SEA-URCHINS AND SEA-CUCUMBERS. 285- 



are broken, the edge of the fracture is not ev r en, but jagged, 

 with holes exactly corresponding with the marks in 

 question; so that the structure is the same as that of the 

 spines and of all the other solid parts of the Urchin. 



We will now examine some specimens of P. tridens, 

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

 reous support better. The head-blades expand at the base 

 into three-sided figures, each of the two interior sides of 

 which is indented with a large cavity, leaving a projecting, 

 dividing rid^e, armed with teeth somewhat remote from 

 each other. The one exterior angle is toothed in a cor- 

 responding manner, but the opposite angle appears plain. 

 The angle of one blade-base fits into the cavity of its- 

 neighbour ; and, so far as I have observed, when the two 

 edges thus overlap it is the toothed one that is on the out- 

 side. Looking from the circumference towards 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. miliaria j for, in the corre- 

 sponding organs of E. sphrera, both sides of the three- 

 cornered base appear untoothecl, except close to the- 

 bottom, where a deep notch indents each margin. 



Viewed from beneath, the head assumes an outline 

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

 the triangle has a very obtuse projecting angle in the- 

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

 accurately, 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 

 divested of its fleshy parts by immersion in caustic pot- 

 ash, and then well cleansed by soaking in clean water, 

 and placing it under a low power of the microscope — 

 100 diameters, for example — with a dark ground, and the 

 light of the lamp cast strongly upon it by means of the 

 Lieberkiihn, or the side condenser, we shall have an object 



