CHAPTER XII. 



CILETOPODA 1 . 



Formation of the Germinal Layers. 



MOST Chastopoda deposit their eggs before development. The 

 Oligochseta 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 diver- 

 sicolor) are viviparous. 



Perhaps the most primitive type of Chsetopod development so far 

 observed is that of Serpula (Stossich, No. 357) 2 . There is a regular 

 segmentation resulting in the formation of a blastosphere 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 Jill up the segmentation cavity 

 (fig. 139 A). The embryo, which has in the meantime become com- 

 pletely covered with cilia, now assumes more or less the form of 

 a cone, at the apex of which is the anus, while the base forms the 

 rudiment of a large proe-oral lobe. The alimentary sack grows for- 

 wards and then bends upon itself nearly at right angles, and meets 

 a stomodaeal invagination from the ventral side some way from the 

 front end of the body. 



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



I. Achseta (Polygordius). 



n. Polycteta. {*?- 

 ni. Oligochaeta. 



2 The observations of Stossich are not thoroughly satisfactory. 



