CHAPTER XII. 



CH^TOPODA 1 . 



Formation of the Germinal Layers. 



MOST Chaetopoda deposit their eggs before development. 

 The Oligochaeta lay them in peculiar cocoons or sacks formed 

 by a secretion of the integument. Some marine Polychaeta carry 

 them about during their development. Autolytus cornutus has 

 a special sack on the ventral surface in which they are hatched. 

 In Spirorbis Pagenstecheri they develop inside the opercular 

 tentacle, and in Spirorbis spirillum inside the tube of the 

 parent. 



A few forms (e.g. Eunice sanguinea, Syllis vivipara, Nereis 

 diversicolor) are viviparous. 



Perhaps the most primitive type of Chaetopod development 

 so far observed is that of Serpula (Stossich, No. 357) 2 . There is 

 a regular segmentation resulting in the formation of a blasto- 

 sphere with a central segmentation cavity. An invagination of 

 the normal type now ensues. The blastopore soon narrows to 

 become the permanent anus, while the invaginated hypoblast 

 forms a small prominence with an imperfectly developed lumen, 

 which does not nearly fill up the segmentation cavity (fig. 139 A). 

 The embryo, which has in the meantime become completely 



1 The following classification of the Chaetopoda is adopted in the present section. 



I. Achseta (Polygordius). 



(Sedentaria. 



ii. Polychseta. j Errantia 

 ill. Oligochgeta. 



2 The observations of Stossich are not thoroughly satisfactory. 



