SEA-URCHINS AND SEA-CUCUMBERS. 287 



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

 jJii/lla, is quite distinct from any of the others. It is 

 very minute, the head being only v^roth °f an inch in 

 height. The head is a kind of lengthened sphere, 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 entirely calcareous, being invested but thinly with 

 the gelatinous flesh. They are filled with the usual oval 

 cavities, set in arched rows. 



The head is set on a hollow gelatinous neck nearly as 

 Avide 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 nu- 

 merous. 



Thus 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 struc- 

 ture, and the perfection of their mechanism, but excite 

 our marvel as to what can be the object which they sub- 

 serve in the economy of the creature, — what purpose can 

 be fulfilled by so many hundreds of organs, so singular 

 and scattered over the whole surface of the shelly body. 



It is very difficult to answer this question. The only 

 organs with which they can be compared are the singular 

 " birds' heads " in so many of the Polyzoa, which we 

 looked at some time ago. But, unfortunately, a like mys- 

 tery enshrouds the use of those processes, and the only 



