470 Report of the State Geologist. 



and 13, Plate C, show its formation in optical sections. In 

 this transformation the two primitive portions of the embryo 

 become more and more separa,ted. In the stage represented 

 by fig. 14, Plate C, the two extremities are enlarged and the 

 middle constricted, forming two visceral masses of the body. 

 The general cavity of the body {cg) and the mantle cavity 

 are reduced to mere slits, and are not as heretofore in direct 

 communication with each other, but their ends are close to- 

 gether at the constricted portion of the body, without being 

 connected. Externally the free larva, at this stage, presents 

 an elongate form {^g. 15, Plate C), with the posterior extremity 

 much the larger, occupied by the mesodermic mass and covered 

 by the mantle. The anterior part is less swelled and con- 

 tains the intestine. The two portions are separated by the 

 constricted portion before mentioned. The two visceral masses 

 are visible by the transparence of the body as two dark spots 

 at the extremities of the free larva. Each pole is occupied 

 by an opening. The first, situated at the superior pole, is the 

 buccal opening. It is capable of contracting in such a manner 

 as to be reduced to a simple point, and frequently it is so small 

 that it is almost impossible to distinguish it ; in this case there 

 are always present, in that part of the skin covering the intestine, 

 radiating striae, which are probably plications produced by the 

 contractions of the buccal opening. At the opposite pole there 

 is another opening capable of expanding and contracting itself, 

 and at the bottom of which the aboral mass is visible. As at 

 the opposite pole there are radiating lines, but finer and situated 

 more closely together. 



Metamorphosis. 

 After the embryo has become fixed the first stage observed is 

 that represented in Plate D, fig. 16, and is composed of an internal 

 mass, ^, and an external layer, &, which are separated from each 

 other by a fatty layer, e; the first constituting a pyriform mass ; 

 the second, soft and transparent ; the third is composed of fatty 

 globules closely enveloping the central mass, and in turn envel- 

 oped by the external layer. The form of the entire embryo is 

 determined by that of the internal mass and is necessarily pyri- 

 form. At this stage it does not present in its interior any cavity 



