BOOK FOURTH. 



ECHINODEEMATA 



CLASSIFICATION. 



§71. 



The Echinoderms have a more or less coriaceous envelope, filled with cal- 

 careous, reticulated corpuscles. These last are sometimes so numerous that 

 they form a real shell, composed of plates, movable, or tightly bound 

 together. In the ray-like, symmetrically-arranged ■ systems of organs, the 

 quinquenary number prevails. 



In many species the digestive canal is asynmietrical. All are marine, 

 and most of them move by means of particular, erectile suckers. Others 

 progress by vermiform motions, and some swim freely by moving their ray^ 

 like oars. Only a few are stationary. All are without copulatory organs. 



ORDER I. CRINOIDEA. 



The calcareous shell, composed of movable pieces, forms a true cutane- 

 ous skeleton. The body is ray-like ; the digestive canal, asymmetrical. 



Family: Encrinidae. 

 Genus : Peiitacrinus. 



Family : Comatülinae. 

 Genus : Comatida. 



ORDER IL ASTEROIDEA. 



The calcareous shell, composed of movable pieces, forms an internal skel- 

 eton. Tlie cutaneous covering is sometimes coriaceous, and sometimes cal- 

 careous. The body is ray-like, and the digestive canal symmetrical. 



FAMfLY : Ophiuridae. 



Genera Astrophyton, Ophionyx, Ophiothrix, Opkiomastix, Opkioccma, 

 Ophiolepis, Ophioderma. 



