398 



ZOOLOGY 



SECT. 



and in the centre of this surface is an opening, the mouth (Fig. 

 337, mo). From the mouth five very narrow ciliated grooves, the 

 ambulacra! or food-grooves, radiate outwards towards the bases 

 of the arms, near which they bifurcate, so that ten grooves are 

 formed, one passing along the oral surface of each of the ten 

 arm-branches to its extremity. The anal opening (an) is likewise 

 on the oral surface, being situated on a papilliform elevation in 

 the interspace between two of the radiating canals. 



The aboral side of the disc is occupied by a large, flat, pentagonal 

 ossicle, the centro-dorsal ossicle (Fig. 336, c ; and Fig. 339, CD), 

 bearing on its outer surface a number of little cup-like depressions, 

 with which the bases of the cirri are connected. The cirri (cirr) 

 consist each of a row of slender ossicles, covered, like all the rest 

 of the animal, with epidermis, and connected together by means 



of muscular fibres. 

 Concealed from v'ew 

 by the centro-dorsal 

 ossicle is a thin plate 

 termed the " rosette " 

 (ros), formed by the 

 coalescence of the 

 basals of the larva. 

 At the sides are five 

 first radial ossicles 

 (R l ), also partly con- 

 cealed by the centro- 

 dorsal ossicle : with 

 each of these articu- 

 lates a second radial 

 (R 2 ), which is visible 

 beyond the centro- 

 dorsal. With each of 

 the second radials arti- 

 culate two third radials 

 (7s! 3 ), each forming the base of the corresponding arm-branch. 



The ossicles of the arms bracliials (Br. 1 , Br. 2 ) are arranged 

 in a single row in each arm. They are somewhat elongated in 

 the direction of the long axis of the arm, strongly convex on their 

 aboral surfaces, longitudinally grooved on the oral surface, and 

 connected together by the investing epidermis and by bundles of 

 muscular fibres, by the contractions of which the movements of 

 the arms are brought about. Fringing the sides of each arm are 

 two rows of side-branches, or pinnules, each supported by its row 

 of connected ossicles, and each grooved along its oral surface. 



The coelome contains numerous strands of connective-tissue 

 which serve to suspend the various organs. 



Extending through the arms and pinnules between the supporting 

 ossicles and the ambulacral grooves are three canals, ciliated in 



FIG. 337. Antedon, oral (upper) surface of the central 

 disc. an. anus ; mo. mouth. (From Vogt and Jung.) 



