426 



THE CIRRIPEDS, TUNICATES AND ECHINODERMS. 



modasal infolding; it never gives rise to coelomic epithelium; and is never coinci- 

 dent with the anus. 



It is evident, therefore, that the terminal infolding of the blastula in the 

 echinoderm, tp., is in nowise comparable with the true gastrula of molluscs and 

 annelids. It does represent, however, the apical infolding of arthropods (telocoele 

 and telopore) and gives rise, as it does there, to the main mass of the postoral 

 mesoderm and endoderm, but which are here temporarily united to form a 

 continuous layer. Whatever remnant of the true gastrula is preserved in the 

 echinoderms must be looked for at the anterior end of the neural surface, at the 

 point where the primitive mouth arises. 



ahcl. 



Fig. 293. — Semi-diagrammatic-iigures illustrating the development of echinoderm larvae. 



The Mesoderm and the Coslom. — After the telocoele is formed, the primitive 

 mesoderm and endoderm that form its walls separate, the latter giving rise to the 

 enteron, the former to the paired anlagen of the mesoderm (hydro-enterocoele). 

 The peculiar way in which these two layers separate is merely a specialized process 

 of differentiating the lateral bands of mesoderm and endoderm, and is pecu- 

 liar to segmented animals with a very small percentage of yolk, and which develop 

 with extreme rapidity (Amphioxus, Balanoglossus, etc.). 



The mesodermic vesicles thus formed then break up on each side into two or 

 three main divisions. They are not to be regarded as primitive mesoblastic seg- 

 ments, but as groups of imperfectly divided ones, corresponding probably to the 

 three groups, or tagmas, so commonly present in the arthropods, one belonging 

 to the head, one to the thorax, and one to the' abdomen. (Figs. 293, 294.) 



The middle section of one side then divides imperfectly into the five chambers 

 of the hydrocoele, h.c, which represent five thoracic somites. If any segmentation 

 occurs in the other vesicles of either side, it is imperfect, and of short duration. 



