48 COMPARATIVE ANATOMY 



fused with the ccelom of the adult worm, being in fact the 

 persistent blastoccele formed in course of segmentation. At 

 the upper pole there is an epidermic thickening which develops 

 into the cerebral ganglion of the adult worm, and a pair of 

 eyespots are developed in connection with this larval nervous 

 plate. 



On each side of the lower end of the stomach is a pro- 

 visional excretory organ in the shape of a delicate tube 

 with flame-cells projecting into the blastoccele. These tubes 

 are known by the name of the head-kidneys or pronephridia. 

 Lastly, there is a plate of cells lying on each side of the 

 intestine and ending posteriorly in a pole-cell or mesomere. 

 These are the mesoblast bands, homologous with the similarly 

 named organs in the earthworm. 



The adult worm is formed by the continued elongation of 

 the conical projection at the lower pole of the larva. As the 

 cone increases in length the mesoblast bands grow back into 

 it and become segmented from before backwards, forming 

 mesoblastic somites which become hollow and give rise to the 

 ccelom, much in the same way as in the earthworm. But 

 whereas in the earthworm the right and left members of each 

 pair of ccelomic pouches fuse dorsally and ventrally, in Poly- 

 gordius they remain distinct and are separated by the dorsal 

 and ventral mesenteries described above. The cavity of the 

 pre-oral lobe becomes the cavity of the prostomium in the 

 adult. A glance at Fig. 10 will show how the body of the 

 worm is evolved out of the trochosphere. 



The Chaetopod worms are so called because of the chaetae 

 borne upon the segments of the body. The class Chaetopoda 

 is divided into two orders, Polychaeta and Oligochseta. The 

 last named group, of which Lumbricus is a member, have 

 small inconspicuous chaetae unsupported by projections of the 

 body-wall; the prostomial region is much reduced, and the 

 gonads are few in number and restricted to certain segments. 

 The Oligochaeta are mostly terrestrial or fresh-water worms, 

 but a few are marine. 



The Polychceta have numerous chaetae borne upon special 

 processes of the body-wall called parapodia. Usually there is 

 a distinct head formed by the fusion of the prostomium with 

 two or three of the following segments. The prostomium 

 usually bears tentacles on its dorsal side and a pair of palps 



