STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 337 
a preparation ; the outlines of the protoplasmic strands are 
probably much more hard and distinct than in life, owing to 
the action of the reagents. In balsam preparations (fig. 13), 
owing doubtless to the greater transparency, the protoplasmic 
strands are not nearly so distinct. 
I may here mention the fact that cystenchyme occurs also in 
the choanosome of Stelospongus, but to this point I shall 
recur later on. 
This form of tissue, or very slight modifications of it, has 
been observed in Sponges of very divers groups. Sollas (16, 
17) has described it in Tetractinellida (Pachymatisma 
Johnstoni), Polejaeff (13) in Ceratosa (Cacospongia 
vesiculifera), and Ridley and Dendy (14) in Clavulina 
(Latrunculia apicalis). 
The stellate mesodermal cells of the ectosome appear to be 
thoroughly typical. They may be seen investing the grains of 
sand in a kind of delicate network, the individual cells being 
mutually connected by long slender processes (fig. 12). The 
body of the cell is somewhat granular and the nucleus is oval 
and of moderate size. 
I have not observed any definite arrangement of the cysten- 
chymatous and stellate tissues with regard to one another, nor 
have I any reason to suppose that such exists. 
(b) The Choanosome. 
I propose to consider the histological characters of the 
choanosome under the following heads : (1) The walls of the 
inhalant and exhalant canals. (2) The walls of the embryo- 
containing cavities. (3) The walls of the flagellated chambers. 
(4) The general mass of mesoderm in which the chambers and 
canals are embedded. (5) The spongoblasts and other meso- 
dermal cells surrounding the skeleton fibres. This arrangement 
is a purely arbitrary one, and I have adopted it merely as a 
matter of convenience, in view of the necessity of some 
definite plan to go upon. It is a matter of no small difficulty 
to classify satisfactorily the various forms of tissues and cell- 
elements which occur in any given Sponge. 
