TYPE ECH1NODERMA. 547 



ated cups, consisting of slight depressions lined by columnar 

 cells each of which bears a long cilium. These cups are 

 especially abundant in the pinnules, and serve to create a 

 circulation of the coelomic fluid, which, as in other Echiuo- 

 derms, contains numerous amoeboid cells floating freely in it. 

 The water vascular system, or hydrocoel, consists of a ring 

 surrounding the mouth, and sending outwards five radial 

 canals (rtv) which lie below the ambulacral grooves and are 

 continued along the arms and pinnules. Occasionally sub- 

 ambulacral calcareous plates are developed in the connective 

 tissue below the radial canals, and in some fossil forms these 

 plates assume a regular arrangement in two rows. At regu- 

 lar intervals along the arms are situated the ambulacral ten- 

 tacles, which are tiugerlike outpouchiugs of the radial canals 

 destitute of terminal suckers and are arranged in groups of 

 three, the canals being somewhat enlarged in the region where 

 they occur, an indication perhaps of the ampullae found in 

 other groups ; in some forms the cavities of the tentacles 

 seem to be united with those of the canals only by exceed- 

 ingly small orifices, which may be closed, since the tentacles 

 in their greatest contraction always remain filled with fluid. 

 In the neighborhood of the mouth are a number of oral ten- 

 tacles (Fig. 251, T) arising directly from the oral ring, and 

 differing from the ambulacral tentacles in not being arranged 

 in groups of three. From the oral ring there also arise in 

 Antedon a number of ciliated tubes (sc) which open into the 

 coelomic cavity, each one corresponding to a stone-canal of 

 the other Echiuoderins. In Antedon there are as many as 

 thirty of these canals in each interradius. and in Pentacrinus 

 an even greater number occurs ; but in other forms they may 

 be fewer, Rhizocrinus, for example, possessing onty five in all, 

 one being situated in each interradius. In the larva of Aiif<'- 

 don there is at an early stage only one, communicating with a 

 portion of one of the primary coelomic cavities, which on its 

 part opens to the exterior by a pore, an arrangement which 

 may be regarded as typical for the Echiuoderins. Later, 

 however, this portion of the coelomic cavity degenerates, and 

 the canal then opens directly into the general ccelom, and 

 this communicates with the exterior by the pore. In subse- 



