CHAPTER - 4 ANIMAL KINGDOM Animals constitues more than 1.5 million species. Of these, species named so far consitutes less than 20% of fall living animals. Animals are unique in many ways. In fact they are the only organisms to have developed ‘active flights’. Even now biologists consider two major groups of animal kingdom the invertebrates without backbone ad the vertebrates called the backboned animals. Of all the liked animals invertebrates consitute 98 percent of them! Animals are the organisms that develop from embryos formed by the fusion of haploid egg and sperm. The multicellular organisms of the kingdom animalia (metazoa) are typically divided into three grades: ANIMALIA (metazoa) Mesozoa Parazoa Eumetazoa (with a single phylum placozoa) (includes porifera, the sponges) This term was given by van The name parazoa means the These true metazoans are Beneden in 1876 who called this "beside-animals". Although the known also as enterozoa. These group as connecting link simplest in organization of all the animals have tissues organized between protozoa (animal metazoa these groups do into organs and organ systems. product) and metazoa. Although compose a higher level of Approximately they constitute 33 mesozoa and parazoa are morphology and physiological phyla according to modern multicellular, their plan of integration than that found in classification of animals. organization is distinct from that protozoan colonies. The mesozoa of the eumetazoan phyla. Such and porifera (or pore bearers) is a cellular layers as they process group of strange animals in this are not found homologous to the world. Many of them are marine, germ layers of the eumetazoa very few live in fresh water. They and neither group has are multicellular, cellular grade developmental patterns in line animals and lack tissues. They with the other metazoa. are grouped in sub kingdom Members of this group are all parazoa. parasites of marine invertebrates eg. Rhopalura and Trichoplax. Their body is made up of about 20-30 cells arranged in two basic layers. The major PORIFERA...The parazoans Phylum porifera includes 10,000 species. The name porifera was coined by Robert Grant. Sponges are included in this group. Sponges are multicellular with cellular grade organization. The main characteristic of this group are as follows : • All are aquatic organisms. Many of them are marine and a few live in freshwater (only 1501 species). Freshwater sponges are included in the family spongillidae. • Sponges are sedentary and attached to the substratum. • They are solitary or colonial. • Their body is cylindrical or vase shaped or cushion shaped. • Sponges are diploblastic animals. Their body wall is made up of outer pinacoderm and inner choanoderm. In between them mesenchyme (mesoglea) is present. • At the centre of the body, a cavity is present called paragastric cavity or spongocoel. • A number of pores are present on the body. They are called dermal ostia. Through these ostia water enters into the body. Hence they are called incurrent pores. Paragastric cavity opens f out via osculum, through which water goes out hence it is called excurrent pore. Sponges bear dermal pores and so they are called pore bearers. • The pinacoderm shows two kinds of cells (Fig.), pinacocytes and porocytes. • The mesenchyme consists of amoebocytes. • Choanocytes and epithelial cells are present in the choanoderm. • The choanocyte (collar cell) has a long flagellum and a collar. With the flagellar action, water enters into the body. This water contains food and oxygen. Collar of the cells filter the food which is digested in the choanocyte (Fig.). It is called intracellular digestion. • Digestive, respiratory, excretory and circulatory systems are absent. • Respiration occurs by diffusion. • Skeleton is made up of spicules which provide strength and support to the animal. Spicules are of three types (Fig.). 1. Calcareous spicules are made by CaCO3 2. Silicious spicules are made by silica. 3. Spongin fibres are spicules are made by spongin. • Poriferan skeleton is used for studying the diversity of sponges. • In the body of sponges, canal system is present. This system helps the animal to draw water in and send it out. The incurrent water brings food and oxygen into the body. The excurrent water takes away waste materials. Incurrent water enters into the body through dermal ostia and excurrent water goes out through the osculum. In sponges Ascon, Sycon, Leucon and Rhagon types of canal systems are present. • Different amoebocytes take up different functions. 1. Myocytes. They work as a muscle and form sphincter around the opening (osculum). 2. Chromocytes. They are pigment bearing amoebocytes. 3. Scleroblasts. They are spicule producing amoebocytes. 