ox UEVELOi'ilENT ETC. OF I'UOIIONIS. 541) 



the iiet'k portion of the gluiid is very sli(jrt, it is difficult to 

 prepare u good longitudinal section of it, in which the canal may 

 be seen opening to the exterior. Fig. 50 c represents the terminal 

 part of the emptying canal, which, as can Ije ascertained hy regu- 

 lating the focus, leads to tlie external pore. Mr. Fizuka tells 

 nie that similar glands of ectohlastic origui arc constantly found 

 on the superior ramus of a parapodium in certain FoIycJiœla. 



Ventral Pouch. As the ventral pouch is one of the juost 

 characteristic structures of Aclinolrocha, its form and fate have 

 been fully studied Ijy many previous oljservcrs. In the 8-armed 

 larva of type A, an ectodermal thickening l)elow the tentacular 

 row represents the origin of the pouch. At the 10-armed stage 

 the thickening becomes more conspicuous, l)ut no invagination has 

 as yet taken place. For the first time in the 12-arnied stage, the 

 wall at the thickenins; begins to sink inwards and backwards 

 (fig. 4ü, ^90.). The invagination is lined with a mesoblastic layer, 

 and, as before noted, is for its whole length suspended by the 

 ventral mesentery, joining it to the somatic and the splanchnic 

 walls. As the growth of the larva advances, the pouch becomes 

 more elongated and bends on itself around the alimentary canal 

 )ligs. 18 and '")7 a, po.). In fully developed larvie of whatever t\']^)e, 

 the inner or ectoblastic wall is thrown into small wavy folds 

 (beginning at the distal portion near the pouch pore), while the 

 mesoblastic layer becomes muscular, so that at the end of larval 

 life, it forms a thick muscular sheath whose constituent cells stand 

 vertically t(j the inner wall. As to the form and positi(jn of the 

 pouch pore, I can öfter no details in addition to what has been 

 observed by Metschnikoef and many other authoi'ities. 



Kervous System. The iiervous system of Actinotroclta, like 

 that of Fhoroiik, is of a very low devcloj)ment, being rc])resented 



