102 MARINE BIOLOGICAL LABORATORY. 



organism with a mouth, endoderm, and mesogloea, and 

 also giv^es a clue to the causes which have brought about 

 the change. The central cells of the hollow planula 

 become transformed into the mesogloea and endoderm, 

 the number of cells contributing to the latter layer being 

 relatively few. The mesogloeal cells have various func- 

 tions, some producing spicules or horny fibres in some 

 sponges, but a large number remain indifferent or un- 

 specialized to a greater or less extent, some becoming 

 reproductive. 



We find too that the endoderm becomes differentiated 

 from the central mass long before the cavity it encloses 

 has any communication with the exterior, and cannot 

 very efficiently function as a nutritive layer since it is 

 separated from the outside world not only by the thin 

 ectoderm, but also by the thick mesogloea. It is neces- 

 sary that respiration should be carried on throughout the 

 entire mass of the sponge. While it is small this is 

 readily effected through the ectoderm, but as it grows, 

 the mass increases so much more rapidly than the sur- 

 face, that this simple method no longer suffices. Cavi- 

 ties appear here and there in the mesogloeal mass, and 

 later communicate with one another, the cells lining them 

 becoming ciliated to produce a more rapid circulation of 

 the water. A large central space into which the various 

 cavities finally open makes its appearance, and last of 

 all this space breaks through to the exterior, forming the 

 Osculiim. The water which at first reached the cavities 

 by filtration through the tissues, later on has ingress by 

 pores, and the complicated canal system of the sponge is 

 established. 



In its first inception, then, the canal system and the 



