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 

 then rises along the side of the cup to 

 the anus ; this system, then, is not 

 affected by radiate symmetry (see 



flnnr 



nooi 



l 2 



FIG. IX. 



Ambulacrals. 1, ventral view of 

 two brachials of Gissocrinus sqiiami- 

 fer, with c.p closed above and re- 17 ,. -ITTT 

 moved below, x 8 diam. 2, side view r Ig. Vll. p. 9). 

 of brachials of Antedon basicu-cva, mi Pr >itViplin1 nprvpe nn 



with c.p open, exposing tentacles (0 ; me epttneiiai nerves on 



c.p are supported on side-plates or Q f tne food-grOOVCS also paSS to the 

 adambulacrals (s.p), between which , . ^ i i- i 



are seen notches for saccuii (p. 137), 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 (hydrocircus) ; 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 



