SECT, ix EXCRETORY AND OTHER GLANDS 135 



simply tubes formed out of the germ-bearing epithe- 

 lium. When we turn to the carnivorous Annelids, 

 we find that the germ-bearing epithelium is simply 

 the ccelom epithelium which covers all the internal 

 organs including the nephridia ; the eggs project from 

 this epithelium into the body cavity, and, falling off, 

 ripen in the body fluid (see Fig. II, p. 54) to find 

 their way out through the nephridia. When we com- 

 pare this process with what takes place in Apus, we 

 find in the latter an epithelium from which the eggs pro- 

 ject into the body cavity. (See Fig. 33, p. 144.) Is not 

 this epithelium homologous with the Annelidan ccelom 

 epithelium ? Instead, however, of dropping off into 

 the body cavity, the eggs are drawn back through the 

 epithelium and find their way out through the canals 

 formed by this ccelom epithelium. Are not these 

 canals, then, in some way, the homologues of the 

 Annelidan nephridia ? To the first of the above 

 questions we give an affirmative, to the second a 

 negative answer ; but we arrive at the conclusion that 

 the germ epithelium is the original ccelom epithelium 

 which covered tJie . nephridia, and that the canals 

 which it now forms once contained the nephridial 

 canals through which the eggs found their way to the 

 exterior. In course of time, the nephridial canals 

 ceased to have any excretory function owing to the 

 sufficiency of the shell gland, and disappeared, leaving 

 only their coverings of ccelom epithelium, which, in 

 proportion as the canals degenerated, itself developed 

 into the pronounced epithelium of the genital glands. 

 The development of the longitudinal canal also out of 



