., are dealt with in greater details while describing the phyla separately.

In the formal scheme of classification all animals are included under the Animal King­dom. The animal kingdom is again divided into two unequal subkingdoms, unicellular animals and the multicellular animals. The unicellular animals are included under the subkingdom, Protozoa and the multicellular animals under the subkingdom, Metazoa. It is believed that life at an early stage was present only at unicellular level.

The origin of multi-cellularity arose from some unicellu­lar stock. The animals called sponges have many cells in their body, but are not regarded to be in the direct line of multicellular ances­try by many zoologists. Some authorities have placed them in a separate subkingdom called Parazoa (almost animals).

The modern trend of classification is that all animals are included under two king­doms—(i) Protista and (ii) Metazoa (multi­cellular animals). Kingdom protista includes unicellular, or acellular eukaryotic animals and plants. The distinction between plants and animals in unicellular level is more dif­ficult. Both groups possess true nucleus, mi­tochondria and membrane bound organelles.

The mode of living in unicellular animals is different from the unicellular plants. The ani­mals can inject solid particles or live as para­sites or lead to symbiotic life. Due to symbi­otic relationships some have acquired chloroplasts and can photo-synthetize.

So all the unicellular animal-like organisms that make up the kingdom protista are called protozoan protists and all these protozoans are grouped sometimes under the subkingdom protozoa for their diverse ultrastructure, mode of living and evolution­ary relationship.

Now the zoologists divide the Protista into different phyla and is in­creasing their number gradually. 

The Kingdom Protista includes the fol­lowing phyla:

1. Phylum Dinophyta or Dinoflagellata (e.g., Noctiluca),

2. Phylum Parabasalia (Zooflagellates) e.g., Trichonympha,

3. Phylum Metamonada (Multiflagellate zooflogellates) e.g., Giardia,

4. Phylum Kinetoplastida or Kinetoplasta (e.g., Trypanosoma, Leishmania),

5. Phylum Euglenophyta (e.g., Euglena),

6. Phylum Cryptophyta (e.g., Cryptomonas),

7. Phylum Opalinata (e.g., Opalina),

8. Phylum Bacillariophyta (e.g., Diatoms),

9. Phylum Chlorophyta (e.g., Volvox),

10. Phylum Haplophyta (e.g., Coccolithus),

11. Phylum Choanoflagellata (e.g., Proterospongia),

12. Phylum Rhizopoda (e.g., Amoeba),

13. Phylum Actinopoda (e.g., Aulacantha, Actinophrys),

14. Apicomplexa (Sporozoa) e.g., Monocystis, Plasmodium,

15. Phylum Labyrinthomorpha (e.g., Labyrinthula),

16. Phylum Microsporidea (Microspora) e.g., Nosema,

17. Phylum Ciliophora (e.g., Paramoecium),

18. Phylum Rhodophyta (e.g., Red alga).

Unicellular Animals (38,000 species ap­proximately):

The unicellular animals (body made-up of a single cell only) are included in the Phylum Protozoa (Gr. Protos = first and zoon = animal). This phylum is again subdi­vided into several classes and orders. The plylum includes free living forms as well as forms which live in association with others.

This phylum includes certain notable parasites causing different diseases to human beings like Malaria, Sleeping sick­ness, Kala-azar, Amoebiosis, etc. Amoeba, Euglypha, Paramoecium, Nyctotherus, Euglena, Chilomonas, Noctiluca, Monocystis, Plasmodium and Trypanosoma, are some of the very com­mon examples of the phylum protozoa.

Kingdom Metazoa (Multicellular ani­mals):

The multicellular animals are also referred to as the metazoa (Gk. meta = after, zoon = animal). This kingdom constitutes a major division of the animal world and includes all groups of invertebrates (except protozoans) and all chordates.

The inclusion of sponges under this category raises strong doubt because some authorities are of the opinion that the sponges are not to be regarded as true metazoa. They are regarded as parazoa and are considered as a blind offshoot in the pathway of metazoan origin.

Multicellular animals are divided into the following phyla:

Phylum Porifera (5,500 species approxi­mately):

The name of the phylum has been derived from Latin words and all the sponges are included in the Phylum Porifera (L. porus = pore and ferre = to bear) as the body of the sponges is perforated by pores. The organisation of sponges presents a level of multi- cellularity, where cells are not organised in definite layers.

Even in such simplified state the sponges show certain specialised organi­sation like canal systems, spicules and gemmules. The skeletal elements are present either in the form of spongin fibres or in the form of spicules.

Phylum Placozoa (e.g., Trichoplax adhaerens), Only a single species constructs the phylum which is minute, marine and composed of upper and lower epithelial cell layers enclosing mesenchyme-like cells in the middle.

