No. 3. ] DEVELOPMENT OF MARINE SPONGES. 2 85 
efferent with afferent canals. One would also be inclined to 
believe that the position of the oscula is not determined by any 
deep-lying (though veiled) division of the sponge body into indi- 
viduals. The homology between pores and oscula would rest 
on the absence of any structural difference between them 
(they differ in size, but the variation in the size of the osculum 
weakens this argument), and on their similarity in the matter 
of local surroundings (comp. Figs. 6 and 7— both pores and 
oscula open into comparatively shallow, spacious cavities strik- 
ingly alike). The homology between the two sorts of canals 
would rest on their entire similarity —there is no discoverable 
difference between the subdermal cavities into which the pores 
open, Fig. 7, and the oscular cavities shown in this figure and 
in Fig. 6. (The development shows also that in this sponge 
they are formed in precisely the same way.) As to the basis 
of the third conclusion, the oscula are distributed with entire 
irregularity, and the oscular cavities cannot be regarded as so 
many centers round which the canal system of the sponge 
groups itself. Rather, it would seem from an examination 
of such portions of the surface as that shown in Fig. 7, that 
circumstances may determine the transformation of a pore 
area into an osculum almost anywhere. The comparative 
anatomy of sponges in general, however, forces upon us the 
conviction that forms like this are phylogenetically colonies, 
even though it be true that new oscula may be formed in an 
individual independently of any process of budding. And, 
further, we are driven to believe that phylogenetically, afferent 
and efferent canals are radically different things, the latter 
being lined with endoderm, while the former are invaginations 
from the exterior. 
Embryological Methods. — lf an Esperella be examined dur- 
ing the summer months, it is found to contain great numbers 
of embryos imbedded in the mesoderm. When these embryos 
are studied they are found not to be egg embryos, but gem- 
mules (z.e. internal buds). Nevertheless, the gemmules in 
sponges kept in aquaria escape through the oscula as ciliated 
larvae, essentially identical in structure with the typical egg 
larva of silicious sponges. After swimming about for a day or 
