SEA-URCHINS AND SEA-CUCUMBERS 325 



which are armed with horizontal hooked spines, some of 

 which are double, and the whole terminates in a sort of 

 ring, formed by the last pair of spines, which unite into 

 the acute horizonal point that I have already mentioned. 

 The skeleton is filled with oval cavities like that of the 

 others. 



The fourth kind of Pedicellaria, which I call P. stereo- 

 phylla, is quite distinct from either of the others. It is 

 very minute, the head being only -g^th of an inch in 

 height. The head is a prolate solid spheroid, cut into 

 three segments, exactly as if an orange were divided by 

 three perpendicular incisions meeting at the centre. Thus 

 the blades meet accurately in every part when closed, but 

 expand to a horizontal condition. These are almost en- 

 tirely calcareous, being invested but thinly with the gelat- 

 inous flesh. They are filled with the usual oval cavities, 

 set in sub-parallel arched series. 



The head is set on a hollow gelatinous neck nearly as 

 wide as itself, and thrown into numerous annular wrinkles; 

 its walls are comparatively thin, disclosing a wide cavity 

 apparently quite empty, as the blue calcareous stem ex- 

 tends only half-way from the base to the head.' At this 

 point the neck contracts rather abruptly, and continues to 

 the base, but just wide enough to invest the stem. 



This sort is confined, so far as I have seen, to the 

 ovarian plates and their vicinity, where they are numerous. 



Tims these tiny organs, so totally unlike anything with 

 which we may parallel them in other classes of animals, 

 do not merely afford us amusement, and delight us by 

 their elegance of shape and sparkling beauty, the variety 

 and singularity of their forms, the elaborateness of their 

 structure and the perfection of their mechanism, but ex- 



