No. 3-] DEVELOPMENT OF MARIAE SPOXGES. 285 



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 e.xterior. 



Enibryolcgical Methods. — If 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 {i.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 



