INVERTEBRA.TE ANIMALS. 53 



larger than the rest, is of a spongy character, and admits 

 water for the supply of the water- vascular system. Five 

 smaller adjacent plates have orifices answering to the 

 eye-specks of star-fishes. On the under side of the 

 Echinus may he seen the points of five projecting teeth, 

 forming the extremities of a marvellously complicated 

 dental apparatus, which has been termed Aristotle's 

 lantern. Estimated number of species : recent, 19 ; 

 British, 3 ; fossil, 26. Cretaceous and Tertiary. 



^ Examples of Sphcerechinns, Lytechinus, Psam- 

 mechinus. 



Group 148.— Genus TOXOPNEUSTES (Agassiz) and allies. 

 To^ov, an arc ; Trvevcnc, respiration. Echinus lividus, the 

 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 continued 

 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. 



iy Examples of Loxechmiis, Toxocidaris, Helioci- 

 daris, Tripneustes. 



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

 pointed ; v-Xalog, 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 ofi" their 

 spines on being removed from the water. Estimated 

 number of species : recent, 6. 



^ Examples of Podophoi'a and two species of Acro- 

 cladia, very fine. 



