OPIIIUROIDEA. 399 



gonia, the recent genus Solaster, comprising the familiar " Sun-stars," 

 is represented by a single known Jurassic species, and is easily re- 

 cognised by its wide disc and short arms, and by the fact that the 

 dorsal integument is studded at intervals with prominent " paxilke." 

 Another still living genus, which is known to have existed in rocks 

 as old as the Lias, is Asterias (or Uraster of many authors). This 

 familiar genus belongs to the group of Star-fishes in which the am- 

 bulacral tube-feet are in four rows, and is the only form of this 

 group which has hitherto been found fossil. The disc is small, and 

 the arms are long, and are variable in number, while the dorsal 

 integument is hardened by netted ossicles, many of which are 

 developed into projecting tubercles or blunt spines. The extinct 

 genera of Mesozoic Star -fishes, such as the Jurassic Plumaster 

 and Tropidaster, do not exhibit any special peculiarities demanding 

 notice here. 



Class III. Ophiuroidea. 



The class of the Ophiuroidea comprises the small but familiar 

 group of the " Brittle-stars " and " Sand-stars,''" and is characterised 

 by the fact that the body is stellate, consisting of a central " disc" in 

 which the viscera are contained, and of elo?igated " arms," which are 

 sharply separated from the disc, do not co?itain prolongations of the 

 alimentary canal, and a?'e not furnished inferiorly with open ambu- 

 lacral grooves. 



The body in the Ophiuroids always has the form of a rounded 

 or pentagonal disc (fig. 276), and carries long slender arms, which 

 are typically simple, though they are branched in many of the Eury- 

 alids, and which are essentially employed as locomotive and prehen- 

 sile organs. The arms do not contain diverticula from the alimentary 

 canal, but they lodge the radiating ambulacral vessels and nerve- 

 cords, these structures not being situated in open "ambulacral 

 grooves,'"' as in the Asteroids, but being covered in by the coriace- 

 ous or plated integument. On the under side of the body, in the 

 centre of the disc, is seen the stellate opening of the mouth (fig. 

 276, c) ; and the reproductive organs open also on the under side 

 by ten fissures or slits, a pair of these being situated at the base of 

 each of the five arms. Owing to the absence of an anus, there is 

 no aperture on the upper surface of the body. 



With regard to the integumentary skeleton of the Brittle-stars, it 

 is impossible to enter here into the interesting features shown in the 

 embryonic condition of certain Ophiuroids, at which stage it can be 

 shown that the exoskeleton of the dorsal surface is in many respects 

 homologous with the apical disc of the Echinoids and the calyx of 

 the Crinoids. In the adult condition, the integumentary skeleton 

 of the Ophiuroids is of a very complicated character ; but the im- 



