IX 



PHYLUM ECHINODERMATA 



385 



represented only by the five angles ; while between these two 

 extremes there are numerous intermediate gradations. The 

 Brisingida: 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 dorsal or aboral, and the ventral or oral surfaces are always 

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

 latter is the mouth, running out from which are five or more 

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

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

 quite in, the middle point of the dorsal surface is the anal 

 aperture, absent in a few 

 instances; and on the 

 same surface, nearer the 

 margin, between the two 

 rays of the bivmm in the 

 five-rayed Starfishes, is 

 the madreporite, a finely 

 grooved calcareous plate 

 perforated by a number of 

 minute apertures. In some 

 fossil Starfishes it is situ- 

 ated on the ventral sur- 

 face. Sometimes instead 

 of one madreporite there 

 are several. 



The wall of the body 

 in the Starfishes contains 

 a number of calcareous 

 ossicles, movably articu- 

 lated together and con- 

 nected 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 con- 

 fined 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 the ossicles of the dorsal surface 

 VOL. i c c 



FIG. 303. Anthenea. View of ventral surface. 

 (After Sladen.) 



