232 PHYLUM ECHINODERMATA. 



a small outgrowth, which has a spongy structure and has been 

 identified as a polian vesicle ; it enters into close relations with 

 branches from the circumoral Vascular ring. The radial vessels 

 give off transverse vessels, each of which opens into an ampulla. 

 The ampullae are placed within the shell plates and each of them 

 communicates by two canals with a tube-foot * (Fig. 169). In 

 those cases in which the pores are single, it would appear that 

 the ampulla is only connected by one canal with its tube-foot 

 The circular vessel is connected by a stone-canal, the walls of 



M A 



FIG. 171. Diagram illustrating the relations of the different systems of organs in an Echinus 

 (after Huxley). A anus ; Am tube-foot ; Aur auricle ; M madreporite ; mouth ; Pe 

 pedicellariae ; Po polian vesicle; pr protractor, re retractor muscles of the lantern; .R radial 

 vessel of the water- vascular system; Rg circumoral water- vascular vessel ; Sc stone-canal ; 

 St spine ; Z teeth. 



which may or may not contain calcareous matter, with an ampulla 

 placed just below the madreporic plate and opening outwards 

 through the pores of this structure. The ampulla communicates 

 with and is really the upper part of the axial sinus, which in 

 Echinoids is surrounded by the axial organ. In Echinocyamus 

 pusillus the madreporite is peculiar in the fact that it is pierced 

 by only one water-pore. 



In Spatangoids the stone-canal is short and its opening into the axial 

 sinus is separated by a wide interval from, the madreporite. 



In the Endocyclica the tube-feet terminate in sucking discs and are 



* See footnote, p. 214. 



