432 EMBRYOLOGY 



from the water-vascular system, the epithelium of which produces an 

 external layer of contractile cells. According to Selenka, the live power- 

 ful longitudinal muscles of the Holothurian arise in this way from the 

 radial ambulacral vessels. This, however, is denied by Semon. Accord- 

 ing to him, the circular and longitudinal muscle layers of the body-wall 

 and the intestine take their origin from the enterocoele. Its two sacs 

 have expanded considerably and applied themselves to the wall of the 

 body and that of the intestine as the somatic and splanchnic layers, 

 thus giving rise to the musculature and epithelial lining of the body 

 cavity. A mesentery is developed on the dorsal and another on the 

 ventral side of the intestine. While, however, the ventral one is broken 

 through, and the two enterocceles become confluent here, the dorsal one 

 persists, and thus throughout life indicates the origin of the body cavity 

 from two separate sacs, as well as the bilateral plan of the body in general. 



The fundament of the nervous system has already been discussed. As 

 regards the development of the genital system, authors are not able to 

 make any very trustworthy statements. 



[The organogeny of the Holothurians has received a new and thorough 

 treatment in Ludwig's memoir on Cucumaria planci. — K.] 



Asteroidea. — Whereas earlier investigators were inclined 

 to believe that the starfish arose as a bud, so to speak, on the 

 larva, we know to-day that even here there is a metamorphosis 

 of the larval body into that of the adult animal. It is true 

 that certain modifications make their appearance in this 

 connection, for the body of the Echinoderm is at first estab- 

 lished in only a comparatively small portion of the larval 

 body. It is but gradually that the larger part of the larval 

 body is employed in the formation of the starfish. la 

 certain cases, it is true, this latter process seems to be 

 omitted, and the Echinoderm then takes its origin from only 

 one part of the body of the larva. In the latter method of 

 development the young starfish is detached from the body of 

 the larva, and the latter is said to be able to live for a con- 

 siderable time (JoH. MiJLriER, Koren et Danielssen). Such 

 a condition might cause the process to be looked upon as 

 budding. This view, however, is disproved by the meta- 

 morphosis of the entire body of the larva in other starfishes, 

 which will be described later. 



The metamorphosis of Bipinnaria or BracJdolaria into the 

 starfish has been studied most exhaustively by A. AoAssiz 

 (No. 1) and Metschnikoff (No. 37), 



