INO;;3.' DEVELOPMENT OF MARINE SPONGES. 353 
sponge shown in Pl. XXV, Fig. 5. Inthe section drawn, three 
main efferent canals (ef.c.) are shown, each with its osculum 
(os.) and its very irregular set of branches (ef. c’., ef. c"’., etc.), 
on the walls of which open the flagellated chambers (/. c.). 
The pores on the surface of this sponge (s. #.) lead into wide 
chambers (s. @.c.), the so-called subdermal cavities, from which 
run the afferent canals (af. c.), carrying water to the pores in 
the walls of the flagellated chambers. The distinction between 
subdermal cavities and afferent canals is more or less artificial, 
for the sharpness with which they are marked off from one 
another varies within wide limits. They are both parts of 
the same system, the subdermal cavity being merely a main 
afferent canal, which is especially enlarged in a tangential 
direction. The water may enter the chambers in some cases 
directly from the subdermal cavities, but for the most part it is 
carried to the chambers by the afferent canals, which branch 
and twist about, following the irregular course of the efferent 
canals. The mesoderm between the two sets of canals is re- 
duced to comparatively narrow trabeculae, in which lie the 
flagellated chambers, arranged in a much folded but still single 
layer. The spicules which are not shown in the figure are in 
the mesoderm, either scattered about or united into a mesh- 
work or a series of bundles. The genital products are also to 
be found in the mesoderm, scattered about, as a rule, in any 
part of the body. 
The structure of a horny sponge, such as the sponge of 
commerce (Euspongia), is essentially similar to that of the 
hypothetical silicious sponge I have just described. The differ- 
ences concern especially the skeleton and the precise manner 
in which the flagellated chambers are connected with the canals. 
In the horny sponges the silicious spicules give place to a 
meshwork of horny fibres, which lie in the mesoderm between 
the canals to which they lend support, and to the course of 
which their arrangement is adapted. The flagellated chambers 
in Euspongia, as in many other horny and silicious sponges, do 
not open directly into spacious efferent canals (as in Fig. 5), 
but indirectly by means of special canals, one of which runs 
from each chamber. And so it is with the afferent canals, 
