292 THE EMBRYOLOGY 



the mouth. The canal corresponds to the madreporic canal 

 which I have described in the Starfish (p. 182, fig. Ill, m 1 ) and 

 Caudina (p. 187, fig. 114, me), and the pit is identical in relative 

 position with the madreporic body (fig. Ill, w, fig. 114, m) of 

 the same. 



In another week traces of a pair of sucker-like feet (fig. 192, s, 

 s 1 ) become visible on the lower side of the body, and 

 near its posterior end. They have the form of hol- 

 lows, and are arranged so that one is on the right, 

 and the other on the left of the inferior mid-line, and 

 close to it. 



At this period the feelers (t, t 1 ) are quite promi- 

 nent, and have a club-shaped form. This figure, which is a 

 view of the lower side of the body, displays their arrangement 

 about the mouth (m) when they are retracted within their 

 sockets. One (P) is in the dorsal mid-line, and the others are 

 placed symmetrically right and left, at equal distances apart. 

 Interiorly the organization has a corresponding degree of devel- 

 opment ; the intestinal canal is a complete digestive organ, 

 with an anterior and posterior opening ; the five tentacles of 

 the second set have begun to develop at alternate points with 

 those of the first set; and the madreporic body has become com- 

 pletely covered over by the skin, in fact, totally included, with 

 the madreporic canal, within the general cavity of the body, and 

 stands in the same relation to the other internal organs, as that 

 of Caudina (p. 187, fig. 114, m). 



If we follow the development of the embryo for a month longer 

 from this time, we shall see the organs assume the positions and 

 proportions represented in this figure (fig. 193), and be enabled 

 thereby to ascertain their homological identity with those of the 

 adult Trepangs. We find here the following organs, namely, the 

 first set of five tentacles (t, t l ) not much larger than the second, 

 and the members of both branched considerably at and about 



Fig. 192. The same as fig. 191, but a little older. A view of the abdominal 

 side, m, the mouth; t, ', the feelers; s, s 1 , the sockets of the first pair of 

 sucker-like feet. From Koren and Danielssen. 



