NO. 19.] ECHINODERMS OF CONNECTICUT. 71 



commonly bear conspicuous spines, termed arm spines. The 

 number of spines is nearly constant on the corresponding 

 parts of the arms in all individuals of the same species, and 

 moreover each species presents its own peculiarities in the size, 

 position, and arrangement of these structures, as is shown on 

 Plates XIII and XIV. Hence their importance in the diagnosis 

 of the species. 



Tube-feet. — The tube-feet are small and inconspicuous, and, 

 as they have no sucking disks nor ampullae, they are of compar- 

 atively little importance in the locomotion of the animal. They 

 function largely as sense organs, and are therefore often called 

 tentacles. They are also of service in respiration by the ex- 

 change of gases which takes place between their fluid contents 

 and the external sea water, and in some cases they aid in pass- 

 ing food particles to the mouth. In certain species the greater 

 portion of the food consists of small organisms which are secured 

 by the tentacles as the slender arms are swept back and forth on 

 the surface of the sand or mud. The food secured is passed 

 along from one tentacle to the next until, on reaching the 

 proximal tentacles, it is pushed between the jaws and into the 

 mouth. 



The tentacles protrude between the lateral plates of the arm, 

 and are protected by delicate scales, called the tentacle scales 

 (Plates XIII and XIV, and Fig. 9). 



Internal structure of Arms. — The arm of the ophiuran is 

 morphologically equivalent to that of the starfish, but the struc- 

 tures have undergone great modification. Instead of a broad 



Explanation of Plate XIV. Skeleton of Ophiuran, Ophiura brevispina. 



(Five times natural size.) 



Photographs of dried specimens, showing aboral and oral surfaces 

 of disk and bases of rays. In the upper figure the granules covering the 

 surface of the disk have been removed on the side toward the top of the 

 page to show the underlying radial shields and other plates. 



The lower figure shows the plates around the mouth and on the 

 under side of the rays. Of the five large oral shields the one on the 

 lower right-hand side is somewhat larger than the others, and is the 

 madreporic plate. The adoral plates lie close beside the oral shields. 

 The figure shows the teeth at the apex of the jaws, and the seven oral 

 papillae on each side of each jaw. The left side of the mouth shows 

 the position of the jaws when closed. The two pairs of large openings 

 near the base of each arm are the genital slits. On the arms the upper 

 and under arm plates, the tentacle scales, and the closely appressed arm 

 spines are shown. 



