NOz3-]| DEVELOPMENT OF MARINE SPONGES. 305 
of which was observed in the living sponge). It is possible 
that the ectodermal amoeboid ‘“ Hof’”’ of Maas corresponds to 
my ‘ectodermal membrane,” but I have never observed the 
peripheral cells to be amoeboid. Even if they were amoeboid, 
their movements could exert no influence on the shape of the 
mes-entodermic mass, merely for the reason that the edge of 
the ectodermal membrane is too far away from this mass. 
Maas does not describe the peripheral zone of amoeboid mes- 
entodermic elements, which is so conspicuous in the sponge I 
studied. The extensive ectodermal membrane I have described, 
which surrounds the body of the sponge, is perhaps confined 
to the silicious sponges, and may not, of course, be universal 
in them. I regard it as an excessive development of a simple 
layer of amoeboid ectoderm, such as clothes the attaching 
Sycandra, Schulze (25). 
Note. —I find that owing to the extreme awkwardness of the term mes- 
entoderm, I have frequently used mesoderm as synonymous with it. No 
confusion will arise from this, if it is remembered that until the canal system 
is formed, the body of the sponge consists solely of two layers, — an outer 
covering (ectoderm) and an inner mass of cells (parenchyma, mes-entoderm, 
or mesoderm). After the canals are formed the term mesoderm is applied 
exclusively to the tissues lying between the ectoderm and the canal system. 
Subdermal Cavities and Canals. — Both the subdermal cavi- 
ties and canals arise as intercellular spaces in exactly the same 
manner. Intercellular spaces appear in the larva while it is 
attaching, 27. sf., Pl. XVII, Fig. 36. There are not many of 
them, and they are small and round. After attachment, PI. 
XVII, Fig. 38, extensive cavities appear in the body, which are 
entirely independent of one another. The cavities formed 
directly beneath the upper surface are especially large, though 
shallow. These, s. d.c. Fig. 38, are the subdermal cavities ; 
the deeper lying spaces, caz., are the canals. At this time the 
mass of cells lying inside the ectoderm, the mes-entoderm, is, 
as has been said, largely composed of formative cells, with 
smaller slender cells scattered about here and there. The cells 
of the mes-entoderm are all connected together by delicate 
processes, and there are many indications that the ectoderm 
