No. 19.] ECHINODERMS OF CONNECTICUT. 73 



mentioned above, although no part of the digestive or other sys- 

 tems extend into it. 



A similar groove on the oral side of the vertebrae contains 

 the radial canal of the water-vascular system, which is connected 

 with each of the tentacles, as with the tube-feet of the starfish. 

 Beneath the radial canal is the radial nerve cord and the peri- 

 haemal canal comparable to those of the starfish. 



The upper arm plates cover the extension of the body cavity, 

 just as the under arm plates cover the organs beneath the ver- 

 tebrae (Fig. 10). 



Viscera. — The visceral anatomy of the ophiuran is not essen- 

 tially different from that of the starfish, except that the digestive 

 and reproductive organs do not extend at all into the rays. The 

 simple digestive organs, mouth and saccular stomach, have been 

 described above. The water-vascular system consists, as in the 

 starfish, of a circumoral vessel and five radial canals, connected 

 with the tentacles of the arms. The madreporic canal is con- 

 nected with the exterior by one or more minute pores situated 

 on one of the oral shields. 



The nerve ring and its branches and the perihaemal vessels 

 have much the same situation as in the starfish, although the 

 nerves are more highly modified by the segregation of the nerve 

 cells into more definite centers, or ganglia. 



HABITS 



The ophiurans are usually nocturnal in habit, some species 

 hiding away in protected places during the daytime, others 

 remaining buried in the mud. When disturbed they hastily swim 

 or creep away. In case the irritation has been too severe some 

 species spasmodically throw off and leave behind one or more 

 of their arms. This causes them only a temporary inconven- 

 ience, however, for such mutilated individuals rapidly regenerate 

 the missing parts. 



Ambulacral grooves, so well developed on the oral surface 

 of the rays in the starfish, are lacking, and the tube-feet are so 

 little developed that the movements of the animal are controlled 

 by the flexible, muscular arms. They provide, however, a much 

 more rapid means of locomotion than is afforded by the tube-feet 

 of starfishes. The ophiuran can move rapidly among seaweeds. 



