Phylum – Aschelminthes and Phylum – Annelida

Phylum – Aschelminthes:

· The body of the aschelminthes is circular in cross-section, hence, the name roundworms.

· They may be freeliving, aquatic and terrestrial or parasitic in plants and animals.

· Roundworms have organ-system level of body organisation.

· They are bilaterally symmetrical, triploblastic and pseudocoelomate animals.

· Alimentary canal is complete with a welldeveloped muscular pharynx.

· An excretory tube removes body wastes from the body cavity through the excretory pore.

· Sexes are separate (dioecious), i.e., males and females are distinct. Often females are longer than males. Fertilisation is internal and development may be direct (the young ones resemble the adult) or indirect.

· Examples : Ascaris (Round Worm), Wuchereria (Filaria worm), Ancylostoma (Hookworm).

Aschelminthes – Roundworm

Phylum – Annelida:

· They may be aquatic (marine and fresh water) or terrestrial; free-living, and sometimes parasitic.

· They exhibit organ-system level of body organisation and bilateral symmetry.

· They are triploblastic, metamerically segmented and coelomate animals.

· Their body surface is distinctly marked out into segments or metameres and, hence, the phylum name Annelida (Latin, annulus : little ring).

· They possess longitudinal and circular muscles which help in locomotion.

· Aquatic annelids like Nereis possess lateral appendages, parapodia, which help in swimming.

· A closed circulatory system is present.

· Nephridia (sing. nephridium) help in osmoregulation and excretion.

· Neural system consists of paired ganglia (sing. ganglion) connected by lateral nerves to a double ventral nerve cord.

· Nereis, an aquatic form, is dioecious, but earthworms and leeches are monoecious.

· Reproduction is sexual.

· Examples : Nereis, Pheretima (Earthworm) and Hirudinaria (Blood sucking leech).

Examples of Annelida : Nereis, Hirudinaria