ECHINODERMATA 255 



consists of small rod-shaped pieces attached to each other by 

 muscles in such a way as to form a network. This skeleton is 

 very flexible and by means of the muscles the arms may be 

 slowly bent through an arc of 180° or more. Short spines, 

 more or less movable, project beyond the general surface. 

 Each radial canal ends at the tips of the arm in a tentacle 

 and at its base there is an eyespot. 



553. The starfish is carnivorous, living largely on shell fish. 

 The mouth is located at the centre of the disc and is capable of 

 opening to an enormous size so that large objects can be taken 

 into the stomach. After digestion the insoluble part is ejected 

 at the mouth. The animal also attacks larger prey than it 

 can swallow. By attaching some of the tube feet to the soHd 

 substratum and others to the two valves of a shell fish the 

 shell may be pulled open. Then another remarkable feat is 

 performed: The stomach is everted though the mouth and 

 its rather voluminous folds are thrown around the soft parts 

 of the prey. Digestion then takes place outside the body of 

 the animal. Five pairs of long retractor muscles connect the 

 wall of the stomach with the interior of the arms; their function 

 is to draw the stomach back into the body. The stomach is 

 connected with a vent at the aboral pole by a small intestine. 

 But intestine and vent are largely functionless and are often 

 rudimentary. Connected with the stomach are five long 

 tubes which project into the cavities of the arms. The walls 

 of these tubes are glandular and secrete a digestive fluid. 

 These glands are called hepatic caeca. The sexes are distinct 

 and the five pairs of gonads lie in the body cavity, a pair in each 

 arm. 



554- Class m. Ophiuroidea.— The serpent stars, or brittle 

 stars, have the general form of the starfish but are more dis- 

 tinctly divided into disc and arms. The arms are propor- 

 tionately longer and more slender and also more actively 

 movable and it is entirely by the sweeping movement of the 