Phylum Cnidaria (10,000 species approxi­mately):

In these metazoan forms, the cells are arranged in two layers, ectoderm and endoderm which are separated by a non-cellular strip—mesoglea. For this reason they are called diploblastic animals. In these two lay­ers the different cells are assigned different functions.

The physiological division of la­bour at cellular level commences from the Cnidarians. These animals possess single opening called the mouth which leads into a cavity called the coelenteron (Gr. Koilos = hollow and enteron = an intestine).

A special kind of cell type called cnidoblast is present which contains a stinging apparatus called nematocyst. The central position of mouth makes it possible to divide it longitudinally into identical halves, in a number of planes. Thus the members show radial symmetry. Hydra, Protohydra, Clytia, Obelia, Aurelia, Corals and Gorgonia are the typical repre­sentatives of this phylum.

Phylum Ctenophora (100 species approxi­mately):

The members of this phylum possess many features common to the phylum Cnidaria. This phylum is characterised by having biradially symmetrical body. The nematocysts are absent, but the adhesive cells or colloblasts are present. Eight rows of cili­ary plates (or comb plates) are present.

A gelatinous ectomesoderm containing mesen­chymal muscle fibres are present. The Phy­lum Ctenophora is a small group embracing about 100 species. The typical examples are Hormiphora, Pleurobrachia, Be roe and many others.

Phylum Platyhelminthes (About 20,000 species):

The members of the Phylum Platy­helminthes (Gr. platys = flat and helminthes = worm) have dorsoventrally flattened body and thus usually exhibit bilateral symmetry which means that their body can be divided longitudinally into two identical halves only through one plane. These animals possess in between ectoderm and endoderm, another cell layer called mesoderm. For this reason they are called triploblastic.

The digestive system is either ill-developed or absent. Most of the animals are parasitic. They are mostly hermaphrodite, i.e., the same individual bears both male and female sex organs. Planaria, Flukes, Taenia, Echinococcus, Schistosoma and Phyllobothrium are some of the representative members.

Phylum Nemertea (= Rhynchocoela), About 900 species:

They live in marine, freshunter or terres­trial habitats. Body is usually elongated or sometimes flattened and called “ribbon worms”. They are also called “proboscis worms” because of the presence of a long eversible probscis used for capturing food. The body is acoelomate, un-segmented and the ectoderm is ciliated. The examples are Carinoma, Micrura, Prostoma, etc.

Phylum Gnathostomulida (Approx. 80 species):

The animals inhabit in marine, intersti­tial environments. Body is elongated, unsegmented and acoelomate. Epidermes is single layered with monociliated epithelial cells. Gut is incomplete with jawed pharynx. They are hermaphroditic and spiral cleavage in their development. The examples are Haplognathia, Gnathostomula, etc.

Mesozoa (Approximately 100 species):

Two phyla of metazoans that represent the mesozoan grade of organization are:

(i) Orthonectida (e.g., Rhopalura) and

(ii) Rhombozoa (e.g., Dicyema).

The common features of these phyla are multicellularity and the body consists of two cell layers—(do not correspond to the ectoderm and endoderm of the metazoa). The true tissues and organs are absent.

Phylum Entoprocta (About 150 species):

Animals are mostly marine, except Urnatella which is an inhabitant of freshwa­ter. They are either solitary or colonial. Pre­viously they were included under Bryozoa but due to lack of coelom and presence of spiral cleavage they have placed under a separate phylum. The body is stalked, cup­like and beset with a circlet of tentacles. The examples are Loxosoma, Myosoma, Urnatella and others.

Phylum Cycliophora:

Single species Symbion pandora has been described and collected from the mouth parts of lobsters. Body is microscopic and the mouth is surrounded by a ring of cilia. They attach to the lobsters by the posterior adhe­sive disc. Probably the animals are acoelomate with U-shaped gut. This phylum was described in 1995.

Aschelminthes:

(Mostly pseudocoelomate animals):

Phylum Gastrotricha (Approx. 430 species):

The members live in the benthic sediment of marine and freshwater. The group is char­acterized by unsegmented and acoelomate body with posterier adhesive tubes.

Cuticle is present and epithelial cells are monocili­ated. The ventral surface bears numerous cilia, hence called “Gastrotricha”. Formerly it was assumed that the group contains pseud ocoelom but recent research shows that they are acoelomate. The examples are Macrodasys, Chaetonotus, etc.

Phylum Nematoda (About 25,000 species):

The representatives of the Phylum (Gr. nematos = thread) are called the round worms. They may be free-living or parasitic. The body is built in the plan of a tube within a tube. Ascaris, Enterobius, Ancylostoma, Wuchereria, Trichinella are some of the members of this phylum.

