THE ECHINODERMATA 



ordinary skeleton of the Sea-Urchin is only appar- 

 ently exterior. As is the case with the ossicles of 

 the Starfish and Sea-Cucumber, the skin lies out- 

 side, and the hard particles belong to the middle 

 layer, or mesoderm. In this the skeleton of 

 Echinoderms differs from the " shell " of a crab 

 or lobster, which is formed by a hardening of the 

 skin itself. 



The Crinoidea, Encrinites or Stone-Lilies, form 

 another group of the Echinodermata. Though 

 still represented by living forms, they attained 



FIG. 39. A, Head of a Stone Lily or En- 

 crinite, Encrinus liliformis, a fossil 

 from the Muschelkalk of Brunswick, 

 natural size. B, Rock with stalks of 

 encrinites. C, Section of a stalk. 



their maximum development in past ages. The 

 English " Mountain Limestone " of the Carbonif- 

 erous period is full of their fossilized remains, 

 which form a marble often used for ornamental 

 purposes. The so-called "Stone Lily" consists 

 of a " head " comparable with the body of a Star- 



