348 E. W. MACBRIDE. 
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. 80, posterior to the gut. The opening 
of the gut into the ccelom has become closed ventrally (figs. 29 
and 80); dorsally, however, it remains open for some consider- 
rable 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, Jpc., which we may call the left 
posterior coelom (fig. 27); this second cavity includes a large 
part but not all of the left coelomic lobe mentioned above ; 
part of this latter is, as is seen in the figure, included in the 
anterior celom. 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 
coclom 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 trabecule are in this stage stretched 
across the left celom. 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 blastoceele between in this stage. These trabeculz 
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 coclom, the larval anus closes, and the short 
rectum (fig. 81) disappears. Shortly before this, however, 
the stomodeum opens into the gut, the main portion of 
which constitutes the larval stomach (/. 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 