Phylum Nematomorpha (About 500 spe­cies):

This group includes a large number of extremely elongated thread-like worms. They are free-living in the sexual stage and are parasitic in asexual stage. The examples are Gordius (freshwater form) and Nectonema (marine form). The excretory organs are ab­sent. The nervous system is represented by a greatly thickened pharyngeal ring.

Phylum Rotifera (About 1,800 species):

The rotifers or the ‘wheel animalcules’ are microscopic animals, e.g., Brachionus. The anterior end gives rise to a retractile trochal disc. A peculiar structure called mastax is present inside the pharynx. The body cavity is spacious and lacks epithelial lining. The excretory system comprises of a pair of nephridial tubes containing flame cells. The sexes are separate.

Phylum Acanthocephala (About 1,100 species):

This group comprises of a number of parasitic forms typified by Echinorhynchus and Acanthogyrus. They are parasitic in the intestine of vertebrates ranging from fishes to mammals. The anterior end of the cylin­drical body is prolonged into a proboscis. The mouth, anus and the excretory pore are absent. The digestive system as a whole is absent. Total lack of sense organs is another important feature.

Phylum Kinorhyncha (About 150 species):

Animals inhabit in marine mud or sand of the marine environments and from the intertidal zone to a depth of thousand me­ters. They are microscopic, segmented and pseudocoelomate animals.

Body consists of 13 segments. First segment is head, second segment is neck and the rest segments are included under trunk. Cuticle forms mov­able, segmental plates. Moulting occurs pe­riodically. The examples are Cateria, Neocentrophyes, etc.

Phylum Loricifera (About 10 species):

Some members of the phylum collected from the coast of France. The animals are microscopic, un-segmented and probably pseudocoelomate. The body is divided into introvert head, thorax and loricate abdomen. The head bears recurved spines on the lat­eral side called scalids. They inhabit in the interstitial marine environments. The exam­ples are Nanaloricus, Piciloricus.

Phylum Sipuncula (About 300 species):

Sipunculans exclusively marine and bur­rowing animals without segmentation and chaetae. The anterior part of the body can be invaginated and the mouth is surrounded by fringed lobes or tentacles.

The posterier cy­lindrical part is called trunk and the cuticle becomes thicker in rock-boring sipunculans. The body surface usually bears warts and tubercles. Trochophore larva is present in their life cycle. Example. Sipunculus, Phascolosoma.

Phylum Echiura (About 130 species):

Echiurans are benthic, marine worms containing long flattend or grooved probos­cis. Body may be oval or cylindrical and is divided into a preoral proboscis and a pos­terior trunk. Trochophore larva is found in the life cycle. Example: Thalassema, Prashadus.

Phylum Priapulida (About 16 species):

Priapulids are exclusively marine animals having superficial segmentation in there vermiform animals. The body is divided into an anterior proboscis (introvert) and a pos­terior trunk (abdomen). The trunk bears cau­dal appendages.

The mouth bears curved spines. The nature of body cavity remains a controversial issue. Previously it was reported that the body cavity is a true coelom but recent researches indicate that the body cav­ity is not lined by coelomic epithelium.

Phylum Mollusca (About 1,00,000 species):

The members of the Phylum Mollusca (L. molluscus = soft) have soft and un-seg­mented body in adult condition. The body is usually provided with a shell. Presence of a mantle covering the body is a characteristic feature. A ventral muscular foot constitutes the organ of locomotion. The examples of this phylum include Pila, Achatina, Loligo, Oc­topus and others.

Phylum Annelida (About 16,400 species):

The members of the Phylum Annelida (L. anellus = a ring) have an elongated body. The entire length of the body is divided into a number of segments by transverse partitions. These partitions are called septa. The body is divided externally as well as internally. Each segment of the body is called a meta- mere.

The mesoderm layer is split—one part adheres to the ectoderm called somatopleure and other part remains attached with the endoderm and is called splanchnopleure.

The space in between them is called the coelom. The animals having the coelom is called the coelomate. The locomotor organs are usually in the form of setae. The locomo­tor organs, if present, are un-jointed. The well-known examples are Pheretima, Lumbricus, Aphrodite, Nereis, Eunice, Placobdella, Polygordius.

Phylum Pogonophora (About 80 species):

The pogonophores or beard worms are very recently studied group. They have many peculiar features and show superficial simi­larities with the hemichordates. Recent re­search suggests that they are closely related to the annelids, specially to the polychaets. The examples of the group are Siboglinum, Lamellisabella, etc.

