82 SPONGES 



other. hand, to be " ectodermal," and formed by flattening out of the 

 ciliated layer of the larva. This mode of interpreting sponge develop- 

 ment was more the result of a priori reasoning than of actual observation. 

 The morphological similarities existing between sponge and Coelenterate 

 larvae on the one hand, and between adult sponges and coelenteratea on 

 the other, led to the assumption that the metamorphoses of the larvae of 

 the two classes were also of an essentially similar type, a belief which was 

 seldom shaken by observation in the case of objects which present so 

 many technical and practical obstacles to microscopic study as do sponge 

 larvae. The development of Sycon alone stood apart, and was always 

 difficult to bring into line with the supposed course of the life-history of 

 other forms ; and it is greatly to be deplored that Metschnikoff, whose 

 accurate investigations first led to a true understanding of the develop- 

 ment of Sycon, should have failed to see that the metamorphosis of 

 Clathrina was of the same type. 



In recent years the careful studies of Maas [11] and Delage [2] 

 have shown the metamorphosis of the larvae of Demospongiae to be 

 of quite an opposite nature to that of the Coelenterate planula, 

 though easily reconcilable with the development 6f such a form as 

 Sycon, since in both cases the flagellated cells give rise to the 

 gastral layer, the inner, or posterior mass of typically non-flagellate 

 cells to the dermal layer of the adult. These observations have 

 been extended by the author to the parenchymula larva of calcareous 

 sponges, and by Maas to the blastosphere of Oscarella. There re- 

 main at present only Halisarca and Plakina, as types in which 

 statements made under the influence of the older views remain 

 uncontradicted and in need of reinvestigation. The more recent 

 researches upon sponge embryology have made it possible, for the 

 first time, to give a consistent and connected account of the de- 

 velopment and to homologise the different types of sponge larva 

 with one another. 



Development of Sponyilla. As an aberrant type of sponge development 

 it is necessary to mention that of the freshwater sponges (Xpongilla, and 

 Ephydatia). About no other form has so much been written ; in no 

 other case are the statements so contradictory or the real facts of the 

 development still so obscure. The questions at issue concern the meta- 

 morphosis, and more especially the origin of the ciliated chambers of the 

 adult, on the one hand, and the fate of the flagellated cells of the larva 

 on the other. Thus, according to Ganin, the flagellated cells of the larva 

 become the " ectoderm " of the adult, and the chambers are derived from 

 the inner mass ; according to Qotte, the flagellated cells of the larva are 

 thrown off entirely, and the whole sponge develops from the inner mass ; 

 according to Delage, the flagellated cells of the larva become the ciliated 

 chambers of the adult, but in a roundabout manner, being first devoured 

 in a phagocytic manner by cells of the inner mass, which then carry them 

 inwards and cast them out again to form the chambers, some, however, 

 being entirely digested and absorbed during the process. Maas at first 



