456 



HOLOTHUROIDEA. 



ME 



FIG. 249. LONGITUDINAL SECTION 

 THROUGH AN EMBRYO OF CuCUMAEIA DOLI- 

 ULUM AT THE END OF THE FOURTH DAY. 



Vpv. vaso-peritoneal vesicle; ME. 

 mesenteron; Blp., Ptd. blastopore, proc- 

 toilasum. 



We must suppose that eventually the madreporic canal loses its 

 connection with the exterior so as to hang loosely in the interior, though 

 the steps of this process do not appear to have been made out. 



The original hinder peritoneal 

 vesicle grows rapidly, and divides 

 into two (fig. 248 C, pi. and pr.~), 

 which encircle the two sides of the 

 alimentary canal, and meet above 

 and below it. The outer wall of each 

 of them attaches itself to the skin, 

 and the inner one to the aliment- 

 ary canal and water-vascular sys- 

 tem; in both cases the walls remain 

 separated from the adjacent parts 

 by a layer of the amoeboid cells 

 already spoken of. The cavity of 

 the peritoneal vesicles becomes the 

 permanent body cavity. Where 

 the walls of the two vesicles meet 

 on the dorsal side, a mesentery, 

 suspending the alimentary canal 

 and dividing the body cavity longi- 

 tudinally, is often formed. In other 

 parts the partition walls between 

 the two sacks appearto be absorbed. 

 The amoeboid cells, which were derived from the invaginated 

 cells, arrange themselves as a layer round all the organs (fig. 249). 

 Some of them remain amoeboid, attach themselves to the skin, 

 and form part of the cutis ; and in these cells the calcareous spicula 

 of the larva and adult are formed. Others form the musculature of the 

 larval alimentary tract, while the remainder give rise to the muscula- 

 ture and connective tissue of the adult. 



The development of the vascular system is not known, but the discovery 

 of Kowalevsky, confirmed by Selenka, that from the walls of the water- 

 vascular system corpuscles are developed, identical with those in the blood 

 vessels, indicates that it probably develops in connection with the water- 

 vascular system. The observations of Hoffmann and Perrier on the com- 

 munication of the two systems in the Echinoidea point to the same con- 

 clusion. Though nothing very definite is known with reference to the 

 development of the nervous system, Metschnikoff suggests that it develops 

 in connection with the thickened bands of epiblast which are formed by 

 :i metamorphosis of the ciliated bands of the embryo, and accompany the 

 five radial tubes (vide p. 462). In auy case its condition in the adult 

 leaves no doubt of its being a derivative of the epiblast. 



From the above description the following general conclusions may 

 be drawn : 



(1) The blastosphere stage is followed by a gastrula stage. 



