410 



ZOOLOGY 



SECT. 



only by the five angles ; while between these two extremes there 

 are numerous intermediate gradations. The BrisingidcB differ 

 from all the rest of the class in having the arms almost as sharply 

 separated off from the central disc as in the Ophiuroids. 



The abactinal or aboral and the actinal or oral surfaces are always 

 distinctly marked off from one another. In the middle of the 

 latter (Fig. 340) is the mouth, running out from which are five or 

 more narrow ambulacra! grooves, one of which is continued along 

 the oral surface of each arm to its extremity. Near to, but not 

 quite in, the middle point of the aboral surface is the anal aperture, 

 absent in a few instances ; and on the same surface, nearer the 



margin, between the two 

 rays of the bivium in the 

 five-rayed Starfishes, is the 

 madreporite, a finely grooved 

 calcareous plate perforated 

 by a number of minute aper- 

 tures. In some fossil Star- 

 fishes it is situated on the 

 oral surface. Sometimes in- 

 stead of one madreporite 

 there are several. 



The wall of the body in 

 the Starfishes contains a 

 number of calcareous ossicles, 

 movably articulated together 

 and connected by bands of 

 muscle, so that, though the 

 body is firm, and in the dried 

 condition often quite rigid, the 

 arms are capable during life of 

 slow movements of flexion 

 and extension, enabling the 

 animal to creep through comparatively small fissures and crannies. 

 A special system of ossicles the ambulacral ossicles are arranged 

 in a double row along each ambulacral groove, the ossicles of the 

 two rows articulating movably with one another at the apex of the 

 groove. At the end of the arm the two rows of ambulacral ossicles 

 end in a terminal ossicle which supports the unpaired tentacle. 

 Spines are invariably present, but are sometimes confined to the 

 margins of the ambulacral grooves, in which position they are 

 movably articulated with the underlying ossicles. Tubercles take 

 the place of spines over most of the surface in many forms. In 

 Astropecten and other Paxillosa the ossicles of the aboral surface 

 take the special form to which the term paxillo3 is applied. Each 

 paxilla is a plate which is produced into a short rod, divided at its 

 extremity into a number of radiating processes. 



340. Anthenea. View of oral surface. 

 (After Sladen.) 



