11th Class Biology Animal Kingdom Important Terms and Classification of Animals

The kingdom animalia or animal kingdom is the kingdom of consumer organisms having ingestive type of nutrition. It is the largest kingdom, with 1.2 million members. It has numerous having different type of form, structure, organisation, complexity and development.

General features of animals

The animals possess several general features, which, taken together, distinguish them from the members of other kingdom.

(1) Animals are multicellular eukaryotes and in most cases their body cells form tissues that become arranged as organs and organ systems.

(2) Animals have heterotrophic mode of nutrition. They get carbon and energy by ingesting other organism or by absorbing nutrients from them. Animals may be herbivores, carnivores, omnivores, parasites, suspension feeders or deposit feeders.

(3) Animals require oxygen for aerobic respiration.

(4) Animals are motile, possess active movement during some stage of their life cycle. Even the sessile sponges have free swimming larval stages.

(5) The animal body cells of nearly all species have diploid chromosome number.

(6) Animal cells lack a cell wall; this provides flexibility of their cells, the most striking characteristic of animals.

(7) Animals are able to make rapid responses to external stimuli as a result of the activity of nerve cells, muscle or contractile tissue or both.

(8) Animals can reproduce sexually. Although some exhibit remarkable diversity of reproductive behaviour, all are capable of sexual reproduction.

(9) Animal life cycle includes stages of embryonic development. Mitotic cell divisions (cleavage) transform the animal zygote into a multicellular embryo.

Terms related to classification

(1) Anaima : Animals without red blood e.g., sponges, cnidaria, mollusca, arthropoda, echinodermata, etc.

(2) Enaima : Animals with red blood e.g., vertebrates.

(3) Vivipara : Animals which give birth to young ones are included in this subgroup e.g., man, dogs, cows, etc.

(4) Ovipara : Animals which lay eggs are included in this subgroup e.g., frogs, toads, lizards, snakes, birds, etc.

(5) Anamniotes : Vertebrates without embryonic membranes e.g., fishes, amphibians.

(6) Amniotes : Vertebrates with embryonic membranes (chorion, amnion, allantois, yolk sac) e.g., reptiles, birds, mammals.

(7) Acraniata or Protochordata : Chordates without cranium (brain box). It includes urochordata and cephalochordata.

(8) Chordates : Animals with notochord dorsal tubular nerve cord, paired pharyngeal gill slits. All urochordates, cephalochordates and vertebrates are called chordates.

(9) Craniata or Vertebrate : Chordates with cranium. It includes cyclostomes, pisces, amphibians, reptiles, birds and mammals.

(10) Nonchordates : Animals without notochord (a rod like elastic structure which supports the body). Phylum Porifera to phylum Hemichordata are called nonchordates.

(11) Invertebrates : Animals without vertebral coloumn (backbone). All the nonchordates, urochordates and cephalochordates are callectively called invertebrates.

(12) Levels / Grades of organization : Four levels of organization are found in multicellular animals.

(i) Acellular or Molecular or Protoplasmic level : It is present in protozoans.

(ii) Cellular level : The body consists of many cells which may be similar or show minor division of labour. Distinct tissues are not formed, e.g., sponges.

(iii) Tissue level :  The body is multicellular. The cells form poorly defined tissues. The cells occur in two distinct layers or tissues of specialized cells e.g., coelenterates.

(iv) Organ-system level : The body is multicellular. The cells are organised into tissues, tissues into organs and organs into organ systems. Except sponges and coelenterates, all the animals of the kingdom animalia have organ-system level of organization.

(13) Animal body plans : It have three types of body plans :

(i) Cell aggregate plan : The body consists of a cluster or aggregation of cells which have rudimentary differentiation but are not organized into tissues or organs. It is found in sponges.

(ii) Blind sac plan : The body has a single cavity which function as digestive tract and coelom both and have one opening to the outside. The single opening functions as both mouth for ingestion (intake of food) and anus for egestion (undigested waste is passed out) such a digestive tract is called incomplete animals having blind sac body plan show tissue grade body organisation. The cells are specialized, organised into tissues and show division of labour. It is found in coelenterates and flatworms.

