viii ECHINODERMATA MORPHOLOGY OF SKELETON 383 



can sometimes be distinguished. The covering plates, which are rarely retained 

 complete, are occasionally continued on to the food grooves of the ambulacra, where 

 they are arranged in two longitudinal rows. Perhaps they could be raised and 

 depressed ; if not, it is difficult to see how the food groves with their lateral furrows 

 could function : the lateral furrows would then have to pass under the covering 

 plates so as to be in communication with the principal groove. In rare cases the 

 covering plates even spread sideways over the spiracles. 



2. Codaster (Fig. 265, p. 314). The arrangements here differ considerably from 

 those of Pentremites, just described. The food grooves are deeply sunk into the 

 lancet plates, which are hollowed out on each side for the reception of the side plates. 

 Spiracles are wanting. A certain number of the hydrospire clefts which run parallel 

 to the ambulacrum always appear at the surface of the calyx, laterally to the 

 ambulacrum (Fig. 333). These clefts run at right angles across the suture between 



FIG. 333. Transverse section through an ambu- 

 lacrum of Codaster (after Etheridge and Carpenter), 



diagram. 1, Deltoid plate or possibly a radial ; 2, anibu- 

 lacral canal ; 3, food groove ; 4, lancet plate ; 5, side plate ; 

 ti, apertures of the hydrospire pouches ; 7, lower lancet 

 plate ; 8, hydrospire pouches. 



FIG. 334. Eleutherocrinus Cas- 

 sedayi, from the oral side (after Ethe- 

 ridge and Carpenter), aa-lb, Axis, 

 passing through the mouth and anus ; 

 r, radials ; i/ 1 , interradials ; TI, the 

 radial of the differently shaped am- 

 bulacrum ; z, y, the two larger basals. 



the radial and the deltoid plates. One or more hydrospire clefts may be covered by 

 the side plates of the ambulacra. At those sides of the two posterior ambulacra 

 which are turned towards the anus, the clefts are altogether wanting. 



3. In Orophocrinus (type 0. stdliformis, Fig. 266, p. 314) there are no hydro- 

 spire pores on the ambulacra, but depressions occur between the consecutive side 

 plates for the reception of the bases of the pinnules. The hydrospire clefts lie quite 

 hidden below the surface within the ambulacral sinus, covered by the lower lancet 

 plate. The spiracles, on the contrary, which are ten in number, appear at the sides 

 of the ambulacra as long wavy slits. The two spiracles of the posterior interradius 

 are distinct from the anus. The ambulacra, round the mouth at least, are covered 

 by covering plates. 



4. The Irregulares (Astrocrinus and Eleutherocrinus) are chiefly distinguished 

 by the fact that one of the four ambulacra is developed quite differently from the 

 others (Figs. 334, and 267, p. 315). 



