No. 3-] DEVELOPMENT OF MARINE SPONGES. 317 



Yves Delage describes an incomplete layer of rounded cells 

 at the surface of the larva, between which protrude the periph- 

 eral ends of the ciliated cells. The ciliated cells migrate into 

 the interior of the larva, subsequently forming the lining epithe- 

 lium of the canals, and are regarded by the author as constitut- 

 ing the endoderm. The superficial rounded cells then form a 

 continuous layer and constitute the ectoderm. These facts 

 enable the author to make a detailed comparison between the 

 amphiblastula of calcareous sponges, and the solid larva of the 

 silicious sponges. The former is hollow, but the cavity of the 

 latter is filled with a mass of mesoderm (precociously formed, as 

 compared with the amphiblastula development). And instead 

 of the endodermic and ectodermic elements being confined to 

 opposite halves of the larva, as in the amphiblastula, they are, 

 in the solid larva, intermingled over the whole surface. Conse- 

 quently, in the latter, the endoderm (ciliated cells) cannot 

 invaginate as a continuous layer, but the component cells have 

 to migrate into the interior separately. The flagellated cham- 

 bers are formed from special mesoderm cells. 



The description of the lar\-ae of Esperella Lorenzi and E. 

 lingua, given by Maas, agrees in essential points with my 

 account of the larva of Esperella fibrex. The resemblance 

 holds good even for many details. On the other hand, Maas 

 describes a cavity or lacuna in the anterior end of the larva, 

 traversed by branching cells, of which I saw no sign in the 

 form I studied. Again, the chelae, which, in Maas's larva as 

 in mine, are united in spherical groups, are in the former 

 differentiated shovels, while in the latter they remain in a 

 more embryonic condition. A more important difference is 

 exhibited in a layer of cells which Maas describes at the 

 anterior end of the larva, with nuclei very close to the periph- 

 ery, and which he believes to be " intermediary cells," i.e., 

 cells lying between the ectoderm elements proper. I have 

 frequently seen nuclei here and there very close to the periph- 

 ery, but saw no reason for regarding them as belonging to a 

 set of cells distinct from the ectoderm. Maas suggests that 

 the arrangement of the spicules in the swimming larva is such 

 that the wei£;ht is evenlv distributed round the long axis — an 



