230 B. W. MACBEIDE. 



lobes of the ccelom have extended so as nearly to meet one 

 another in the mid-ventral line ; the mesentery formed by their 

 apposition is seen in fig. 30, posterior to the gut. The opening 

 of the gut into the coelom has become closed ventrally (figs. 29 

 and 30); dorsally, however, it remains open for some consider- 

 able time yet. On the left side the coelom becomes segmented 

 into an anterior portion^ a., into which the pore-canal opens, 

 and a left posterior portion, Ipc, which we may call the left 

 posterior coelom (fig. 37) ; this second cavity includes a large 

 part but not all of the left ccelomic lobe mentioned above j 

 part of this latter is, as is seen in the figure, included in the 

 anterior coelom. The septum between the two cavities is first 

 formed dorsally, and then extends in a ventral direction; 

 fig. 28 shows it in process of formation. 



At the same time one can notice the first indication of that 

 predominance of the organs of the left side which is the key 

 to the whole ontogeny of the star -fish. We see in fig. 30 that 

 the septum between the right and left coelomic sacs is pushed 

 over to the right, owing to the tendency of the left posterior 

 coelom to extend over to the right on the ventral side. At no 

 time, so far as I have seen, however, does this septum break 

 down. Some curious trabeculse are in this stage stretched 

 across the left ccelom. They are easily distinguished from the 

 septum between the two sacs, as they consist of solid strings 

 of cells, whereas the septum has two layers of epithelium with 

 a slit of blastocoele between in this stage. These trabeculse 

 are very transitory ; in figs. 28 and 29 (Stage B) we see them 

 being formed, and in fig. 33 is the last trace of them (Stage C). 



As development proceeds the gut becomes more completely 

 separated from the coelom, the larval anus closes, and the short 

 rectum (fig. 31) disappears. Shortly before this, however, 

 the stomodseum opens into the gut, the main portion of 

 which constitutes the larval stomach (Z. stom.), the rectum 

 being very short ; but it is only for an extremely short time 

 that the larva possesses both mouth and anus. 



Stage C is reached about the end of the fifth day, or the 

 commencement of the sixth day. The division of the left 



