Phylum Aschelminthes

Distinguishing Features

1. Many roundworms live as parasites in plants and animals. They cause serious agricultural, veterinary and human health problems. A large number of nematodes are free living and occur in freshwater, seawater and soil.
2. They are called roundworms because they appear circular in cross-section. They are unsegmented.
3. The body wall consists of a firm, non-living resistant cuticle, syncytial epidermis (a continuous layer of cytoplasm with scattered nuclei) and muscle layer. The muscle layer consists of longitudinal fibres only. The epidermis lacks cilia.
4. The body cavity is pseudocoel or pseudocoelom.
5. Digestive Tract is complete with an anterior mouth and posterior anus with the muscular pharynx. The intestine is non-muscular.
6. Skeletal, respiratory and circulatory systems are absent. Pseudocoelomic fluid present in the pseudocoelom maintains body shape. It is called hydroskeleton. Gaseous exchange in aerobic respiration occurs by diffusion through the body surface. The pseudocoelomic fluid transports materials.
7. Ammonia is the main excretory matter. Ascaris excretes both ammonia and urea.
8. The nervous system consists of a circumpharyngeal ring that gives rise to nerves, forwards as well as backwards.
9. Papillae and amphids are present on the lip and phasmids on the posterior end of the body.
10. The roundworms are usually dioecious(unisexual). Generally, they show sexual dimorphism. 
11. Fertilization is internal. There is no asexual reproduction.

Animals are those organisms which are eukaryotic, multicellular and heterotrophic in their mode of nutrition. Animal cells do not have cell walls. Except for a few, most animals are mobile. Multicellular animals are often called metazoa and have been divided into two branches: Parazoa and Eumetazoa. In Parazoa, the animal’s body is formed of loosely aggregated cells. These animals have poorly differentiated tissues and have no organ or digestive cavity (e.g., sponges). However, in Eumetazoa, the cells are properly arranged into tissues and organs. Their digestive track is the mouth but with or without an anus. 

Eumetazoa is further classified on the basis of 

• the number of germ layers present in the embryo
• the symmetry of the body of the organism
• the mode of origin of mouth

Those animals which have two germ layers in the embryo are called diploblastic animals and those having three germ layers are called triploblastic animals. Further classification of animals is on symmetry, on basis of mouth origin, based on the presence or absence of body cavity or coelom. Animals do not have cell walls and chlorophyll. The central vacuole is absent. They store food as glycogen and fat. Animals can transmit impulses due to the presence of nerve cells. Sponges do not have nerve cells. Centrioles are present.