SEA-UECHINS AND SEA-CUCUMBEBS. 287 



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

 pliylla, is quite distinct from any of the others. It is 

 very minute, the head being only ^J-^th of 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 cavi- 

 ties, set in arched rows. 



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 numer- 

 ous. 



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 subserve 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 



