ANNELIDA 



339 



its lumen, and lies as a cellular cord in the ventral median 

 line of the embryo. 



On each side of the nerve cord lies a ribbon-like cord of 

 cells, the mesodermal band. The two mesodermal bands are 

 united to each other by. a median part. They have arisen 

 from the common ento-mesodermal mass, the origin of which 

 we have previously traced, by the separation of a ventral 

 layer, the mesoderm, from the dorsal layer, the entoderm. 

 A segmentation, like that 

 in the ventral cord, also 

 makes its appearance in 

 the mesodermal bands, 

 which separate into the 

 primitive segments. The 

 processes thus effected, as 

 well as the formation of 

 the body cavity and the 

 septa, take place in a 

 manner similar to that 

 described for the other 

 Annelida. The internal 

 segmentation is late in 

 finding expression on the 

 exterior of the body, and 

 is suppressed in its an- 

 terior and posterior parts, 

 where the primitive seg- 

 ments for the present 

 acquire no cavities, and 

 therefore remain in an 

 embryonic condition. 

 Each segment exhibits 

 externally a division into 

 a broader and a narrower portion (Fig. 157 E). The former 

 corresponds to a ganglion, the latter to a septum. In front 

 of the anterior end of the ganglionic chain lies a part of the 

 mesoderm, which forms the head cavity ; but regarding this, 

 Salensky could not determine whether it likewise arose from 

 the mesodermal bands. 



Fig. 157.—^ to E, embryos of Branchio- 

 hdella in vatious stages (after Salensky). 

 ect, ectoderm ; gr, pit in the ectoderm on 

 ibe dorsal side ; ma, macromeres ; m, mouth- 

 opening ; n, neural groove ; s, sucker. 



