176 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



calcified body, which occupied the vertical axis of the body cavity, and was 

 contracted into a narrow tube toward the base (Fig. 280). 



In all of the Recent Crinoids (excepting in certain species belonging to the 

 family Coraasteridae where they may be quite absent from the arms arising from 

 one, two, or even three of the posterior rays) each arm, and each pinnule 

 which it bears, carries on its ventral surface an open ambulacral fui'row lined 

 with ciliated epithelium. At the base of the arms these ambulacral furrows 

 unite and form five large furrows, which, traversing the tegmen, converge at 

 the central mouth, or, as in the Comasteridae, lead to a horse-shoe shaped 

 furrow just within the border of the tegmen in the centre of which is the 

 mouth. Just below the floor of these furrows runs a nerve band (absent from 

 ungrooved arms), and under this the genital rachis (especially developed in 

 ungrooved arras), and the canals of the water and blood vascular systems ; 

 below these, deeply buried within the substance of the calcareous plates, lies 

 the large nerve cord of the dorsal nervous system. All but the last of these, 

 which leads to a central nervous mass in the dorsal apex of the calyx, run to 



a in 



Fig. 271. 



.. 



Isocrimia asteriu (Linn.). Ventral 

 disk, protected by very thin peri- 

 somic ])lates, witli central nioutli (»), 

 exposed ambulacra, and eccentric 

 anus {A). 



.aJL 



Fig. 272. 



Hyocrinus hethelianuf: (Wyv. Tliom.). Re- 

 cent. Ventral disk, enlarged, o. Orals ; p, 

 Mouth (iieristome) ; s, Covering plates ; c, 

 Dorsal canals of the anus ; am, Ambulacra! 

 furrows of the arms ; an, Anus (after Wy ville 

 Thomson). 



ring-like structures surrounding the oesophagus. The circumoesophageal ring 

 canal of the water vascular system is in communication with the body cavity, 

 which in turn communicates with the exterior by means of very numerous 

 (five only in Rhizocrinus) interradial perforations. The margins of the 

 ambulacral grooves are bordered with a series of small lajipets, and at the 

 base of each of these is a group of three tentacles ; these tentacles, which are 

 absent from ungrooved arms, are connected with the canal of the water 

 vascular system ; they also secrete a more or less poisonous fluid which serves 

 to paralyze the small organisms which serve as food. 



In all Recent Crinoids five (occasionally four) open ambulacral furrows 

 lined with epithelium conduct from the mouth to the tips of the arms, 

 remaining either simple or subdividing as often as there are arms. Under- 

 neath the floor of the grooves runs an ambulacral vessel filled with water ; 

 and accompanying this are the blood and vascular canals and a nervous cord. 

 Distensible tentacles pass out from alternate sides of the ambulacra, and the 

 latter unite to form a circumoral ring canal. From the ring canal five short 

 open tubes {stone or water canals) extend downwards into the body cavity and 

 supply the ambulacral system with water. 



