Define the following:

Define the following:
(a) Nephridium, (b) Hirudin,
© Prostomium, (d) Peristomium,
(e) Parapodia, (f) Mantle,
(g) Ctenidia, (h) Radula.

(a)Nephridium : These are delicate, coiled, respiratory units of Annelids. They collect waste matter from the body cavity and discharge the same into the alimentary canal or outside the body.
(b) Hirudin : It is an anticoagulant substance secreted by Hirudinaria (leeches). When it bites some animal to suck it blood, it prevents clotting of blood.
© Prostomium : A flat like structure found hanging over the mouth of earthworm.
(d) Peristomium: It is region or segment that surrounds the mouth. In earthworm and other annelids, peristomium is the first segment of the body.
(e) Parapodia : These are paired jointed, leaf like locomotory appendages of polychaetes (a group of annelids). The parapodia bear bristles, spines, and setae. It helps in locomotion in water and on land.
(f) Mantle : It is the outer, soft fold which covers the visceral hump of molluscs. It also secretes the outer shell in them.
(g) Ctenidia : The respiratory organs (gill like) present in mollusca are termed as ctenidia.
(h) Radula : It is a short chitinous curved structure having a row of teeth on both sides and is present in the buccal mass of Gastropods.