No. 3-] 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. 



Siibdcrmal 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, in. sp., PL XVII, Fig. 36. There are not many of 

 them, and they are small and round. After attachment, PL 

 XVII, Fig. }i^, 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. 1%, are the subdermal cavities ; 

 the deeper lying spaces, can., 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 



