CRINOIDEA. 



271 



calyx-cover between the ambulacral grooves are called the inter- 

 ambulacral regions, and in one of these the so-called posterior- 

 is situated a papilla, at the end of which is the anus (Fig. 188). 

 The stalk carries whorls of jointed appendages, the cirri ; the seg- 

 ments to which such whorls are attached being called nodal, and 

 the intervening segments internodal. In the unattached forms, 

 the apex of the calyx 

 where the stalk 

 would be attached, 

 if present, carries 

 more than one whorl 

 of similar jointed 

 cirri. The ambula- 

 cral grooves are con- 

 tinued from the 

 calyx-cover along 

 the whole length of 

 the arms and their 

 branches (excepting 

 in a few cases, e.g. 

 some of the posterior 

 arms of Actinometra 

 and some of the 

 proximal pinnules of 

 Antedon), and tube- 

 feet project from 

 their sides. As in 

 Ophiurids the tube- 

 feet are without 

 ampullae and are 

 not used as locomo- 

 tive organs. The 

 arms and pinnules contain prolongations of all the most im- 

 portant organs of the calyx, excepting the alimentary canal, 

 which as in Ophiurids is confined to the disc. The mouth is 

 surrounded by small tentacles, the cavities of which open 

 directly into the circumoral vessel of the water- vascular system. 

 Orientation and numbering of the rays. As stated above the 

 interradius in which the anus is placed is called posterior. If 

 the animal be drawn from the ventral surface with the anal in- 



FIG. 187. Pentacrinus caput medusae (from Clans, at'tc: 

 J. Miiller). O mouth ; A anus. The lower figure is the 

 tegmen calycis, the arms heiug cut off. 



