No.3.] DEVELOPMENT OF MARINE SPONGES. i7 
oO 
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 larvae 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,” z.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 weight is evenly distributed round the long axis —an 
