46 COMPARATIVE ANATOMY 



by a dorsal and a ventral mesentery. But the most important 

 feature in Polygordius is the almost complete repetition of the 

 organs in every segment of the body, so that the animal might 

 be likened to a chain each link of which is exactly similar to 

 the links in front of and behind it, with the exception of the 

 first and last, the prostomium and anal segment. 



Thus, in every somite, except the first three and the last, 

 there is a section of the alimentary canal and the ventral nerve 

 cord. There is a pair of ccelomic cavities, lined by a flattened 

 ccelomic epithelium, a mass of somatic muscles consisting of 

 longitudinal fibres (the circular layer is not developed in 

 Polygordius), a pair of nephridia, and a pair of generative 

 organs or gonads. When all the organs are repeated in 

 successive segments in this manner the segmentation is said 

 to be homonomous. In the earthworm, on the other hand, 

 the segmentation is heteronomous. Certain of the organs are 

 suppressed in some of the segments, and others attain to a 

 greater degree of elaboration in some segments than in others. 

 Thus the gut of the earthworm is considerably modified in the 

 anterior segments and enlarged to form the pharynx, crop, 

 gizzard, etc. The generative organs are much reduced 

 and are confined, in normal cases, to the loth, nth, and 

 1 3th somites. But it is not uncommon to find rudi- 

 ments of a pair of gonads in the twelfth segment, and a case 

 has been described in which an earthworm had gonads in 

 nine somites viz. two pairs of testes in somites 10 and 

 n, and seven pairs of ovaries in segments 12 to 18 in- 

 clusive. From these and other considerations we are able 

 to infer that the earthworm is derived from an ancestor in 

 which all the segments were alike, as in Polygordius, and 

 that the heteronomy exhibited by existing earthworms is due 

 to specialisation involving the suppression of the gonads in 

 most of the somites, and the elaboration of other organs in 

 some few of the somites. 



There are no generative ducts in Polygordius. The sexes 

 are separate, and the generative products when ripe escape by 

 rupture of the body wall, sexual maturity involving the death 

 of the animal. The ova are shed into the water and there 

 fertilised. The ovum segments and develops into a curious 

 larval form known as the trochosphere. This larva is shaped 

 something like a humming-top, having the form of a flattened 



