The Development of Phascolosoma. 143 



Development of the Trochophore. The trochophores of 

 Ph. vulgare and of Ph. gonldii are positively phototactic to the direct 

 rays of the snn. as well as to diffnse daylight. The trochophore of 

 Ph. vulgare is pelagic in its habits, that of Ph. gouldii rises but 

 little above the bottom. 



The apical plate consists of a circular area of small cells, in the 

 middle of which is a rosette of four large cells, which bear long 

 flagella. A pair of pigment spots occur in the dorsal side of the 

 apical plate, and the peripheral cells are provided with a circlet of 

 preoral cilia, which are far more prominent in Ph. gouldii than in 

 Ph. vulgare. 



The prototroch consists of nineteen cells, sixteen of which are 

 "primary", three "secondary". They are covered with short adorai 

 •cilia. Through the dorsal interruption of the prototroch extends a 

 narrow band of cells, the dorsal cord, which corresponds to a 

 similar band, which underlies the dorsal amniotic canal, in Sipunculus. 



A postoral circlet of long cilia occurs in Ph. vulgare, almost 

 immediately behind the prototroch. 



The ventral nerve cord arises as a median unpaired thickening 

 of the ectoderm of the trunk. The supraoesophageal ganglion is 

 formed independently, from the deeper cells of the apical plate. 

 Outgrowths extend forward from the anterior extremity of the ventral 

 nerve cord on each side of the stomodaeum, and unite with the 

 rudiment of the supraoesophageal ganglion, forming the circum- 

 oesophageal connectives. 



Transitory division of the ventral nerve cord and of the meso- 

 blast into from two to four metameric segments was observed in 

 Ph. goiddii. 



The retractor muscles are formed from ectomesoblast (primary 

 mesoderm) cells, upon each side of the apical plate, and probably 

 from others in the zone immediately behind the prototroch. They 

 retain connection in front with epithelial sensory cells of similar 

 ■origin. Twp pairs of accessory retractors, derived from ectomesoblast, 

 are associated with the four chief retractors. The circular muscle 

 fibres form a middle sphincter of the body wall. They also arise 

 from primary mesoderm cells. 



The proctodaeum is formed in an advanced stage of the trocho- 

 phore. A terminal process of the solid mass of endoderm growls 

 dorsad, and becomes attached to a cluster of ectoderm cells, which 

 later are in vagin ated. 



