BRITTLE-STARS 455 



Class II.— BRITTLE-STARS (Ophiuroidea) 



The members of this class, though to some extent they 

 resemble ordinary star-fishes, and are by some naturalists included 

 with them in a common section of the phylum, differ in a number 

 of important particulars. There are a number of British species, 

 of which one of the commonest [Amphiura squamata) is a little 

 creature found on the under sides of stones near low-tide mark 

 (fig. 279). It is at once evident that the five arms are sharply 

 marked off from the central disc. 

 They are indeed more of the nature 

 of appendages, and are here the 

 organs of locomotion, for which their 

 extreme flexibility eminently fits 

 them. Upon the under side of the 

 disc, as in an ordinary star-fish, is 

 situated the mouth in the form of a 

 five- rayed slit. There are no ambu- 



^ ^ . ^ J ■ 1 r ^^S* °79' — Brittle-Star {A^nphiura sgitaviata), 



lacral grooves, and the under side 01 seen from above. Enlarged 



each arm is covered by a regular 



series of flat plates, at the sides of which the tube-feet project. 

 They are not, however, used as feet, but serve as organs of 

 touch and respiration. There are no eye-spots on the tips of 

 the arms. Turning to the upper surface, no trace of a madreporite 

 can be seen; it is represented by one of the plates situated inter- 

 radially on the under side. There is no intestinal opening at all. 

 Each arm is covered by a single series of plates, and a similar 

 series runs along each of its sides. The margins of the arms 

 are spiny, but there are no pedicellariae. 



There are important differences as to internal structure between 

 a brittle-star and an ordinary star-fish. The mouth, for example, 

 is armed by a number of modified plates, and it leads into a 

 spacious stomach, which bulges out into ten very short pouches. 

 But there is no intestine and no digestive structures in the arms. 

 Indeed each of these is traversed by a special series of ossicles, 

 which have been called vertebral, because they are jointed together 

 something like the successive vertebrae making up a backbone. 

 Each of them is formed by the fusion of two ossicles, which are 

 equivalent to a pair of the ambulacral ossicles of an ordinary 



