172 



INVERTEBRATA 



CHAP. VIII 



to a prae-oral position ; the other appendages develop in order from 

 before backwards. Into each rudimentary appendage an outgrowth 

 of the corresponding somite with its coelomic cavity extends. Then 

 the endodermic tube shrinks away from the ectoderm and leaves 



som 



-ps 



FIG. 124. Stages in the division of the blastopore and the formation of the mesoderm of 

 Peripatus capensis. (After Sedgwick.) 



A, the blastopore elongated but unconstricted ; the primitive streak is seen behind the blastopore. 

 B, the blastopore has just divided into mouth in front and anus behind. The mesodermic' bands 

 have been formed and have already budded off somites in front. C, the embryo has become concave 

 ventrally. The appendages are beginning to grow out from the somites, a, anus ; ap, appendage ; 

 at, rudiment of antenna ; blp, blastopore ; in, mouth ; p.s, primitive streak ; som, somites. 



spaces which eventually form the body-cavity of the adult, a cavity 

 which is totally distinct from the coelom and is termed the haemo- 

 coele since it becomes filled with blood (Gr. haema, blood). It corre- 

 sponds exactly to the blastocoele or primary body -cavity of the 

 Annelid larva. Of these spaces three primary ones may be dis- 

 tinguished, namely, one median ventral and two dorso-lateral (Fig. 



