M. Coste on Fresh-water Polypi. 73 
nal folds.) The rectum commences towards the anterior part 
of the stomach, a little behind the cesophagus, by an aperture 
furnished with a sphincter, and opens externally upon the 
back of the animal. The coats of the three intestinal divi- 
sions are somewhat thick,—have a structure which appears 
glandular, and exhibit a muscular coat formed of circular 
fibres, which endows them with a strong contractile power, 
and which enables the stomach to operate on the aliment by 
its very marked movements, facilitating, of course, the process 
of digestion. In the Paludicella this power of contraction 
being much less, there is substituted a number of very long 
cilia, which are placed round the opening of communication 
between the stomach and rectum, and which, by their vibra- 
tions, agitate the alimentary molecules. 
The Apparatus of Reproduction. Besides the power of pro- 
pagating themselves by means of gemme, the fresh-water 
polypi also propagate by ova, which are produced in a filiform 
ovary, situate at the posterior part of the stomach, to which 
it is attached by one of its extremities, whilst by the other it 
is fixed to the inner surface of the cell, between the two points 
of attachment of the retractor muscles of the animal and those 
of the intestine. All the members of the same polypidon ap- 
pear to produce ova, and consequently, if there are two sexes, 
they must be united in the same individual. In general, the 
ova of the Alcyonella produce two individuals which are 
united, soft, and contractile throughout their whole extent ; 
so that the new polypidon which results from the co-existence 
of these two individuals, is capable of displacing itself up to 
the moment when ¢he superficial layer of its external envelope 
becomes solid. The external envelope of the Cristatella never 
becomes solid, but always maintains the power of contraction. 
We have observed in the common visceral cavity which is com- 
mon to the two individuals which proceed from the ova of the 
Alcyonella a well-defined spheroidal mass, which diminishes in 
size in proportion as the young polypidon augments, and final- 
ly disappears entirely. This temporary organ appears to be 
connected with the posterior extremity of the stomach, and 
should be considered, we believe, a kind of umbilical vessel. 
The young of the Cristatella have the whole of their visceral 
