EEPORT ON THE TETEACTINELLIDA. 
229 
of the same sponge, or even different individuals of the same species of sponge, are 
brought in contact, and thus we may conclude that the pleats wherever they touched 
each other coalesced ; in this manner a complex folded sponge-wall, traversed by sinuses, 
and much thicker than of the earlier stage, would result (Fig. 2, d). Obviously this wall 
may by a continuation of the same process increase in thickness and complexity to an 
almost unlimited extent. A transverse section (PL XXVI. fig. l) through the actual 
sponge presents appearances which are entirely consistent with the foregoing explanation. 
The diagram (Fig. 2, d) might well stand for a generalised representation of its structure ; 
there are sinuous passages of the most irregular form, some still merely deep extensions 
of the outer folds, others converted into complete canals opening by circular mouths to 
the interior ; some communicating with the inner surface of the cup, but more with the 
outer. All are lined by a thin layer of sterrasters, thus demonstrating their superficial 
origin, and proving that they are in no way connected with the true water-canals of the 
sponge. 
The passages are seldom empty, usually they are closely packed with a substance 
which looks like cotton wool, but which really consists of sponge spicules densely matted 
together, every one of them identical in character with those which build up the 
skeleton of the sponge. They are deciduous spicules, which have been secreted by the 
sponge and shed from its free surface ; and they owe their preservation in these cavities 
to the accident that a part of the free surface has here become involved in the interior 
of the wall. In most sponges the effete spicules when they are shed from the sponge 
fall to the sea-floor and dissolve all connection with their source. Here, owing to 
fortunate circumstances, they are preserved to suggest the existence of a process which 
might otherwise have been overlooked. Whilst there are other sponges which also 
furnish evidence as to the deciduous nature of the spicules, there are none which would 
have led us to suppose that the quantity of deciduous material is so large as from this 
sponge we learn to be the case, for the quantity of dead spicules accumulated w'ithin its 
