Ö40 I. IKEDA : 



the septum (see figs. öS n and d, s.r.c). Tliis, togetlier with 

 .several other larval organs in the larval collar cavity, liad hetter 

 be treated at a more suitable place in the sequel. 



Masterman has described a d(jrsal mesentery running along 

 the mid-dorsal line of the œsophagus, and separating dorsally the 

 larval collar cavity into two lateral halves. In the Actinotrochae 

 of all the types observed by me and at every stage of the larval 

 growth, no such mesentery is present. It is true that the liody 

 walls and the œsojihageal walls very frequently come close together, 

 especially in the young larva after preservation, so as to greatly 

 narrow the collar cavity in this region (figs. 49 and 50 a). But 

 a mesentery is never to l)e found. Its absence is quite clear in 

 the large Actinotrocha belonging to type D, in which the skin 

 and the œsophagus lie w^ell separated l)y a consideral)le space 

 (figs. öSrt and 58/>). 



Trunh cavity. The trunk cavity occupies the interior of the 

 third body-division — the trunk. It is completely separated 1)y the 

 joostoral septum from the collar cavity, and since the septum is oblique 

 in position, it extends dorsally nearly to the l^ase of the œsopha- 

 gus. The ventral mesentery extends along the median ventral line 

 of the body wall and of the alimentary canal, and is wholly con- 

 fined to the trunk cavity. In fig. 4'"5, which shows a median 

 sagittal section of a young larva of type A, a portion of this 

 mesentery (ernes.) is represented as a thin cellular membrane 

 extending between the alimentary canal and the ventral pouch 

 ip.o.), the latter being still shallow at this stage. The whole 

 extent of the ventral pouch is stretched by the ventral mesentery 

 to the skin as well as to the digestive canal. This relation re- 

 mains the same as the pouch grows in length and finally winds 

 around the digestive canal. A tr«nnsverse section through the 



