ECHINODERMATA OF NEW ENGLAND. 



59 



of the body. Our common sea-urchin sometimes exca- 

 vates cavities in the solid rock. 



In the "sea-cucumbers" the structure of the ambula- 



SEA-URCHINS IN EXCAVATIONS. 



cral plates is obscure. In some genera a foot is formed by 

 the modification of three of these series ; in Cucumaria we 

 find five double rows, and in Thy one the suckers are ir- 



