326 EVENINGS AT THE MICROSCOPE 



cite 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 singu- 

 lar, 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 

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

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

 mystery enshrouds the use of those processes, and the 

 only light that we have as yet upon either form is that 

 of dim conjecture. It has been supposed that, in both 

 cases, the function of the prehensile forceps is to seize 

 minute animalcules or floating atoms of food, and pass 

 them to the mouth: but the supposition is involved in 

 great difficulties; as the organs, however fitted for pre- 

 hension, seem peculiarly unsuited for transmitting objects; 

 besides that the great majority of them are placed very 

 remote from the mouth. I can only repeat the conjecture 

 which I have hazarded in the case of the Polyzoan "birds' 

 heads' ' ; viz. , that the Pedicettarice are intended to seize 

 minute animals, and to hold them till they die and de- 

 compose, as baits to attract clouds of Infusoria, which, 

 multiplying in the vicinity of the Urchin, may afford it 

 an abundant supply of food. 



There is yet another series of organs which stretch out 

 from every part of the periphery of this living box; 

 scarcely less numerous than either the spines or Pedicel- 

 larice, but very different from both. They are what I 

 alluded to just now as the feet. Let us pay a moment's 

 attention to' their appearance and action, before we ex- 

 amine their structure. 



