ECHINODERMATA : ASTEROIDEA. 205 



members of the order. In the common Star-fish (Uraster 

 rubens) the disc is small, and is furnished with long, finger-like 

 rays, usually five in number (fig. 97). In the Cribella the 

 general shape of the body is very much the same. In the 

 Solasters the disc is large and well marked, and the rays are 

 from twelve to fifteen in number, and are narrow and short 

 (about half the length of the diameter of the body). In the 

 Goniasters (fig. 98) the body is in the form of a pentagonal 

 disc, flattened on both sides ; the true " disc " and rays being 

 only visible on the under surface of the body. In the singular 

 genus Brisinga, we have in some respects a transitional form 

 between the Asteroids and Ophiuroids, the arms being much 

 longer and more slender than is the case in the typical Aster- 

 oids, at the same time that they are much thicker and softer 

 than is the case amongst the latter. In none of the true Star- 

 fishes, however, are the arms ever sharply separated from the 

 disc, as in the Ophiuroidea, but they are always an immediate 

 continuation of it. 



The principal groups of Asteroidea are the following : 



Family \. Asteriadae or Asterocanthiidce. Four rows of ambulacral 



feet. 

 Fam. 2. Astropectinidce. Two rows of ambulacral feet ; back flattish, 



netted with tubercles, which carry radiating spines at the tip 



("paxillae"). 

 Fam. 3. Oreastridce. Two rows of ambulacral feet ; skin granular, 



pierced by minute pores. 

 Fam. 4. Asterinidce. Two rows of ambulacral feet ; body discoidal 



or pyramidal, sharp-edged ; skeleton of imbricate plates ; dorsal 



wart single, rarely double. 

 Fam. 5. Brisingida. Arms long and rounded, sharply marked off 



from the disc. Ambulacral grooves not reaching the mouth ; two 



rows of ambulacral feet. 



ORDER OPHIUROIDEA. Body stellate, consisting of a central 

 "disc" in which the viscera are contained, and of elongated 

 "arms" which are sharply separated from the disc, solid, not 

 containing prolongations of the viscera, and not furnished inferi- 

 orly with ambulacral grooves. Larva generally pluteiform, with 

 a skeleton. 



This order comprises the small but familiar group of the 

 "Brittle-stars" and Sand-stars," often considered as belong- 

 ing to the Asteroidea, to which they are nearly allied. The 

 body in the Ophiuroidea (fig. 100) is discoidal, and is covered 

 with granules, spines, or scales, but pedicellariae are wanting. 

 From the body which contains all the viscera proceed long 

 slender arms, which may be simple or branched, but which do 

 not contain any prolongations from the stomach, nor have 



