CHAPTER IX. 



ECHINODERMATA. 



" Ultra magis pisces et Echinos aequora celent." — HoR. Ep. 



In their " Natural History of the Echinodermata," Messrs. Hupe 

 and Dujardin divide this vast natural group into five orders or families, 

 namely: i, Crbwidece, stone lilies, calcareous, stem composed of 

 movable pieces ; 2, Asterotdece, which includes the true star-fishes ; 

 3, OphiuridecE, having the disc much depressed, the rays simple, 

 sometimes much divided ; 4, Echmidce^ comprehending the animals 

 known as sea-eggs, or sea-urchins, distinguished by their rounded 

 form and absence of arms ; 5, Holothuroidece, with soft lengthened 

 cylindrical body, covered with scattered suckers. 



The Echinodermata (from the Greek words ixivos, rough, and 

 Se'p/io, skin, indicating an animal bristling with spines like the 

 hedge-hog) are animals sometimes free, sometimes attached by a 

 stem, flexible or otherwise, and radiating, that is presenting an 

 appearance more or less regular in all its parts, after the manner of a 

 circle or star, its form being globular, egg-shaped, cylindrical, or like 

 a pentagonal plate ; or lastly, hke a star, -with more or less elongated 

 arms, which secrete either in all their tissues or only in the in- 

 tegument, very numerous symmetrical calcareous plates of solid matter, 

 sometimes forming an internal skeleton or regular shell covered with 

 a more or less consistent skin, often pierced with holes, from which 

 the feet issue ; they are frequently furnished with appendages ot 

 various kinds, such as spines, scales, &c. 



The organisation of the Echinodennata is among the most perfect 

 of all the annulose animals, serving as a transition between them and 

 animals of more complicated frame. They have a digestive and 

 vascular system, and a muscular system is almost always present; they 

 have also either internal or external respiratory organs, and a rudi- 

 mentary nervous system has been detected in many of the species. 

 The nutritive system is very simple, presenting in some families a 



