336 
ARTHUR DENDY. 
in shape, about 0 0048 mm. in diameter, and in preparations 
stained with borax carmine they stand out very sharply. I 
have endeavoured to demonstrate the outlines of the epi- 
dermic cells by means of silver nitrate staining, but possibly 
owing to the fact that I had only spirit material to work with 
without success. Doubtless this epithelium is continued in- 
wards through the pores to line the subdermal cavities, but 
I have not succeeded in detecting it here. 
A very large proportion of the ectosome is occupied by the 
sand grains above mentioned, but surrounding these is a con- 
siderable quantity of mesodermal tissue. 1 This is for the most 
part made up of cystenchyme (fig. 13), but stellate mesodermal 
cells (fig. 12) are also present. 
The term cystenchyme has been applied by Sollas (17) 
to a peculiar form of tissue not uncommonly met with in the 
ectosome of Sponges. This tissue consists essentially of a 
number of more or less spherical cells, each provided with a 
distinct cell wall, and containing a very much vacuolated 
protoplasm in the interior. The nucleus appears to be sus- 
pended in the centre of the cell in a central protoplasmic mass 
connected with the cell wall by radiating strands of proto- 
plasm. The whole structure resembles very much an ordinary 
vegetable parenchyma cell. The individual cells are packed 
more or less closely together, and the spaces between them 
are filled with a granular or sometimes fibrous substance, 
which is probably chiefly of an intercellular nature. 
In Stelospongus the cystenchyme cells (fig. 13) are oval 
or subglobular in shape, measuring about 0'024 mm. in 
diameter, and the nucleus is small and granular. The proto- 
plasmic strands connecting the nucleus with the cell wall are 
best seen in unstained preparations mounted in glycerine ; 
they are in such seen to form a network, branching and anasto- 
mosing inter se. Fig. 14 represents a single cell from such 
1 I use the term mesoderm here and elsewhere because it is in such very 
general use amongst spongologists, and not because I believe the tissues 
thereby designated to be homologous with the mesodermal tissues of other 
animals. 