Phylum Arthropoda (About 11,33,000 species):

The Phylum Arthropoda (Gr. arthros = joint and podos = foot) includes largest number of animals. These animals are characterised by their metameric segmentation.

The body is covered by chitinous exoskeleton. Paired jointed appendages are present. Cephalisation is well marked. This group exhibits wide range of adaptive variations. Palaemon, Peri- planeta, Limulus, Scolopendra and Peripatus are a few members of this great phylum.

Phylum Tardigrada (About 800 species):

The phylum Tardigrada includes minute, cylindrical to dersoventrally flattened animals with four pairs of short legs ending in claws or adhesive pads. They have a remarkable power of surviving in adverse environmen­tal conditions. The limbs are lobopod-like and features of the cuticle share both with the arthropods and annelids. Example. Halobiotus, Styraconyx.

Phylum Onychophora (About 200 species):

The members of this phylum have a soft worm-like body with many lobopod-like appendages, variable with their body seg­ments (13-43 pairs). The head bears 3 pairs of segmental appendages. Respiration is per­formed by tracheae and spiracles. Excretion is by nephridia, a pair in each leg bearing body segment except the genital opening bearing segment. The examples are Peripatus, Peripatopsis.

Phylum Phoronida (About 20 species):

The example of this group is Phoronis. They are all marine. The body is worm-like, cylindrical and unsegmented. The body is encased within a membranous tube. The lophophore is horseshoe-shaped.

Phylum Brachiopoda (About 335 species):

The members of this phylum are marine and the body consists of dorsal and ventral valves like the lamellidens of Mollusca and is divided into an anterier Prosome, a mid­dle mesosome and a posterior metasome. Some species attach to the substrate by means of a fleshy stalk, called pedicel. The shape of lophophores varies from circular to coiled. The example is Lingula, Dallina, Lacozella.

Phylum Bryozoa (Ectoprocta) (About 4,500 species):

The example of this group is Plumatella. They are branched colonial forms. The individuals are minute and are placed in separate chambers. Ciliated tentacles are present around the mouth. The digestive tract is U-shaped.

Phylum Echinodermata (About 700 spe­cies):

The adult species shows radial (Penta- radial) symmetry but the larvae exhibit bilateral symmetry. Calcareous ossicles are in the form plates, spines or pincer-like pedicellariae. Locomotion is carried by nu­merous tube feet.

Water vascular system, an important feature which is formed by a network of vessels through which the water flows is present. The typical examples are Asterina, Astropecten (starfishes), Ophiura, Ophiothrix (brittle stars), Echinus (sea urchin), Holothuria, Cucumaria (sea cucumbers) and Antedon (feather star).

Phylum Chaetognatha (About 100 species):

They are all marine. The body is stream­lined with lateral and tail fins. Mouth is surrounded by spines and jaws used for food capture. All possess a hood-like structure situated anterolaterally of the body. The ex­amples are Spadella, Zahonya, Sagitta, etc.

Phylum Hemichordata (Approx. 85 spe­cies):

Animals of this phylum are deuterostomate coelomates. They are either solitary or colonial. The body is soft and divided into proboscis (prosome), collar (mesosome) and trunk (metasome). The preoral buccal diver­ticulum is called the stomochord rather than notochord.

The nerve cord of the collar re­gion is dorsal and hollow, and probably homologous with the nerve cord of chordates. They are all marine. Some zoologists place the group under the phylum chordata. An­other group of zoologists place the group within invertebrates and called “Invertebrate Chordates”. The examples are Ptychodera, Balanoglossus, etc.

Phylum Chordata (Approx. 46,700 species):

The phylum chordata (L. chorda = chord) gets its name from the presence of notochord, (a dorsal rod-like supporting structure) at some stage of development or throughout life.

In higher groups the notochord is re­placed by the vertebral column. The other characteristic features are pharyngeal gill- slits, dorsal hollow tubular nervecord, post and tail, endostyle or thyroid gland, etc. The post anal tail is present in most cases except the adult sessile urochordates.

The endostyle, the iodine secreting tissue is present in urochordates and cephalochordates which runs along the ventral side of the pharynx and thyroid gland in vertebrates. The mus­cular post anal tail is present in some stage of development. Heart when present, is situ­ated ventrally. Sexes are separate (dioecious) or hermaphroditic. Cleavage is radial, holoblastic or meroblastic.

The phylum is divided into 3 subphyla. These are—(i) Urochordata (ii) Cephalochordata and (iii) Vertebrata. The urochordates and cephalochordates are jointly called Protochordates (Protochordata) and they are characterised by the presence of only notochord in some stages but without true brain, and heart in some cases. The examples are Ascidia, Branchiostoma, Myxine, Labeo, Rana, Calotes, Columba, Bos, etc.