4. Archaeocytes. They are also called sex cells. These amoebocytes are called totipotent cells. They can change into any type of cell. • Asexual reproduction occurs by budding (gemmation) and gemmule formation (Fig.). In fresh water sponges, to over come unfavourable conditions, gemmules are formed. • Sexual reproduction is performed by sponges. Life history includes parenchymula or amphiblastula larva. • The power of regeneration in sponge is very high. Systematic Position Although they are multicellular sponges they share few of the characterisitics of other metazoanphyla. They seem to be outside the line of evolution leading from protozoa to metazoa, therefore, they are called parazoa. Resemblances with Protozoans A colonial flagellate protozoan called Proterospongia resembles a sponge. Hence sponges are believed to evolve from colonial protozoans. Both of them show intracellular digestion. Differences with Protozoans 1. In Proterospongia, flagella are present outside, but in sponges they are present inside. 2. Sponges are multicellular whereas protozoans are unicellular. 3. In sponges division of labour is seen but in protozoans it is absent. Hence sponges are more advanced than protozoans. Resemblances with Cnidaria 1. Both are multicellular. 2. Both of them are diploblastic. 3. Both of them have central hollow cavity. Differences with Cnidaria (radiate metazoa) 1. Cnidarians show tissues (tissue grade organization). 2. Cnidarians exhibit inter and intracellular digestion. 3. Cnidarians have diffused nerve-net. Peculiar Characters of Sponges 1. Presence of dermal ostia. 2. Presence of spicules. 3. Presence of canal system. Diversity of Porifera (classification) 1. Phylum porifera is divided into three classes . Class: Calcarea • They may be solitary or colonial (Fig.) • They are small sized calcareous sponges not exceeding 10 cm. in height. • Body shape may be cylindrical or vase shaped. • They are exclusively marine and mostly live in shallow waters. • Canal system may be of asconoid or syconoid type. • Skeleton is made of calcareous spicules. • Some examples include Leucosolenia, Sycon, Grantia and Clathrina. Class: Hexactinellida • They are exclusively marine and many are found in deep sea (Fig.). • Some are moderate sized glass sponges, reaching 1 metre in length. • Body shape may be cup like, urn or vase like. • No dermal epithelium or exopinacoderm are present in them. • Choanocytes are restricted to finger shaped chambers. • Skeleton consists of six rayed triaxon siliceous spicules. • Examples include Euplectella, Hyalonema and Pheronema. Class : Demospongiae • They may be solitary or colonial (Fig.). • They are small to large sized animals. • They are mostly marine, some are in fresh water. • Their body shape is vase like, cup like or cushion like. • Body organizations is leuconoid with choanocytes restricted to small rounded chambers. • Skeleton may be absent or madeup of silicious spicules or of spongin fibres or of both. • Examples : Oscarella, Cliona (boring sponge), Halichondria, Spongia (fresh water sponge), Chalina and Hippospongia (horse sponge). Economic Importance of Sponges Sponges are of great economic importance. They are both beneficial and harmful to mankind. a. Beneficial sponges: Skeleton of some silicious sponges like Euplectella is famous for beauty and is in great demand for marriage ceremonies in Japan. Scraps of sponges are of great utility, particularly in packing and in linoleum. Some marine sponges such as Euspongia and Hippospongia, after removal of the living parts, form what we call the sponges of commerce. These are used for bathing, washing, mopping and ceramic work etc. Sponge fishing is one of the industries in the mediterranean Florida and West Indies. b. Harmful sponges: Cliona is the boring sponge destroying several molluscan shells. This way several edible molluscs such as oysters and clams are destroyed. CNIDARIA ... The radiate animals. The two phyla, cnidaria and ctenophora makeup the radiate animals which are characterized by primary radial or biradial symmetry, that are seemingly ancestral for the eumetazoans. Radial symmetry, in which the body parts are arranged concentrically around the oral-aboral axis is particularly suitable for sessile or sedentary animals and for free floating animals because they approach their environment (or it approaches them) from all sides equally. Biradial symmetry is basically a type of radial symmetry in which only two planes through the oral-aboral axis divide the animal into mirror images because of the presense of some part, that is single or paired. All other eumetazoans have a primary bilateral symmetry; that is, they are bilateral or were derived from an ancestor that was bilateral. Neither phyllum has advanced generally beyond the tissue level