SEA-URCHINS AND SEA-CUCUMBERS 323 



"We will now examine some specimens of P. tridens, 

 treated with potash, which will enable us to see the cal- 

 careous support better. The head- blades expand at the 

 base into three- sided prisms or pyramids, each of the two 

 interior sides of which is indented with a large cavity, 

 leaving a projecting dividing ridge, armed with teeth some- 

 what remote from each other. The one exterior angle is 

 toothed in a corresponding manner, but the opposite angle 

 appears plain. The angle of one blade-base fits into the 

 cavity of its neighbor; and, so far as I have observed, 

 when the two edges thus overlap, it is the toothed one 

 that is on the outside. Looking from the circumference 

 toward the centre of the head, it is the left angle that is 

 toothed and external, the right being plain and sheathed. 

 This observation, however, applies only to E. miliaris; 

 for, in the corresponding organs of E. sphcera, both sides 

 of the trigonal base appear untoothed, except close to the 

 bottom, where a deep notch indents each margin. 



Viewed from beneath, the head assumes an outline 

 which is rondo- triangular; but yet such that each side of 

 the triangle has a very obtuse projecting angle in the mid- 

 dle, where the blade-bases meet each other. They fit accu- 

 rately, and each has a deep oblong cavity in its bottom, 

 which does not, as I conceive, communicate with the 

 interior. 



By selecting one of these heads, which has been di- 

 vested of its fleshy parts by immersion in caustic potash, 

 and then well cleansed by soaking in clean water, and 

 placing it under a low power of the microscope 100 ,di- 

 ameters, for example with a dark ground, and the light 

 of the lamp cast strongly upon it by means of the Lieber- 

 kuhn, or the side-condenser, we shall have an object of 



