226 SCIENCE PROGRESS. 



While there is thus a close agreement, extending even 

 to details, between the life-histories of the great majority 

 of both calcareous and siliceous sponges, the development 

 of the most primitive forms in the two groups presents 

 great difficulties. This is especially the case in the Ascons, 

 where, on account of the primitive nature of the adult 

 forms, we might expect to find an equally primitive mode 

 of development, which would furnish the key to that occur- 

 ring in other sponges. But if the description hitherto given 

 of the development of Ascetta be correct, any comparison 

 with the larva of Sycon or of Cornacuspongice is out of the 

 question. 



According to the accounts of Schmidt 1 and Metschni- 

 koff, 2 the larva of Ascetta is hatched as an oval blastula 

 composed of a single row of liagellated cells enclosing a 

 large cavity. The cells at the hinder end differ in some 

 characters from the remainder, being in particular more 

 granular. From the hinder pole an immigration of cells 

 goes on into the interior, until the internal cavity becomes 

 packed with large, granular cells, with nuclei considerably 

 larger than the nuclei of the ciliated epithelium covering 

 them. The larva thus constituted fixes, by which pole 

 is not certain, and the covering of flagellated cells is said 

 to furnish the flattened ectoderm of the adult, while the 

 cells of the internal mass form the collar-cell layer. 



If the destination of the two cell layers of the larva 

 of Ascetta be really as described, there is no possibility 

 of comparing it with the amphiblastula, or with the larvae of 

 Cornamspongice. In the face, however, of the close resem- 

 blance of the larva of Ascetta just before fixation and the 

 completely ciliated larvae of some Cornacuspongice, it is 

 difficult to escape the conviction that the metamorphosis 

 of Ascetta has been quite wrongly described. In both larvae 

 we have a layer of finely granular flagellated cells sur- 



1 Schmidt, Das Larvenstadium von Ascetta primordialis and A. 

 clathrus, Archiv f. mikr. Atiat., bd. xiv., 1877. 



2 Metschnikoff, Spongiologische Studien, Zeitschr. f. wiss. Zoo/., bd. 

 xxxii., 1879. 



