THE LAEVA OF ASTERIAS VULGARIS. 109 



bands of Annelids, as described in other Echinoderms by 

 Hatschek, Selenka, and Pleischmanu. As the invagination 

 progresses the spherical form of the larva changes to ovoid, 

 the long axis corresponding with the autero-posterior axis of 

 the future Bipinnaria. The gastrula travels through the 

 water with two motions, one of translation in the line of the 

 long axis of the body, the blastopore directed forwards ; the 

 other of rotation around this axis. At the completion of 

 invagination the gastrula is much elongated (fig. 9). The 

 archenteron, extending backwards about two thirds of the 

 length of the embryo, is somewhat tubular in form ; its blind 

 end is bent towards that side which later proves to be the 

 dorsal surface of the Bipinnaria. At this blind end is a con- 

 siderable enlargement, where the cells by becoming flattened 

 and losing their cilia acquire a character difierent from the 

 columnar ciliated cells of the rest of the archenteron. 



In section the ectoderm-cells are seen to be flatter and 

 smaller than those of the entoderm, but the one grades insen- 

 sibly into the other (fig. 9) . At the pole opposite the blas- 

 topore is a point where the cells are distinctly more columnar 

 than in any other part of the ectoderm. This point is found 

 to become the apical pole of the Bipinnaria (fig. 9, a. p.). 



Formation of the Enteroccels. — The mesoderm in As- 

 terias as in most other Echinoderms has a twofold origin, 

 though morphologically a sharp distinction between them in 

 this case is not to be made : (1) mesenchyme formation ; (2) 

 enterocoel formation. The enterocoels arise as two bilaterally 

 symmetrical diverticula of the blind end of the archenteron 

 (fig. 10, el.). In position they are lateral and slightly dorsal. 

 The time of complete separation shows much individual varia- 

 tion, in some cases being complete before, in others just after 

 the formation of the larval oesophagus. A. Agassiz (2), work- 

 ing upon Asterias vulgaris, fouud that the stomodaeal 

 invagination united with the archenteron before the ente- 

 rocoels separated from the latter. Metschnikofl" (26) agrees 

 with this, as does also Gotte (10) for Asterias glacialis. 

 Ludwig (25) says, " In most Echinoderms the separation of 



