with a well- developed 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 (Roundworm), Wuchereria (Filaria worm), Ancylostoma (Hookworm). . .
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) (Figure .
). 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). Figure .
Examples of Annelida : (a) Nereis (b) Hirudinaria Male Female Figure . Example of Aschelminthes: Roundworm . . Phylum – Arthropoda This is the largest phylum of Animalia which includes insects.
Over two-thirds of all named species on earth are arthropods (Figure . ). They have organ-system level of organisation. They are bilaterally symmetrical, triploblastic, segmented and coelomate animals.
The body of arthropods is covered by chitinous exoskeleton. The body consists of head , thorax and abdomen . They have jointed appendages (arthros-joint, poda-appendages). Respiratory organs are gills, book gills, book lungs or tracheal system.
Circulatory system is of open type. Sensory organs like antennae, eyes (compound and simple), statocysts or balancing organs are present. Excretion takes place through malpighian tubules . They are mostly dioecious.
Fertilisation is usually internal. They are mostly oviparous. Development may be direct or indirect. Examples: Economically important insects –