STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 335 
lacunar type of canal system, as it occurs in his Euspongia 
canaliculata, and this species appears to have the clear 
transparent ground substance usually found in association 
with eurypylous chambers. 
I have endeavoured to show in a previous paper (9) that we 
cannot draw any hard and fast line between flagellated 
chambers with and flagellated chambers without special incur- 
rent and excurrent canaliculi, and I believe with Polejaeff that 
these two types graduate into one another. I have also stated 
above that the chambers of Stelospougus usually have 
short, relatively wide cameral canaliculi, but that we must not 
lay too much stress upon this fact. Now, according to Pole- 
jaeff, as we have just seen, Schulze describes the flagellated 
chambers of Euspongia as being provided with special 
cameral cananiculi and embedded in a granular ground sub- 
stance, but to judge from Schulze’s figure, as copied by 
Vosmaer (18), the exhalant canaliculi at any rate are only very 
slightly develoved, and the arrangement of the chambers in 
Euspongia agrees very closely indeed with that found in 
Stelospongus. 
Histology of the Soft Tissues. 
(a) The Ectosome. 
The ectosome (fig. 2, ect .) forms a relatively thin external 
layer all over the body of the Sponge. Owing to the presence 
in it of a large amount of sand, especially abundant in the 
raised ridges, it is very hard and tough, and forms an excel- 
lent protection against the attacks of parasitic crustaceans, 
worms, & c., to which Sponges are subject. 
The outermost portion of the ectosome is formed by an 
extremely thin and delicate epidermis, which I have succeeded 
in making out chiefly in the pore-areas, where the sand grains 
are absent. In preparations such as that represented in 
fig. 4, one can easily distinguish the nuclei of the epidermic 
cells ( n . e. c.) scattered in the transparent, pore-bearing mem- 
brane. These nuclei are small granular bodies, round or oval 
