XIV] ECHINOIDEA 355 



surrounds tne axial sinus in the lower part of its course, so that this 

 space appears like a cavity excavated in the stolon. Hence by one 

 author the axial sinus and stolon were mistakenly described as a 

 kidney with glandular walls, and the madreporic vesicle, which 

 is here much enlarged and extends parallel with the axial sinus, 

 was called by the same author the "accessory kidney." 



The Echinoidea are divided into three orders : 



I. The ordinary Sea-urchins, such as we have described, con- 

 stitute the order ENDOCYCLICA, which live chiefly on rocky and stony 

 ground. The other two orders live in sand or mud and have under- 



FIG. 167. Aboral system of plates of Echinus esculentus x 4. From Chadwick. 



1. Genital plate. 2. First plate in the radius pierced for the eye-spot. 



3. Madreporite. 4. Periproct, or region round anus. 5. Anus. 



6. Genital pore. 



gone singular modifications in order to fit them for this kind of life. 

 They are termed the Irregular or Exocyclic Sea-urchins, because 

 whereas the anus has become shifted from the upper pole of the 

 body down one side to the edge, or even to the under surface of the 

 more or less flattened body, the madreporite and genital plates still 

 retain their position. In both orders the tube-feet of the upper 

 part of the ambulacra are the main breathing organs, and are 

 greatly flattened and expanded at the base, while the pores through 

 which they pass are arranged in two converging curves on each 

 ambulacrum, the figure produced being compared to a petal of a 



232 



