64 INVERTEBRATE ANIMALS. 



Group 150.— Genus ECHINOMETRA (Breynius):; and 

 allies. On the integument of many of the Echino- 

 dermata are found curious little appendages called 

 PedicellariaB, in shape not unlike the headj and stem 

 of an Encrinite, but possessing only two or three arms, 

 which open and close with much activity. As in the case 

 of the avicularia of the Polyzoa, their true character is 

 yet unknown. Estimated number of species in the 

 present group : recent, 14 ; fossil, 195. 



Family CLYPEASTRIDiE. clypeus, a shield ; astrum, 

 a star. 



Group 151.-^Genus ECHINARACHNIUS (Van Phels) and 

 allies. sxhoi, apocKVYj, a spider's web. The globose 

 form of the common Sea-urchin differs very widely from 

 the flattened disc of the Cake-urchin, Echinarachnilis 

 placenta, a northern form rarely found in British 

 seas ; but upon examination the true ambulacral pores 

 of the Cake-urchin may be seen, forming a rosette 

 on its upper surface ; and other very curious modifica- 

 tions of the ordinary echinoid structure may be recog- 

 nised as associated with an extreme reduction in the size 

 of the body cavity of the animal. Estimated number 

 of species : recent, 32 ; British, 2 ; fossil, 63. 



H Examples of Melitta, Dendraster, Rwmphia, 

 Lobophora, Scutella. 



Group 152.— Genus CLYPEASTER (Lamarck) and allies. 

 Shield-stars. Between the ClypeastridtB of the present 

 and the SpatangidcB of the succeeding group occurs the 

 family Cassldididce, in which the mouth is central, but 

 the dental apparatus is wanting. The species, of which 

 M. DuJARDiN recognises more than 260, begin to appear 



