WATER-VASCULAR SYSTEM. 129 



tubular prolongations as there are radii. These are the radial 

 water- vascular vessels ; they give off lateral branches to the 

 tube-feet all along their course (Fig. 133). 



The tube-feet are hollow, cylindrical or conical processes of 

 the body wall, and the space within them is continuous with the 

 water- vascular system by the just mentioned lateral branches of 

 the radial vessels. At their inner ends they are connected with 

 small vesicles the ampullae (Fig. 129, 26). Ampullae are absent 

 in Crinoids and Ophiuroids ; they have muscular walls and their 

 function is to drive the fluid into the tube-foot and so to cause 

 its extension. The retraction of the foot is brought about by 

 the contraction of the muscles in its wall, the fluid in the tube- 

 foot passing into the ampulla.* In the forms with ampullae 

 the tube-feet are locomotive and adhesive organs. Their free 

 ends terminate in sucker-like discs which adhere to foreign 

 bodies. When ampullae are absent (Crinoids and Ophiuroids) 

 the tube-feet are purely sensory and respiratory in function 

 and cannot be used for adhesion and locomotion. Calcareous 

 bodies are often present in the connective tissue layer of the 

 tube-feet, particularly at the sucker-like termination. 



The circumoral vessel frequently possesses accessory structures 

 opening into it in its interradial portions ; these are the bladder - 

 like polian vesicles (p. 184) and the gland-like Tiedemann's 

 bodies (p. 185). 



The water- vascular system is indirectly connected with the 

 exterior by a canal called the stone- or sand- canal (Figs. 133, 136). 

 The stone-canal owes its name to the fact that it frequently 

 contains in its walls a large amount of calcareous matter, which 

 readily breaks through the lining and falls into the canal, where 

 it is found as gritty matter. The connexion with the exterior is 

 effected in the following way (Fig. 132) : the stone-canal passes 

 off from the circumoral vessel in one of its interradii to open, 

 in the larva, into the anterior coelom which is a portion of the 

 splanchnocoel. The anterior coelom, which, as already explained 

 (p. 127), opens to the exterior by the water-pore, persists into 

 the adult in Asteroids, Ophiuroids, and Echinoids as a distinct 

 space, the axial sinus, f In Crinoids it is a distinct space in the 



* For the valve assisting in this process, see p. 182. 



f In the adult many of the pores of the madreporite come to open directly 

 into the dorsal end of the stone-canal, but the opening of the stone-canal 

 into the axial sinus is always maintained. 



Z IIT K 



