24 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BuU. 



it contains is returned to the ampulla, and at the same time the 

 foot tends to shorten. As the terminal disk adheres to the ex- 

 ternal object by the pressure of the water, the shortening of the 

 foot will tend to draw the body of the starfish towards the point 

 of adhesion. If many such tube-feet work tc^ether, an actual 

 movement of the body occurs. By the coordination of the 

 nervous system of the animal all the tube-feet of any arm or 

 of all the arms may be extended in the same direction, either to 

 the right or left, backwards or forwards, as the case may be. 

 The result is a slow, steady movement of the body. 



Suitable muscular contractions of the other body muscles, 

 combined with the working of the multitude of tube-feet, en- 

 able the animal to bring about a great variety of movements, by 

 which it seeks its food, migrates from shallow to deep water or 

 the reverse, rights itself when accidentally thrown upon its aboral 

 surface, and forces apart the valves of the shells of the moUusks 

 on which it feeds, as described on page 30. By such means, 

 moreover, the starfish can so change the shape of its body as to 

 occupy small crevices in the rocks, creep through small open- 

 ings, and adapt itself to any surface with which it may come 

 in contact. 



Ossicles. — The roof of the ambulacral groove is formed by 

 a series of interlocking movable plates, the ambulacral ossicles, 

 arranged in a double row articulating with on,e another at the 

 apex of the groove in such a way as to open or close the groove 

 when required (Plate VIII). 



On each border of the ambulacral grooves is a regularly 

 arranged row of plates, adambulacral ossicles, which bear the 

 adambulacral spines (Plate VIII). These' spines may all be 

 turned toward the middle line of the groove in such a way as to 

 form a protection for the tube-feet. 



The two rows of ambulacral ossicles end in a single terminal 

 ossicle at the tip of the arm. This ossicle supports a terminal 

 tentacle, endowed with a sensory function, presumably both 

 tactile and olfactory in nature. At the base of the tentacle the 

 radial nerve ends in a minute, orange-colored, eye-spot. 



Between the ambulacral ossicles are the pores by which each 

 of the tube-feet is connected with the corresponding ampulla, 

 situated internal to the ossicle. The firmly interlocking ossicles 



