ECHINODERMATA 403 



Holothurians. There also the mesenchyma cells separate from the apex 

 of the archenteron. To be sure, according to Selenka, only two cells — 

 namely, the primitive mesenchyma cells— arise in this way. Metschm- 

 KOFF declares that his search for such a stage with two cells has always 

 been in vain. His observations allow him to reject even in the case of the 

 Holothurians the idea of the origin of the mesenchyma from two primi- 

 tive cells, and to assume instead a continuous emigration of cells from the 

 entoderm, especially since certain observations of Selenka seem to him 

 to corroborate his own view. For Selenka found larvae in which the free 

 end of the archenteron was irregularly outlined or covered with stellate 

 cells. Selenka explains this phenomenon as being pathological, whereas 

 Metschnikoff considers it warrantable to look upon such larvas as indi- 

 viduals in which an emigration of numerous entoderm cells is now taking 

 place. 



The mouth-opening and the stomodaeam in Asterina arise 

 in the form of a hollow plug, which is invaginated at the 

 front end of the ventral side of the embryo, and fuses with 

 the entoderm at that point (Ludwig). At this stage of 

 development the embryo abandons the egg-membrane, and, 

 as an approximately pyriform larva, swims about free in the 

 water by means of cilia, which cover its entire external 

 surface. 



Ophiuroidea. — Cleavage appears to take place in the 

 same way as in the Asteroidea (Ludwig, Selenka). The 

 blastula, which also exists here, exhibits a thickening at its 

 vegetative pole. The mesenchyma is formed within it in 

 the same way as in the Echinoidea ; according to Selenka, 

 it originates from the two primitive cells, but according to 

 Metschnikoff from a continuous emigration of cells from 

 the thickened wall. 



Not much weight should be given to the view, advocated by Fewkes, 

 that there is a bilaterally symmetrical arrangement of the mesenchyma 

 in OphiophoHs, especially since the author himself did not find the 

 mesenchyma bands in the sea-urchin {Echinarachinus parma) studied by 

 him (No. 13). 



Crinoidea. — Nothing is known of the earliest develop- 

 mental processes of the Crinoidea, except the statements 

 concerning Antedon. In this form, too, a blastula is deve- 

 loped after equal cleavage, and the gastrula is formed by 

 invas^ination. The formation of the mesenchyma takes 



