374 



ZOOLOGY 



SECT. 



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

 ossicle, the centro-dorsal ossicle, 1 (Fig. 304, C, D) 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 view 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 (H. 1 ) ossicles, 

 also concealed by the centro-dorsal ossicles: with each of these 

 articulates a second radial (-R. 2 ), which is visible beyond the centro- 

 dorsal. With each of the second radials articulate two third 



radials (-ft. 3 ), each 

 forming the base of 

 the corresponding 

 arm -branch. 



The ossicles of 

 the arms braehials 

 (r. l , t r. z ) are ar- 

 ranged 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 dorsal sur- 

 faces, longitudinally 

 grooved ventrally, 

 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 con- 

 nected ossicles, and each grooved along its ventral surface. 



The coelome contains numerous strands of connective tissue 

 which serve to suspend the various organs. 



Extending through the arms and pinnules between the sup- 

 porting ossicles and the ambulacral grooves are. three canals which 

 are prolongations of the coelome (Fig. 303, ccel. can.). Two of these 

 the sub-tentacular canals form a pair separated from one another 

 by a median septum underlying the ambulacral groove. The 



1 This has nothing to do with the dorso-central ossicle referred to above 

 (p. 362) as one of the primary apical plates of the Starfish : the dorso-central 

 is not represented in the Feather-star. 



FIG. 302. Antedon, ventral (upper) surface of the central 

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



