XXVI NOTES KT UEVUi: 



bands. Wlien the closure of tlie sides of the body beneath the 

 prototroclial area lias been coinpleLcd, the last remnants of the yolk 

 membrane hâve already been cast olï, and the head (apical région 

 of trochophore) is intermittently and completely involuted by 

 retraction into the newly formed cœlom. 



The varions parts of the trochophore ofPhascolosoma correspond 

 closely with those in Sipunculus, viz. the définitive rosette, apical 



Fig. 7. 



FiG. 7.— Obliquely longitudinal section of larva of Ph. vulgare, five days old, showing 

 one of the ventral retractor muscles, postoral circlet of cilia, supra-a^sophageai 

 ganglion (gn), yolk granules {vt) in coelom, etc. 



plate, dorsal cord, somatic plate and the prototroch (serosa, in 

 Sipunculus), but the amnionic cavities, which are formed in 

 Sipunculus by the sinking of the apical plate, dorsal cord and 

 somatic plate from beneath the yolk membrane, do not exist in 

 Phascolosoma, although the apical plate does show a tendency to 



