796 
DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
zooid. This is illustrated by figs. 204 to 207. In fig. 204 the free edge (e.) is still 
some distance behind the zooid (ex.). In the next figure it has advanced further 
forwards and in fig. 207 has nearly passed the zooid. The edge finally unites with 
the dorsal wall at about the stage of fig. 187 and the canal is completely closed. 
These stages in the forward movement of the edge of the septum are shown also in 
dorsal view in figs. 181, 186 and 185. 
As the septum travels forwards its edge stretches between the bases of the dorsal 
pah' of septa and the latter gradually extend down upon the lower side of the 
peduncular septum in a manner which it is difficult to describe, and is scarely shown 
in the figures. In fig. 181 the edge may be seen stretching between the dorsal septa 
far behind the dorsal mesenterial filaments ( d.f .). In fig. 205 the edge of the septum 
and the filaments have nearly met. In fig. 206 the lower ends of the dorsal filaments 
lie below the peduncular septum, and in fig. 207, finally, the filaments and septa lie 
for more than half their length on the lower side of the peduncular septum. 
b. The ventral canal. 
The closure of this canal is effected by an entirely different process which I have not 
been able to follow out completely. It has already been mentioned (p. 786) that the 
partition between the first two sexual buds on each side ends below by a free edge 
(fig. 204). From this point a delicate flap or fold of membrane extends for a short 
distance inwards on the under side of the peduncular septum (fig. 204The latter 
bends rather suddenly upwards at this level to terminate by the free edge (e). in front. 
In a later stage these flaps extend still further inwards so as nearly to meet on the 
under side of the septum. The edge of the flap also extends obliquely upwards and 
forwards across the base of the bud (fig. 205). Still later the two flaps unite below 
the peduncular septum and form a single membrane extending entirely across the 
upper part of the ventral canal and ending by a smooth round edge below (fig. 206). 
At the sides the membrane is slung to the body-wall by fibrous strands like those 
which suspend the peduncular septum (see p. 768), and it now extends nearly across 
the base of the bud. 
In the latest stage observed (fig. 207) the membrane extends at the sides nearly 
around the body and has united with delicate irregular outgrowths from the ventral 
and ventro-lateral septa. Thus the ventral ca^.al is separated by an incomplete 
partition from the anterior part of the axial polyp where the septa, mesenterial fila¬ 
ments and other organs are situated. There is still, however, a very large rounded 
opening in the middle of the partition through which the currents of the gastric fluid 
still flow. In later stages this opening closes up completely, probably by the 
approximation and union of the edges of the membrane, but I have been unable to 
follow this since the walls of the body become very opaque through the appearance of 
great numbers of spicules. 
In the adult the peduncular septum appears to be split horizontally in front into two 