(iii) Tube-within-a-Tube plan : The body has two tubes, one formed by the body wall and the second formed within it by the digestive tract. Digestive tract is a continuous tube-like structure that has two opening, a mouth for ingestion and anus for egestion such a digestive tract is called complete. In between two tubes is present coelom in which are present a number of organs. Food is digested and absorbed in the digestive tract.

(14) Animal symmetry :  Body symmetry is the similarity of parts in different regions and directions of the body. When the body is not divisible into equal halves by any plane it is called asymmetrical or asymmetric as found in Amoeba and some sponges. An animal is said to be symmetrical if its body is divisible into equal halves by one or more planes. Four types of symmetry found in animals are –

(i) Spherical symmetry : In this type of symmetry, any plane passing through the centre divides the body into equivalent or mirrored, halves. It is found in animals whose body resembles a sphere. e.g., Protozoans such as Volvox, Heliozoa, Radiolaria.

(ii) Radial symmetry : In this type of symmetry, a number of similar parts radiate out from a central axis. The body of the individual can be divided into equal halves by any plane passing through the centre from top to bottom. This type of symmetry is found in some sponges (Sycon), coelenterates (e.g., Hydra, jelly fish), echinoderms (e.g., star fish).

(iii) Biradial symmetry : In this type of symmetry, only two planes passing through the longitudinal axis. The body can be divided into two similar halves by one or two vertical planes only. This type of symmetry is found in sea walnuts (phylum ctenophora) and sea anemones (Anthozoa). The animals which show radial and biradial symmetry have oral and aboral sides. The oral sides is that which has mouth, whereas the aboral side is one which is opposite to oral side.

(iv) Bilateral symmetry : In this type of symmetry, the body can be divided into two equal halves by a median longitudinal or sagittal plan only. The appearance of bilateral symmetry in animal evolution was a major advancement, because bilateral animals are much better fitted for directional (forward) movement than in radially symmetrical animals. This type of symmetry is found in many invertebrates and all vertebrates.

(15) Germ layers : They are primary layers of cells which differentiate in the animal embryos at the gastrulation stage. The germ layers give rise to all the tissues/organs of the fully formed individual. The embryos of poriferans and coelenterates have two germ layers, the ectoderm and endoderm. These animals are called diploblastic. The embryos of all other animals (from phylum Platyhelminthes to phylum Chordata) have three layers – the ectoderm, mesoderm and endoderm. These animals are called triploblastic animals.

(16) Segmentation : Segmentation is a type of body form having a linear sequence of units of segments possessing a similar or modified structure. It occurs in three animal phyla–Annelida, Arthropoda and Chordata.

(17) Metameric segmentation (True metamerism or True segmentation) : It is a type of segmentation where external divisions correspond to internal divisions. The body is often divided both externally and internally into a number of segments (metameres) e.g., annelids. Segmentation is mostly external in arthropods and mainly internal in man and other chordates (vertebrae, body muscles, some blood vessels and nerves).

(18) Pseudometamerism (False segmentation) : It is found in tapeworms. In tapeworms, the proglottides (segments of tapeworms) are budded off from the neck, hence this segmentation is called pseudometamerism (pseudosegmentation). It differs from true segmentation of embryonic origin as found in annelids, arthropods and chordates.

(19) Body cavity or Coelom : A body cavity or coelom is a fluid-filled space between the gut and the outer body wall of an animal. It contains the major internal organs.

(i) Acoelomates : The animals which do not have coelom are called acoelomates e.g. sponges, coelenterates, ctenophorans and flat worms.

(ii) Pseudocoelomates : The animals which have body cavity, called pseudocoel (false coelom) derived from blastocoel of the embryo are called pseudocoelomates. Round worms (Nemathelminthes) are psuedocoelomates.

(iii) Eucoelomates (Coelomates) : The animal which possess true coelom are called eucoelomates or coelomates. The true coelom is a body cavity which arises as a cavity in embryonic mesoderm. In this case, the mesoderm of the embryo provides a cellular lining, called coelomic epithelium or peritoneum, to the cavity. The coelom is filled with coelomic fluid secreted by the peritoneum. True coelom is of two types; schizocoelom or schizocoel and enterocoelom or enterocoel.

Schizocoelom. It develops as a split in the mesoderm sheet. It is found in annelids, arthropods, molluscs.

