POEIFERA 



27 



complete loss of the ectoderm, which breaks up and is detached 

 from the surface in shreds, so that the entire body of the 

 young sponge consists of entoderm. In this now solid mass 

 (for the entodermic cavity has disappeared in the process 

 of attachment) ampullae arise as separate fundaments ; they 

 are produced by groups of cells (each of which has ai-isen 

 from a single cell) acquiring cavities within them. The 

 canals and cavities of the body develop in the same manner 

 in many separate regions, and afterwards unite with one 

 another and with the ampullfe. The most superficial layer 

 of the body acquires the character of a pavement epithelium, 



^c 



-- ZW 



Fig. 9.— Free-swimming larva of Spongilla fluviatilis (after Goeitk). The 

 covering of flagella has been omitted. Ec, ectoderm; En, entoderm; d, ento- 

 dermic cavity. 



and forms the permanent epidermis of Spongilla. All or- 

 gans therefore arise by histological differentiation from a 

 single layer, the primitive entoderm. 



In Goette's account, which we have given here, several points must 

 appear to be still doubtful, thus, above all, the alleged complete casting off 

 of the ectoderm. To be sure, a rupture and partial loss of the ectoderm 

 has also been maintained for other sponges (Beniera, Esperia) by earlier 

 observers, to which attention has already been called (p. 24). But such 

 appearances are probably for the most part to be referred to pathological 

 or abnormal processes. Ganin has assumed that in Spongilla the ecto- 

 derm of the larva becomes the permanent epidermis of the sponge ; and 



