MATERIALS FOR A MONOGRAPH OF THE ASCONS. 325 
cells and the actinoblasts as derived from the porocyte-layer, 
then it is not impossible that the excretory cells might arise 
from the actinoblasts in some cases. Certain appearances 
suggest that the actinoblasts first become very granular and 
excretory in nature (fig. 59), and then give rise to separate 
excretory cells (fig. 69). On the other hand, I have fre- 
quently found the excretory cells near the base of young 
gastral rays, in a situation that suggests the possibility that 
they might migrate on to the rays (figs. 62, 63). I am 
obliged to leave this point undecided, and may remark that 
it would be better to study the question in surface-views of 
the inner aspect of the sponge-wall (as I had hoped to do) 
than in sections, since by the latter method it is never 
possible to be certain how much has been cut away in the 
preparation. Hence negative evidence is indecisive when 
derived from the study of sections. 
With regard to the function of these cells, I may recall, in 
this connection the fact that Bidder (1892) regarded certain 
cellsin Clathrina clathrus as excretory in function, basing 
this conclusion upon the reactions of these cells to stains. 
Bidder termed the cells in question Metschnikoff’s cells. In 
my memoir on the Clathrinide (1898) I pointed out that 
the Metschnikoff’s cells were contracted porocytes, and 
argued (l.c.,p. 527) against the probability of the porocytes 
being excretory on the ground that any excretions produced 
by them would be carried into the sponge by the water- 
currents, whereas excretions are usually produced in situa- 
tions where they are carried out of the body. I may point 
out, however, that the cells which I am describing in 
Leucosolenia, and which are also of the nature of poro- 
cytes, would be very favourably situated for exercising an 
excretory function, especially when they occur in the oscular 
rim or on the gastral rays of the quadriradiates. The cells 
of sponges, and especially of Ascons, are extremely general- 
ised in function, and capable of exerting at different times 
activities which in other Metazoa are exerted by different 
cells, specialised each in a particular direction. Thus in 
