C1LETOPODA. 



2<)5 



The alimentary canal soon differentiates itself into three regions 

 oesophagus, (2) stomach, and (3) intestine. With these changes 



FIG. 139. Two STAGES IN THE DEVELOPMENT OF SERPULA. (After Stossicb.) 

 in. mouth; an. anus; al. archenteron. 



the larva, which in the meantime becomes hatched, assumes the 

 characters of a typical Annelid larva (fig. 139 B). In front is a 

 large pree-oral lobe, at the sides of which the eye-spots soon appear. 

 The primitive segmentation cavity remains as a wide space between 

 the curved alimentary tract and the body walls, and becomes tra- 

 versed by muscular fibres passing between the two. The original 

 chorion appears to serve as cuticle, and is perforated by the cilia. 



The further changes in this larval form do not present features of 

 general importance. A peculiar vesicle, which in anomalous cases is 

 double, is formed near the anus. If it were shewn to occur widely 

 amongst Chwtopoda, it might be perhaps regarded as homologous with 

 the anal vesicles of the Gephyrea. 



Serpula is one of the few Chsetopoda at present knoAvn in which 

 the segmentation is quite regular 1 . In other forms it is more or 

 less unequal. The formation of the germinal layers has been far 

 more fully studied in the Oligochoeta than in the Polychseta, and 

 though unfortunately the development is much abbreviated in the 

 former group, they nevertheless have to serve as our type; and unless 

 the contrary is indicated the statements in the remainder of the 

 section apply to the Oligochreta. The segmentation is nearly regular 

 in Lumbricus agricola (Kovvalevsky) and results in the formation of a 

 flattened blastosphere, one of the sides of which is hypoblastic and the 

 other epiblastic, the hypoblast cells being easily distinguished from 



1 According to Willemoes-Suhm, Terebellides strosmii is also characterised by :i 

 regular segmentation. 



