BRITTLE-STARS. 



307 



objects or rolled towards the mouth ; in the Streptophiwrce, the faces of the ossicles 

 have slight pits and processes, but none sufficient to prevent the ossicles being so 

 twisted on their neighbours that the arms may be rolled up towards the mouth ; in 

 the Zygophiurce, the faces of the arm-ossicles have articulating knobs and pits, 

 which prevent the arms from being rolled up towards the mouth. These vertebral 

 arm-ossicles are encased in the tough outer skin of the arm, in which are developed 

 granules, plates, and spines, which are least definite and regular in the Cladophiurce, 

 most definite in the Zygophiurce. The spines, which are clearly shown in the 

 annexed figure of Opliioihrix, are borne on the side-plates of the arm, and aid the 

 animal in locomotion. The integument of the disc also bears plates or scales of 

 various sizes, often more or less covered 

 with granules and minute spines. The 

 precise arrangement of the plates on the 

 top of the disc varies in different species ; 

 but five pairs of plates, known as the radial 

 shields, are always present at the base of 

 the arms, and are shown in the annexed 

 figure. On either side of the arms where 

 they join the disc, there is seen on the under 

 surface a slit-like opening. These openings, 

 known as the genital slits or clefts, are 

 usually single but sometimes double ; they 

 lead into thin- walled pouches or bursse at 

 the sides of the rays. In a living ophiurid, 

 the disc alternately expands and contracts, 

 and thus water is pumped into and out of 

 the pouches, through the slits. The enter- 

 ing water brings oxygen, which it ex- 

 changes, through the thin walls of the 

 pouch, for the carbonic acid contained in 

 the water of the body -cavity, and then 

 goes out again by the return current. 

 Hence the pouches are called respiratory 



bursse. But they have another function, since the ovaries enter into them, and the 

 ripe ova may either be carried out by the current through the slits, or they may 

 remain and undergo direct development in the pouches themselveSc Around the 

 mouth are a number of short flat processes, or papillae, serving as strainers, and 

 keeping foreign bodies that are not wanted for food from entering the stomach. 

 Round the mouth are also twenty tentacles, which are really the modified tube-feet 

 of the two first arm-segments of each arm. They are in a state of continual move- 

 ment, assisting the food to enter, and clearing away the undigested residue, which 

 is ejected from the mouth. 



The branched ophiurid s, or Cladophiurce, are sedentary, attaching themselves 

 by coiling their branching arms around corals and suchlike animals ; but can move 

 when they please. The same mode of life is also affected by a few of the simpler 

 forms ; but, as a rule, ophiurids have considerable powers of locomotion, of which 



COMMON BRITTLE-STAR ( nat. size). 



