504 THE ANATOMY OF INVERTEBRATED ANIMALS. 



eral diverticula enlarge, and apply themselves to the rest of 

 the archenteron, now become the intestine, from which they 

 are soon completely shut off, and converted into peritoneal 

 sacs. The left sac thus formed lies on the ventral side of the 

 intestine, the right sac on its dorsal side. The walls of the 

 two sacs become applied together, and form a circular mesen- 

 tery. The peritoneal sac of the aboral side sends a pro- 

 cess into the hinder end of the bodv, which has be^un to 

 elongate, in order to give rise to the stem of the Pentacrinoid 

 form. 



The third, or ventral, diverticulum is shut off from the 

 alimentary canal much later than the other two. It grows 

 round the mouth, and gives rise to the circular ambulacral 

 vessel, whence the tentacular canals are given off. 



Ten plates, each consisting of a calcareous network, and 

 arranged in two rows of five each, next appear in the sub- 

 stance of the Echinoposdium around the alimentary canal. 

 From the centre of the posterior row, eight calcareous rings 

 extend through the length of the body of the larva, inclosing 

 the backward prolongation of the aboral peritoneal sac ; and 

 the series terminates by a broad, discoidal network, which lies 

 on one side of the posterior end of the larva. This discoidal 

 plate is that which occupies the attached end of the stem of 

 the future Crinoid ; the rings become the stem, and the two 

 circles of plates the basal and oral ossicula of the calyx, re- 

 spectively. As the stem elongates, new rings (articuli) are 

 added at the junction of the stem with the calyx. 



The larva now fixes itself bv the discoidal end of its stalk, 

 which becomes relatively longer and narrower ; while the 

 part of the body which contains the basal and oral plates, and 

 is to be converted into the calvx, remains thick and short. 

 Its broad end becomes five-lobed, ench lobe answering to an 

 oral plate. These plates separate like the petals of a flower- 

 bud, and discover, in the centre, the wide, permanent oral 

 aperture. Between the margins of this and the oral plates, 

 tentaculiform pedicels, at first only five, but eventually ar- 

 ranged in groups of three, between every pair of oral plates, 

 make their appearance. 



The alimentary cavity is still a mere sac, without intestine 

 or anus. 



Five radial plates next appear in the wall of the calyx, be- 

 tween the basal and the oral plates, and alternating with both ; 

 and, in correspondence with them, the arms grow out as rap- 

 idly-elongating processes, in which the other radials are sue- 



