229 



as in Fig. 86, C (after Thompson). The walls of the stomach then 

 separate from the body-wall. The animal now represents the pri- 

 mary stage of the crinoids, that which is the permanent stage in 

 the Pentacrinus and its fossil allies. The Antedon, however, in 

 after life separates from the stalk and moves about freely. 



Development of the Starfish. We will select as a type of the 

 mode of development of the starfishes, that of the common five 

 finger, Asterias (Fig. 87), as worked out with great thoroughness 



Fig. 87. 



M 



by Mr. A. Agassiz, and given in the " Seaside Studies." The 



inal memoir, through the kindness of the author, whose descrip- 

 tion is here freely used. 



Fig. 88 shows the transparent spherical egg, enclosing the ger- 

 minative vesicle and dot, and Figs. 89, 90, illustrate the segment- 

 ation of the yolk into two and eight and more cells, enclosing a 

 central cavity. After this the embryo hatches and swims about as 

 a transparent sphere (Fig. 91). A depression (Fig. 92, ma) then 

 begins to appear, the body elongates, and this depression forms 

 an inversion of the outer wall of the body (ectoderm), constitut- 

 ing the body cavity (d, Fig. 93, a), being the provisional mouth- 



