194 COLLEGE ZOOLOGY 



pores between the ambulacral ossicles (Fig. 133, 11). Sea-water 

 is forced into this system of canals by cilia which occur in grooves 

 on the outer surface of the madreporite and in the canals which 

 penetrate it. Arising from the ring-canal near the ampullae of 

 the first tube-feet are nine vesicles called, after the name of 

 their discoverer, " Tiedemann's bodies." These structures pro- 

 duce amcebocytes which pass into the fluid of the water- 

 vascular system. Polian vesicles (Fig. 134, ap) are present in 

 some starfishes, but not in Asterias. 



The most interesting structures of the water- vascular system 

 are the tube-feet. They are primarily locomotory and function 

 as follows: "When the tube-foot is to be stretched out, the 

 ampulla contracts and drives the fluid downwards. The con- 

 traction of the ampulla is brought about by muscles running 

 circularly around it. The tube-foot is thus distended and its 

 broad flattened end is brought in contact with the surface of 

 the stone over which it is moving and is pressed close against it. 

 The muscles of the tube-foot itself, which are arranged longi- 

 tudinally, now commence to act, and the pressure of the water 

 preventing the tearing away of the sucker from the object to 

 which it adheres, the starfish is slowly drawn forward, whilst 

 the fluid in the tube- foot flows back into the ampulla." Tube- 

 feet are also sensory (p. 197). 



A number of other spaces and canals have been considered as 

 parts of the ccelom and at one time were supposed to be a " blood"- 

 vascular system. These are the axial sinus lying along the 

 stone-canal and opening to the outside through the madreporite, 

 the inner circumoral perihcemal canal, the outer perihcemal canal 

 beneath the ring- canal, the aboral sinus, and the peribranchial 

 spaces. The functions of these various cavities are not 

 clear. 



Digestion. — The alimentary canal of the starfish (Fig. 135) 

 is short and greatly modified. The month opens into an oesoph- 

 agus which leads into a thin-walled sac, the stomach. Follow- 

 ing this is the pyloric sac. From the pyloric sac a tube passes 