Enterocoelom. The mesoderm arises from the wall of the embryonic gut archentron or enteron as hollow outgrowths which form this type of coelom. It occurs in echinoderms and chordates.

(iv) Haemocoelomates : The primary body cavity or blastocoel persists to some extent in many animals either enclosed with in narrow blood vessels as in annelids or open as blood-containing space called a haemocoel and such animals are called haemocoelomates. Haemocoels occur in Mallusca and Arthropoda.

(20) Protostomes and Deuterostomes : The terms protostome and deuterostome denote the differences in the embryonic origin of the mouth.

In protostomes (first mouth) the mouth forms from the first opening of the embryo namely blastopore, the opening from outside into the archenteron, cleavage is determinate and spiral e.g. Platyhelminthes, Aschelminthes, Annelida, Arthropoda and Mallusca.

In deuterostomes (second mouth) the mouth never develops from the blastopore, although the blastopore may give rise to the anus cleavage is indeterminate and radial. e.g. Echinodermata and chordata.

Outline classification of animal kingdom

The animal kingdom is subdivided into two sub-kingdoms, namely Protozoa and Metazoa.

1. Protozoa : It includes microscopic, unicellular animals. It contains a single Phylum called protozoa. e.g. Euglena, Amoeba, Paramecium etc.

Subkingdom 2. Metazoa : This subkingdom includes multicellular animals. e.g. Porifera to Chordata. The subkingdom Metazoa is divided into three branches, namely Mesozoa, Parazoa and Eumetazoa.

Branch 1. Mesozoa : It is intermediate between Protozoa and Metazoa. It includes endoparasitic animals. e.g. Dicyema, Rhopalura etc.

Branch 2. Parazoa : It includes sponges.

Branch 3. Eumetazoa : It includes true multicellular organisms. They have tissue organ and organ system grade of organization. e.g. Coelenterata to Chordata. Eumetazoa is further divided into two grades, namely Radiata and Bilateria.

Grade A. Radiata : It includes radially symmetrical animals. e.g. Coelenterata.

Grade B. Bilateria : It includes bilaterally symmetrical animals. e.g. Platyhelminthes to Chordata. The grade Bilateria is further divided into two divisions namely proterostomia and deuterostomia. 

Division 1. Proterostomia : In this group of animals, the blastopore develops into the mouth. e.g. It is further divided into 3 subdivision.

Subdivision 1. Acoelomata : In this group of animals, a coelom (Cavity lying between the gut and the body wall) is absent. e.g. Platyhelminthes

Subdivision 2. Pseudocoelomata : In this group of animals, a false coelom (cavity not lined with coelomic epithelium) is present. e.g. Aschelminthes or Nematoda.

Subdivision 3. Schizocoelous Coelomata : In this group, a true coelom is present. e.g. Annelida to chordata.

Division 2. Deuterostomia : In this group of animals, the blastopore develops into the anus. It consist of one subdivision.

Subdivision 1. Enterocoelous coelomata : Coelom is enterocoel which originates as pouches of embryonic gut (archenteron)

Characters of Non Chordata (Invertebrates) : The animals which lack vertebral coloumn are called invertebrates. e.g. Amoeba, sponges, Hydra, worms, insects, etc., Invertebrates are characterised by the following salient features –

(1) The vertebral column is absent.

(2) The nerve cord is solid in nature.

(3) The nerve cord is present on the ventral side and never on the dorsal side.

(4) When alimentary canal is present, it lies dorsal to the nerve cord.

(5) Invertebrates may be acoelomate or pseudocoelomate or true coelomate.

(6) They have either asymmetry or radial symmetry or bilateral symmetry.

(7) The circulatory system is open type or closed type.

(8) They exhibit all possible types of reproduction.

The invertebrates are grouped into about 30 phyla. These phyla are of two types, namely minor phyla and major phyla.

Minor phyla : (1) Mesozoa (2) Nemertinea (3) Endoprocta (4) Acanthocephala (5) Rotifera (6) Gastrotricha (7) Kinorhyncha (8) Nematomorpha (9) Ectoprocta (10) Brachiopods (11) Phoronida (12) Chaetognatha (13) Priapulida (14) Sipunculida (15) Echiuroidea (16) Pogonophora etc.

Major phyla : It, include following phylum