3274 



ECHINODERMS 



is the arms themselves, and not the tube feet that are used for locomotion. The 

 tube feet accordingly have no terminal suckers, but are very sensitive to touch, and 

 probably assist respiration. The greater part of each arm is formed by a central 

 axis of successive calcareous segments, not unlike the vertebrae of a backbone. 

 Each arm ossicle or vertebra is, however, composed of two parts, one on either 

 side and united in the middle line; the successive ossicles being connected by pairs 

 of strong muscular bundles, and articulating with one another by tenon- and-mortice 

 joints. According to the degree of development of these joints, the arms have varied 

 powers of coiling. Thus, in the Cladophiurce , the ossicles have more or less saddle- 

 shaped faces, so that the arms can be twisted round foreign objects or rolled toward 

 the mouth; in the Streptophiurce, the faces of the ossicles have slight pits and 

 processes, but none sufficient to prevent the ossicles being so twisted on their 

 neighbors that the arms may be rolled up toward the mouth; in the Zygophiurce 

 the faces of the arm ossicles have articulating knobs and pits, which prevent 

 the arms from being rolled up toward the mouth. These vertebral arm ossicles 

 are incased 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 Ophiothrix, 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 bursa; 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 

 entering 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 



COMMON BRITTLE STAR. 

 (Two-thirds natural size.) 



