THE ANIMAL KINGDOM 



417 



dwellers. The body is distinctly segmented, the head with 

 sense organs being well developed in the free-swimming forms. 

 A characteristic structure of the latter is the parapodium. This 

 is a fleshy outgrowth of the body wall usually consisting of two 



Fig. 242. Fig. 243. 



Fig. 242. — A, Anterior end of Nereis, with proboscis extended. (After Ehlcrs.) 

 B, enlarged view of parapodium from posterior aspect. {After Quatrefages.) 

 a, acicula (large bristles embedded in parapodium); d, dorsal cirrus; h, head 

 (prostomium) bearing four simple eyes; J, jaw; n, notopodium; ne, neuropodium; 

 p, palp; pt, peristomal tentacles; T, tentacles; v, ventral neuropodium. 



Fig. 243. — Diagram of alimentary canal of leech, a, anal opening; g, nerve 

 ganglia; h, head with eyespots; p, muscular pharynx; s, stomach; su, sucker. 



lobes: a dorsal notopodium and a ventral neuropodium (Fig. 

 242), both of which may bear cirri and setae, and are supported 

 internally by heavy bristles called acicula. Parapodia are 

 reduced or lacking in the sedentary forms. They are used in 

 locomotion and in aerating the body by producing a flow of 

 water over it when the animal is in its burrow or in a tube. 



