Structure of Marine Sponges. 333 
appears to me to be undescribed, and which I have just found on 
the rocks here, the black colour is owing to the ampullaceous 
sacs, which, although scattered throughout every part of the 
sponge, are brought together in much more close approxima- 
tion on the surface, where they forma layer 1-12th of an inch 
thick, of intense blackness; and when a portion of this layer 
is torn to pieces, the black colour is found to arise from the 
presence of one or more black granules in each of the sponge- 
cells, which thus collectively give the black colour to the am- 
pullaceous sac, and the latter, in great numbers and close "ad 
proximation, to the uniform and characteristic blackness of the 
surface of the sponge. 
(This layer, which is supported on another internally, formed 
ofstout quadriradiate spicules, and covered externally by a 
thin transparent dermal layer charged with short bacillary 
spinous ones, the spicules of the body being 4-radiate and tri- 
curvate, I propose to notice more in detail in a future com- 
munication, describing and illustrating the whole sponge.) _ 
ut to return to the ampullaceous sac in the living marine 
sponges, I have, besides observing it in Halichondria simu- 
lans, seen it in all the rest that I have examined where I have 
looked for it particularly. I have dried pieces of the Geodide, 
too, in which it is obvious ; but in the dried sponges the am- 
pullaceous sac or little globular group of sponge-cells which 
represents it will not be preserved if the specimens have not 
been dried while fresh and living. Putridity destroys them ; 
and therefore all weathered specimens, such as are chiefly 
found on beaches, will probably fail to exhibit them. 
Calcareous Sponges. 
sponges, that the same kind of ampullaceous sac with its cilia 
waving internally exists in all, as in the siliceous Isodictya 
