292 PHYLUM ECHINODERMATA. 



and the left posterior coelom, but it continues its movement, 

 carrying with it the left coelom and hydrocoel, until it comes 

 to lie beside the anus at the posterior end (see account of develop- 

 ment). Having reached this point, far removed from the 

 preoral lobe, the hydrocoel ring closes. As a result of this move- 

 ment, which must be due to a torsion of the whole of the pos- 

 terior part of the body, the right posterior coelom has also shifted 

 and come to lie on the aboral (originally anterior) side of the gut, 

 and the preoral lobe of the larva becomes enclosed by the row 

 of skeletal elements (apical plates) which are developed outside 

 the right posterior coelom in all Echinoderms except Holo- 

 thurians. These plates as is well known are laid down at first in 

 the larva in an open curve, which later closes, as does the hydro- 

 coel, to form a complete ring. In Asteroids the closure of this 

 curved row of plates is effected far from the point of origin of 

 the preoral lobe, on the right or right dorsal (larval) side of the 

 body. In Crinoids it is effected at the anterior (larval) end of 

 the body and encloses the preoral lobe, just as the hydrocoel does 

 in Asteroids. If these considerations are sound, it follows that 

 the rejection of the hypothesis as to the homology between the 

 apical plates of Crinoids and those of other Echinoderm classes, 

 in so far as that rejection depends upon the difference in the 

 point of origin of the stalk in Asteroids and Crinoids, is not 

 justified. 



The fact that in Crinoids alone is the rudiment of the posterior 

 coeloms given off at the posterior end of the larva and separately 

 from that of the anterior becomes to a certain extent intelligible 

 in view of the foregoing considerations. This peculiarity must 

 be regarded as a back-thrust from the adult form, in which the 

 posterior end conies to hold all the viscera. Pursuing the same 

 line of thought the fact that the hydrocoel does not share in 

 this impress of adult arrangements and in like manner develop 

 from the posterior end of the enteron but comes off in front with 

 the anterior coelom is significant as showing that the connexion 

 in origin of the hydrocoel and anterior coelom is a fundamental 

 feature of Echinoderm morphology.* 



The fact that the left posterior coelom does not at an early stage 

 come to exceed the right in size is another feature of the larva of 



* This holds even in Ophiurids, see p. 151, and MacBride, Q.J.M.S. 

 51, 1907, p. 557. 



