INVERTEBKATE ANIMALS. 55 



Purple Sea-urchin, was seen by the writer, lying hun- 

 dreds together in tide-pools on the coast of Kerry, 

 Ireland. This species frequently lives in cup-shaped 

 cavities of rocks corresponding with the size of the 

 animal. It seems to perforate the rock by the con- 

 tinued motion of its spines, which, being covered by a 

 living membrane, are renewed at the tip as the points 

 are worn away. Estimated number of species : recent, 

 30 ; British, 1 ; fossil, 9. 



IT Examples of Loxecli'mus, Toxocidaris, Helioci- 

 daris, Tripneustes. 



Group 149.— Genus ACROCLADIA (Agassiz). axpoc, 

 pointed ; xAaSoc, the young shoot of a tree. The 

 species of this group, distinguished by their massive 

 spines, are found in Tropical seas, and during life are 

 superbly coloured ; but they are much disposed to cast 

 off their spines on being removed from the water. 

 Estimated number of species : recent, 6. 



^ Examples of Podophora and two species of Aero- 

 cladia. 



Group 150. — Genus ECHINOMETRA (Breynius) and 

 allies. On the integument of many of the Echino- 

 dermata are found curious little appendages called 

 Pedicellariae, in shape not unlike the head and stem of 

 an Encrinite, but possessing only two or three arms, 

 which open and close with much activity. These pre- 

 hensile organs, much resembling the avicularia of the 

 Polyzoa, are supposed to be modified spines. Estimated 

 number of species in the present group : recent, 14 ; 

 fossil, 195. 



