SEA-URCHIN. 105 



columns placed closely side by side, and connected by transverse 

 rods forming an exceedingly delicate pattern. Beside the tentacles 

 and the spines, they have other external appendages, of which 

 the function long remained a mystery, and is yet but partially 

 explained ; these are the so-called pedicellariae ; they consist of a 

 stem (s, Fig. 137), which becomes swollen (j?, Fig. 137) into a 



Fig. 136. Fig. 137. 



thimble-shaped knob at the end (, Fig. 137) ; this knob may 

 seem solid and compact at first sight, but it is split into three 

 wedges, which can be opened and shut at will. When open, 

 these pedicellarias may best be compared to a three-pronged fork, 

 except that the prongs are arranged concentrically instead of on 

 one plane, and, when closed, they fit into one another as neatly 

 as the pieces of a puzzle. 



If we watch the Sea-urchin after he has been feeding, we 

 shall learn, at least, one of the offices which this singular 



' ' O 



organ performs in the general economy of the animal. That 

 part of his food which he ejects passes out at an opening on the 

 summit of the body, in the small area where all the zones con- 

 verge. The rejected particle is received on one of these little 

 forks, which closes upon it like a forceps, and it is passed on from 

 one to the other, down the side of the body, till it is dropped off 

 into the water. Nothing is more curious and entertaining than 

 to watch the neatness and accuracy with which this process is 

 performed. One may see the rejected bits of food passing rapidly 

 along the lines upon which these pedicellaria3 occur in greatest 

 number, as if they were so many little roads for the conveying 



Fig. 136. Transverse section of spine; magnified. 



Fig. 137. Perticellaria of Sea-urchin ; s stem, p base of fork, t fork. 



14 



