272 



COMPARATIVE ANATOMY 



CHAP. 



with the ciliated tuft would answer in position to the neural plate of the Trocho- 

 phora. The further development of the Pedicellina larva involves a peculiar 

 metamorphosis (Fig. 181, A-D). The larva attaches itself by the oral region, 

 while at the same time the vestibulum closes by the growing together of its free 

 edges over the mouth and the anus. Thereupon the whole enteric canal, with the 

 altered vestibulum turns round, inside the sac-like ectoderm which encloses them, 

 so that later, reversing the larval order, the vestibulum, with the stomodsemn and 

 proctodseum entering it, is turned to the free end, i.e. the original aboral region 

 of the larva. The vestibulum becomes connected with the exterior by means of a 

 new invagination of the ectoderm ; at the point of junction the tentacles appear. 

 According to this ontogenetic observation the anus would lie, not dorsally, but 

 ventrally, behind the mouth, and so would the ganglionic mass, which would thus 

 not be homologous with the brain of other worms. Further investigations, however, 



FIG. 182. A, B, C, Three stages of the development of the larva of Phoronis (Actinotrocha), 

 from the side, sp, Neural plate; wk, ciliated organ; st, stomodaeum ; t, larval tentacles; ti, 

 definitive tentacles ; md, mid-gut ; ra, rudiment of the trunk (stalk), invaginated in the larva A, pro- 

 truded in B, developed into the trunk in C ; a, anus ; awk, anal ciliated ring ; hd, hind-gut (partly 

 after Metschnikoff). 



especially as to the development of the nervous system, are needed to elucidate these 

 points. 



The larva of Phoronis (Fig. 182) is known by the name of Actinotrocha. The 

 mouth and anus lie at opposite ends of the ciliated larval body. Over the mouth a 

 large prostommm hangs down, whose edge carries stronger cilia which probably 

 correspond with the preoral ciliated ring of the Trochophora. A larval ganglion 

 (neural plate) lies in the ectoderm of the prostomium, and is in one species provided 

 with 4 eye-spots. Behind the mouth lies a ring of larval tentacles, and im- 

 mediately behind this the rudiments of the definitive tentacles, at whose bases the 

 nerve ring of the adult Phoronis begins to form. Around the anus we find a strongly 

 ciliated ring. Behind the definitive tentacles on the ventral side lies the rudiment 

 of the trunk, invaginated into the larval body. The secondary body cavity is well 

 developed. In front of the invaginated rudiment of the trunk a nephridium like the 



