1H 



THE SEA-URCHINS. 



ECHINID^, 



OR CIRRHO-SPINIGRADE ECHINODERMATA. 



Of equal importance to zoologist and geologist is the 

 study of the Sea-Urchins : to the former they present the 

 perfection of radiism, as well as the first steps towards a 

 symmetrical or bilateral form ; to the latter the know- 

 ledge of their habits and organization is necessary in order 

 to understand the relations and associations of the numerous 

 species which abound in many of the earth's strata. Of 

 all the Eadiata they are most perfectly preserved in a fossil 

 state. Their hard calcareous integument, or shell, as it is 

 popularly but inaccurately termed, the parts of which are 



