SEA-UECHINS AND SEA-CUCUMBEBS. 285 



edge of the fracture is not even, but jagged with holes ex- 

 actly 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 ridge, 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. miliaris; for, in the corre- 

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

 cornered 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 

 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 

 Ldeberkuhn, or the side-condenser, we shall have an object 



