CH^ETOPODA. 34! 



becomes hollowed out. The dorsal vessel in Lumbricus and 

 Criodrilus is stated by Kowalevsky and Vejdovsky to be formed 

 by the coalescence of two lateral vessels ; a peculiarity which is 

 probably to be explained by the late extension of the mesoblast 

 into the dorsal region. 



The layer from which the sacks for the setae and the 

 segmental organs spring is still doubtful. The sacks for the setae 

 are believed by Kowalevsky (No. 342) to be epiblastic invagina- 

 tions, but are stated by Hatschek (No. 339) to be mesoblastic 

 products. For the development of the segmental organs the 

 reader is referred to the chapter on the excretory system. 



In marine Polychaeta the generative organs are no doubt 

 mesoblastic products, as they usually spring from the peritoneal 

 epithelium, especially the parts of it covering the vascular 

 trunks. 



The Alimentary Canal, In Lumbricus the enteric cavity 

 is formed during the gastrula stage. In Criodrilus the hypoblast 

 has at first no lumen, but this becomes very soon established. 

 In Euaxes on the other hand, where there is a true epibolic 

 gastrula, the mesenteron is at first represented by a solid mass 

 of yolk (i.e. hypoblast; cells. As the central amongst these 

 become absorbed a cavity is formed. The protoplasm of the 

 yolk cells which line this cavity unites into a continuous polynu- 

 clear layer containing at intervals masses of yolk. These masses 

 become gradually absorbed, and the protoplasmic wall of the 

 mesenteron then breaks up into a cylindrical glandular epithelium 

 similar to that of the other types. 



In Lumbricus and Criodrilus the blastopore remains as the 

 mouth, but in Euaxes a new mouth or rather stomodaeum is 

 formed by an epiblastic invagination between the front end of 

 the two mesoblastic bands. This epiblastic invagination forms 

 the permanent oesophagus; and in Lumbricus trapezoides and 

 Criodrilus, where the oral opening is at first lined by hypoblast, 

 the epiblast soon becomes inflected so as to line the cesophageal 

 region. The splanchnic mesoblast of the cephalic region subse- 

 quently invests the oesophagus, and some of its cells penetrating 

 between the adjoining epiblast cells give rise to a thick wall for 

 this part of the alimentary tract ; the original epiblast cells being 

 reduced to a thin membrane. This mesoblastic wall is sharply 



