ON DEVELOPMENT ETC. OF PHORONIS. 547 



trunk of a Inu'hlv advanced larva of type C', is ^iven in fig. ~>7 «r, 

 in wliicli tlie nnieh elongated and eonvoliited poneh is seen cut 

 into several sections (po.), connected witli one another l)y tlie 

 mesentery (I'./nes.). 



Very freqnently it happens that the ])eritoneal niesoblastic 

 ephhelinm, which lines the perianal ciliated helt, is detached from 

 the ectohlastic wall. 'J'liis is a pnrely artificial appearance cansed 

 hv the killing reagent. It seems probable that Mastekman has 

 erroneonsly considered the space thus formed by splitting to be a 

 vascular space (the " perianal sinus "). The same author states, 

 though with nnich reserve, that he has discovered a third pair of 

 nephridia in this trunk cavity, which is considered to be a modi- 

 fied part of the b(xly-cavity, and also to l)e rudiments of the adult 

 nephridia. I can at jn-esent say no more than that these are cer- 

 tainly absent in every type of the Actinotrocha studied by myself. 



2. Orgam of Ectohlaxtlc Orir/in. 



The epidermis of Adinoiroclm is represented by a single 

 laver of cubical or cylindrical cells, those of the collar wall and 

 of the upper and the lower walls of the hood l)eing ja'ovided with 

 well developed cilia. Besides, there are three specially ciliated 

 regions : the preoral belt, the tentacles, and the perianal helt. The 

 last is the lai'val locomotory organ ; on it the cilia are very long, 

 thick, and somewhat bristle-like when in active motion. At places, 

 where cilia are strongly developed, [e.g., the nerve ganglion, the 

 sensory spot if jiresent, the ciliated ])elts, etc.) the constituent cells 

 are cvUndrical, the nucleus generally lying near the basal end. 

 The body wall of the trunk region is very thin and is formed of 

 greatly attenuated cells (es]X'cially slender in the advanced larva*). 



