1876.] 15 [Hyatt. 



appearance to the mechanical action of the presence of water which 

 accumulates in the recently formed canals. The hollowing out of 

 these canals in the endoderm, succeeding the formation of the am- 

 pullaceous sacs, and for the secondary purpose of supplying these 

 extraordinary organs with food and aerated water, is not a very close 

 resemblance to the formation of the gastrula in the Polyps, and 

 completely demolishes Haeckel's theory of the connection of sponges 

 and corals. In fact, there is no need of argument in this direction; 

 the fact stands that in the larger number of sponges there are no 

 gastrula forms at all in the young. Besides this, it is even doubtful 

 whether in the Calcispongise and Halisarca this stage is not a tempo- 

 rary condition of the young, as Barrois seems disposed to think it is 

 in the former. He had himself often observed what seemed an invo- 

 lution of the ectoderm in the ciliated larva of various species of sili- 

 ceous sponges, and had only been prevented from figuring it as such 

 by the fact that the centre was already solid with the included endo- 

 derm. Upon farther investigation, it became evident that the ap- 

 pearances were due to the extraordinary changes of form undergone 

 by the larva in confinement. 



Even supposing that the presence of the true gastrula stage were 

 to be demonstrated in some sponges, and the identity of the first 

 formed canals with the stomach of the gastrula be shown, there would 

 still remain the most extraordinary differences. The development 

 and structure of those singular organs, the ampullaceous sacs, con- 

 taining cells so formed that each one performs the function of an 

 independent, monad-like individual, taking in and digesting food on 

 its own account; the fact that this monadigerous layer which lines the 

 sac, or the interior of the canals, as in the Calcispongige, is not the 

 endoderm as supposed by Haeckel, but a supplementary layer, as 

 shown by Schultze, in the Calcispongise, and frequently seen by the 

 speaker in the siliceous and keratose sponges, would still remain to 

 be accounted for. The monadigerous layer, as shown by Barrois, is 

 an aggregation of cells, arising from the plasma of the endoderm 

 and resting upon it, as shown by Schultze, when the secondary cel- 

 lular membrane, which properly represents the endoderm, is fully 

 formed and lines the canals in the adult. The monadigerous layer 

 and these ampullaceous sacs are organs without homologues in the 

 remainder of the animal kingdom; for the attempt to trace any 

 homology between them and the nettle cells has not the slightest 

 element of plausability, either in the structure, uses or location of 



