Grade Bilateria

This group includes animals with bilateria symmetry. In an animal where the right side is approximately the same as the left side and where there is a distinct anterior end is said to have bilateral symmetry. The animal can be divided into two equal parts by an imaginary line only in one plane. In most multicellular animals there is a clearly differentiated head present at the anterior end and a distinct posterior end.also there are clearly difined dorsal and ventral surfaces.

The animals belonging to phyla, platyhelminthes, Nematoda, Annelida, Mollusca, Arthropoda, Echinodermata, hemichordate and Chordata are included in this grade. It must be mentioned here that the animals belonging to phylum echinodermata, have developed bilateral symmetery, as is evident, from the study of structure of their larvae. However, the adult Echinoderms, have secondarily developed radial symmetry, due to their special mode of life. All the animals included in grade bilateria are triploblastic. These may be acoelomate, pseudocoelomate or coelomate.

A spiral and determinate cleavage is that in which the lines or planes of cleavage are not symmetrical between poles instead these are diagonal to the polar axis and produce unequal cells around the axis of polarity and all the balstomeres have determined role to play in the formations of embryo. The fate of each blastomere is foretold.

In radial and indeterminate cleavage the planes of cleavage are symmetrical to the polar axis and produce tiers of cells on top of each other and the fate of each blastomere is not pre-determined. In some anyone blastomere can produce a complete embryo.

Diploblastic and Triploblastic Organization

Diploblastic animals belong to division radiata. The body of these animals consists of two layers of cells, ectoderm and endoderm. There is a jelly like mesenchyme or mesogloea which in most cases is non cellular. Diploblastic animals show lesser degree of specialization and they do not form specialized organs. There is no special transport system is these animals. Most substances are distributed within their body by process of diffusion. There is no central nervous system in these animals. A neuron net is present. These animals have radial symmetry.

There is only one cavity in the body called gastrovascular cavity which has only mouth which serves for the entry of food and water and also for the removal of wastes along with water. This is known as sac like digestive system. Diploblastic animals are included in phylum Cnidaria (coelenterate) which would be discussed in detail later in the chapter.

Triploblastic animals are included in phyla which have been placed in grade bilateria. The body of these animals is made from three layers ectoderm, mesoderm and endoderm. After embryonic development these layers in most triploblastic animals are not distinct as separate layers of cells, but are represented by the structures formed from them. The cells of these animals show greater degree of specialization. These have specialized organs and organs systems. Special transport systems i.e. blood vascular system is present in most of the cases.

The systems such as integumentary and nervous develop from ectoderm. Mesoderm gives rise to muscular, skeletal and reproductive systems. Endoderm forms the lining of digestive tract and glands of digestive system, such as liver. The digestive systm is of tube type i.e. having mouth at the anterior end and the anus at the posterior end. Triploblastic animasl may be acoelomate, pseudocoelomate or coelomate.

Acoelomates, Pseudocoelomates and Coelomates

The following account would help to explain the above mentioned terms.

Acoelomates

In phylum platyhelminthes there is no body cavity or coelom, and the mesoderm forms a loose, cellular tissue called mesenchyma or parenchyma which fills the space between the ectoderm and endoderm. If forms a packing around the internal organs of the animals to support and protect them. Such animals are called acoelomates (Fig. 10.3).

In acoelomates the gut is sac-type and there is no special transport system. Only excretory system is developed for the transport of excretory products. This system consists of flame cells, excretory ducts and excretory pores. However the nervous system is well developed.

Pseudocoelomates

In aschelminthes the space between the body wall and the digestive tube is called pseudocoelom (false body cavity). Pseudocoelom is not homologous to true coelom because: it is not lined by coelomic epithelium. It has no relation with the reproductive and excretory organs. It develops from the blastocoels of the embryo and it is bounded externally by the muscles and internally by the cuticle of the intestine. The animals having pseudocoelom are called pseudocoelomates.

Coelomates

Coelom is cavity present between the body wall and the alimentary canal and is lined by mesoderm. The mesoderm splits into outer parietal layer which under lined the body wall and the visceral layer which covers the alimentary canal and thecavity between them is the true coelom. It is filled with with fluid called coelomic fluid. The animals which possess coelom or true body cavity are called coelomates e.g. animals from annelids to chordates.

In coelomates gut attains more complexity and neuro-sensory system is well developed along with excretory system, circulatory system, respiratory and reproductive systems.

Fig. 10.3: General body plan of acoelomate, pseudocoelomate and coelomate.