INVERTEBRATE ANIMALS. 49 



Sub-Class ASTEROIDA. Star-Fishes. 



Order OPHIUEOIDEA. ocf^ir, snake ; oiipa, tail ; 

 sl^of, form. 



Group 136.— Family OPHIURID^. Part I. Sand-stars. 

 In the Opliiurida3 the number of arms is always five ; 

 the arms do not, as in the true Star-Fishes, contain 

 extensions of the body cavity, but are simply locomotive 

 appendages. Estimate of species : recent, 80 ; British, 

 2 ; fossil, 9. 



^ Examples of Ophioderma, Ophiura, Ophlolepis, 

 and other genera. 



Group 137.— Family OPHIUKID^. Part II. Brittle-stars. 

 The Brittle-stars are much more active than the Sand- 

 stars, and have a tendency to break themselves up into 

 little pieces when captured. They die instantaneously 

 when placed in fresh water. The OpMuridiB inhabit 

 almost all seas, and are found occasionally at great 

 depths. Estimated number of species ; — recent, 65 ; 

 British, 11 ; fossil, 11. Silurian and upwards. The 

 total number of species of Ophiuride, in a list by 

 Theodore Lyman, of Harvard College, U.S., is 164. 



IT Examples of Ophiocoma, Ophiothrix, Ophiacan- 

 tlia, OpMomyxa, and other genera. 



Group 138.— Family ASTEROPHYDE^ (Dujardin). This 

 group includes species with arms varying from perfectly 

 simple stems to the highly branched appendages of 

 Asterophyton, one species of which, known as the 

 Shetland Argus, when curled up somewhat resembles 



