Development of the Calcispongia3. 49 



reality Hackel has furnished no proof at all of the homology 

 of the ectoderm and the skeleton-forming layer. But it would 

 also not be difficult, by the aid of facts already sufficiently 

 well known, to convince ourselves that no such homology 

 exists in nature. We need only take into consideration the 

 known points in the developmental history of the marine sili- 

 ceous sponges*. It is known that in the embryo of these 

 animals the whole cell-mass divides into two portions, of which 

 the outer becomes the ciliated epithelium, whilst the inner takes 

 on the character of a skeleton-forming cell-aggregate. The 

 topographical position of this inner cell-mass (beneath the 

 ciliated layer), the circumstance that it appears as an aggregate 

 of compact spiculigenous elements, and, further, the fact that 

 these cells never appear as ciliated epithelial cells, furnish us 

 with sufficient data for rejecting their supposed homology with 

 the ectoderm of the Coelenterata. To this of course must also 

 be added the argument above cited, that the ectoderm of the 

 Coelenterata never produces skeletal structures, which always 

 appear as derivatives of the cutical layer. I have designedly 

 left out of consideration the facts observed by me in the deve- 

 lopmental history of the Sycon, in order to show that by careful 

 consideration of the known material it is impossible to arrive at 

 the erroneous notion of the agreement of the skeleton-forming 

 layer with the entoderm. But if we will also consider the 

 facts above described, we shall see at once that the develop- 

 ment of the Calcispongise is likewise opposed to Hiickel's 

 interpretation. It was established that it is the hinder un- 

 ciliated half that furnishes the calcareous skeleton, and con- 

 sequently that the skeleton-forming elements never appear in 

 the form of flagellate or ciliated epithelial cells, which are 

 characteristic of the ectoderm of the Coelenterata. 



From the reasons adduced, I venture to draw the conclusion 

 that the skeleton-forming layer of the sponges, or the so-called 

 " syncytium," of Hackel, does not represent the ectoderm, but 

 the skeleton-forming layer of many other animals, especially 



* See, e. </., the investigations of Lieberkiilm. I have myself made 

 some observations upon the development of the siliceous sponges, which 

 I shall publish elsewhere. Here I limit myself to the remark that the 

 larvoe ot four genera {Reniera, Esperia, Raspailia, and an undetermined 

 genus) are essentially of similar structure. As an example I have figured 

 the larva of Reniera (Plate II. tig. 15), which is chiefly distinguished 

 from the others by the presence of a posterior circlet of cilia. I have 

 observed the metamorphosis in an Esperia. The external epithelial layer 

 is gradually lost, so that for a time the young sponge appears to be com- 

 posed of an irregular aggregation of parenchyma-cells. It is only subse- 

 quently that the so-called ciliary baskets ( Wimperkorbe) appear, in the 

 form of closed spheres, which as yet are in no way connected with each 

 other. 



Ann. & Mag. N. Hist. Ser. 4. Vol. xvi. 4 



