102 



THE CRINOIDEA 



called " ambulacrals " (Amb), which can open or close as occasion 

 demands (Fig. IX.). 



We may now trace the various 

 extensions of the Body Systems into 

 the Thecal Cavity. 



The food -grooves and associated 

 structures, except the axial cord, pass 

 over the tegmen to the mouth, into 

 which the food -grooves drive their 

 streams of water. The mouth leads 

 into a gut, which makes a dextral coil 

 down to the bottom of the cup, and 

 Pio IX then rises along the side of the cup to 



Ambulacrals ' 1, ventral view of the anUS '> this system, then, is DOt 



two brachiais of Giaomnvs sqnami- affected by radiate symmetry (see 



fer, with c.p closed above and re- T ,. _ 7TT nx * \ 



moved below, x 8 diain. 2, side view r Ig. V 11. p. y ). 



The epithelial nerves on the floor 

 of the food -grooves also pass to the 

 are'seen'"notche v s'for isaccuiiVisf),' mouth, where they join an epithelial 



ridge encircling the mouth ; from 



this "oral ring," nerves pass to the walls of the gut. The 

 paired subtentacular nerves run down to a subepithelial, 

 " circumoesophageal nerve-ring," below the oral nerve-ring. From 

 this ring proceeds, in each interradius, a pair of nerves which 

 innervate the tegmen and the mesenteries of the body-cavity. This 

 nerve-system is connected with the aboral nerve-system in a manner 

 explained below. 



The radial pseudhaemal canals join a " pseudhaemal ring " 

 round the oesophagus beneath the oral nerve-ring ; these structures 

 are hard to distinguish, and even in other classes, where they are 

 better developed, their origin is not yet clear. There is, however, 

 surrounding the oesophagus a " lacunar plexus " belonging to what 

 is generally called the blood-vascular system. The circumoeso- 

 phageal ring is connected with two vascular trunks leading from 

 the plexus that surrounds the intestine and that absorbs nutrient 

 substances therefrom ; these substances appear to be worked up 

 into corpuscles by a "spongy organ" in the oesophageal ring. The 

 ring is also connected with a plexus that passes down the vertical 

 axis of the theca, through the coil of the gut, to the base ; this 

 surrounds the " axial organ " (vide infra). 



The water- vessels (perradial ambulacral canals) meet in a 

 circumoesophageal water -vascular ring (hydrodrcus) ; these struc- 

 tures have longitudinal muscle-bands, as well as muscle-fibres 

 traversing the lumen ; no ampullae or valves are differentiated, 

 as they are in forms where this system has a locomotor function. 

 In so simple a crinoid as is here in question, there is good reason to 



