150 PHYSIOLOGY 



ventrally (Fig. 84). It has two enlargements which 

 somewhat obscure its tube-like character, each of which 

 is made of five pouches, one for each ray. The pouches 

 of the thin-walled stomach on the dorsal side are further 

 subdivided, and are continued to 

 form the ducts of the hepatic 

 glands, or livers, found in each 

 ray. The other organs are also 

 arranged with reference to the di- 

 gestive tract. The nervous, circu- 

 latory and water-vascular systems 



Fig. 84.— Dorsal view of the d . 



alimentary canal of a take the form of rings which Slir- 

 Btarflah. Diagrammatic. ^^ ^ ^p^g and give off 



an arm for each ray. At the end of each radial nerve is 

 a sense organ called an eye-spot that is sensitive to light. 



Irritability. — Irritability manifests itself in change 

 of form and position and in movement from place to 

 place through the activity of muscles whose cells are 

 specially modified for contraction. These are aided by 

 nerves whose special form of irritability consists in their 

 power of carrying stimuli quickly. They thus increase 

 the rapidity of movement. 



Mechanics of Locomotion. — In moving from place to 

 place, a starfish contracts the muscles which bend a ray. 

 The ray moves in a definite direction and then holds 

 fast by means of its tube feet until the body of the ani- 

 mal is adjusted to the new position. The tube feet 

 stiffen, and force the air from beneath them. They are 

 then held fast to the damp surface by the weight of the 

 surrounding air. The pressure exerted on them by the 

 air may be found approximately by comparing the ap- 



