o62 CRESSWl'lI.L SllKAHKR. 



inner surface of the body-wall, and commences behind the 

 teloblast cells of the mesoderm bands, and runs forward in 

 the region of the oral ciliated ring. In the pre-oral region 

 one finds a nnmber of these mesenchyme cells under the 

 body-wall, where they form a portion of the head division of 

 the mesenchyme in relation with the pi'ototroch and apical 

 organ. 



The median trunk mesoderm begins behind in front of 

 tlie anal vesicle, and is continued forward in the median line 

 under the gut into the region of the stomatodasum between 

 the mesoderm bands. The functional larval muscles of the 

 mesenchyme type consist of a ventral and dorsal longi- 

 tudinal set and the pre-oral circular muscles of the proto- 

 troch. With the growth of the larva the greater part of the 

 lai'val mesenchyme is converted into the definite musculature 

 of the adult. The mesoderm bands in no instance give rise 

 to mesenchyme cells, and the two can be sharply distin- 

 guished throughout the course of the larval development. 



The development of Spirorbis borealis has been briefly 

 described by Schively (33). There is a very small blastoctcl, 

 and the blastopore is a median ventral slit. It closes from 

 the posterior end forwards until nothing remains but a small 

 nperture at the anterior end, which becomes the future mouth. 

 The endoderm on invagination forms the archenterou. The 

 mesoderm can be traced to the left posterior macromere, 

 which sinks into the segmentation-cavity, giving rise by 

 a bilateral division to the primitive mesoderm cells. No 

 mention is made of the larval mesoblast. 



The early development of Serpula infundibulum has 

 been studied by Soulier (38) in 1002. The main outcome of 

 his work has been to confirm very closely Hatschek's results 

 for Eupomatus. The mesoderm cells are recognisable as 

 two large cells in the endoderm at the time of invagination. 

 They arise at the point of union of ecto- and endoderm, and 

 pass into the segmentation-cavity, whore they give rise to 

 the mesoderm bands. Their relation to the irregular cells of 

 the mesenchyme was not determined. 



