OPHIUROIDEA. 



401 



(fig. 276, c), which are homologous with the "oral plates" of the 

 Crinoids. In the majority of the Brittle-stars one (or more) of the 

 oral plates is enlarged, and represents in function the " madreporite " 

 of the Star-fishes. 



The arms are, typically, protected by four rows of calcareous 

 plates, one dorsal, one ventral, and two lateral (fig. 277). The 

 lateral, or " adambulacral," plates carry rows of spines, and not only 

 cover the sides of the arm, but also encroach upon the inferior sur- 

 face. The ventral shields (" superambulacral plates ") are so related 

 to the lateral plates, that a pair of pores is u 



formed on each side of each of the former, 

 by means of which the tube-feet gain the 

 exterior. In the Euryalids, the arms are 

 covered with a leathery skin, containing 

 minute granules and scales. 



In addition to the proper integumentary 

 skeleton, the Ophiuroids possess an internal 

 or ambulacral skeleton, consisting of a linear 

 series of large calcareous discs or " vertebral 

 ossicles," which occupy the greater part of 

 the interior of each arm, and are grooved 

 inferiorly for the reception of the ambulacral 

 vessel and radial nerve (fig. 277). These 

 discs correspond with the "ambulacral ossi- 

 cles " of the Star-fishes, but the ossicles of 

 each pair are anchylosed with one another. 

 Successive vertebral discs are movably arti- 

 culated with one another, and the entire 

 series is largely supplied with muscles. The 

 first two pairs of ambulacral ossicles in each series have their lateral 

 elements disjunct, instead of being fused in the middle line, the 

 pieces of the first pair taking part in the formation of the calcified 

 peristome, and thus becoming connected with the armature of the 

 mouth. 



The ambulacral system of the Ophiuroids is constructed upon 

 essentially the same type as in the Asteroids and Echinoids, but its 

 place as a locomotive apparatus is taken by the arms, the tube-feet 

 being tentacle-like, without terminal suckers, and with no specially 

 developed " ampullae." The special peculiarity of the ambulacral 

 system in the Ophiuroids, as compared with that of the Asteroids, is 

 that the grooves on the under side of the row of ambulacral ossicles, 

 in which lie the radiating ambulacral vessels, are not open, as they 

 are in the latter, but are closed in by the passage over them of the 

 integument. Another peculiar feature, in all except the extinct 

 Protophiurida, is the position of the " madreporite " on the inferior 



vol. 1. 2 c 



Fig. 277. — Diagram of the 

 cross-section of the arm of an 

 Ophiuroid (slightly altered 

 from Sladen). ao, Ambulacral 

 ossicle, immovably united with 

 the corresponding ossicle of the 

 opposite side ; u, Superior 

 plate of the arm ; s, Lateral 

 plate ; /, Inferior plate ; n, 

 Radial nerve-cord ; b, Radiat- 

 ing blood-vessel ; w, Radiating 

 ambulacral vessel ; /, Tube- 

 foot ; a, Ampulla. 



