130 



LIFE BY THE SEASHORE. 



twenty times that attained by the common starfish. The 

 arms, or rays, are very long and slender, so slender that 

 there is no difficulty in realising that they do not, like 

 those of the starfish, contain prolongations of the digestive 

 organs, which here, like the reproductive organs, are con- 

 fined to the disc. The tube-feet are also reduced, are no 

 longer placed in an open ambulacral groove, are not used 

 in locomotion, and are small and tentacle-like in appearance. 

 Between the rays there are peculiar bursse (b in Fig. 43), or 



pockets, which open to 

 the exterior by deep 

 slits placed at the sides 

 jof the rays. These are 

 breathing organs, and 

 are very characteristic 

 of brittle -stars. 



This general survey 

 of a brittle-star should 

 make the main points of 

 contrast with starfish 

 clear, but the details of 

 structure are difficult ; 

 we can only indicate the 

 more important points. 



FIG. 43. Diagrammatic view of the under First, as to the ravs : 

 surface of the disc in the sand-star , , -i i 



(Ophiura). Of the five rays three are cut among the dried Wreck- 



short close to the disc, the other two just OCTP npnr Vn'rrh rirlp mirL- 



beyond its limits. In the centre is the star- a 6 near HlgH-tlde mark, 



like mouth; the tiiangular plates which yOU Will always find 



project into it are fringed by mouth- olrolafnTn'onrl V.ffl of^-r, 



papillae. The presence of teeth is also Skeletonised brittle-stars 



indicated. ft, one of the bursal slits ; ms, which will show that 



one of the mouth-shields; s. arm-spines. ,1 , 



the arms are made up 



of a series of segments or vertebrae, jointed together. Out- 

 side these segments there are a series of plates, one of 

 which is placed on the dorsal surface of each segment, one 

 on the ventral, and two at the sides. These last, the lateral, 

 plates bear spines. The delicate tube-feet emerge at the 

 side of the rays, and there are one or two little plates, 

 called the tentacle scales, at the point of exit. In regard 

 to the disc the dorsal surface is variously marked in the 

 different species, but there are usually two distinct plates, 

 called radial shields, at the origin of each ray. On the 



