Hyatt.] 82 [March 5, 



which press the pigmented cells outward into a circular band. 

 As stated by Barrois the collar appears to homologize with the 

 crown of large cells in Sycandra raphanus which surrounds the 

 true blastopore, and according to Schultze is useful in assisting 

 the larva to fasten itself to surfaces. These coronal cells are^ 

 however, really formed from modified ectoblast cells, and, as 

 Barrois pointed out, must be considered as a third differentiation 

 occuring in the embryo between the ectoblast proper and the 

 endoblast, though not as supposed by that author necessarily 

 mesodermic cells. The position occupied by the collar, its sharp 

 outline, color, and functions indicate, that it is a third part in 

 the larva, and Keller's observations support Barrois' opinion 

 that silicious spicules arise in the collar or from cells belong- 

 ing primitively to this part, and that the collar is a primitive 

 mesodermal layer. The color in Keller's figure appears to make 

 this probable, but on the other hand there are no convincing 

 specific observations either by Barrois or Keller, which would 

 do away with the impression made by the constant presence of 

 spicules only in the central uncolored cells of the blastoporic 

 area and after the larva becomes plugged from the interior. 

 They are parts of the central layer proper coming to the surface 

 in the central core of the blastoporic area, and the collar appears 

 to us in the larvae we have studied to be an entirely inde- 

 pendent derivative formed out of the older ectoblastic cells. 

 The internal outlines in Spongia graminea and Chalinula lim- 

 bata when the collar is fully developed are sharply defined, and do 

 not permit us to imagine any connection between the collar cells 

 and spicules. 



If it had been possible to illustrate this paper we could have 

 given figures of several larvae of Halichondria with the spicules 

 unquestionably gathered into a sheaf in this core, with their ends 

 coming to the surface, and also similar cases in Chal. limbata. In 

 this last, threads, evidently horny and more or less sinuous, occu- 

 pied the place of straight spicules. This, to us, is a strong argu- 

 ment for the derivation of the cells in the plug through an inva- 

 gination of the outer layer of the larva. We have held (Mem. 

 Bost. Soc. Nat. Hist., vol. n, p. 482) that keratode was found 

 in invaginations of the ectoderm, and have now drawings of 

 the internal structure of Chal. arbuscula w 7 hich leave no doubt 



