ECHINID.E. 381 



our sea- shores, lurking among the rocks, where they entrap 

 their prey. Their spines and suckers are used as feet, or 

 as a mode of progression, even to the climbing of rocks, in 

 order to feed upon corallines and zoophytes : they march 

 along with ease where apparently no footing could be found, 

 or dig holes with their spines to bury themselves in the 

 sand, to escape pursuers, or hide from observation. Echi- 

 nodermata, 1 sea-urchins, or sea-eggs, derive their name 

 from these curious spinous processes ; they are divided 

 into two principal orders, the ISckinoidea, and Holothurina. 

 These again are subdivided into families, as HJchinida, 

 Asterida, Ophiurida, Crinoidea, Cystidea, Holothuriada, 

 and Synaptida. 



The E chinodermata belong to the family Annuloida, the 

 most familiar of examples of which are star-fishes and 

 sea-urchins. The labours of that distinguished comparative 

 anatomist and physiologist of Berlin, Johannes MUller, 

 have made us better acquainted with the structure and 

 development of these remarkable animals, than with those 

 of most classes of the animal kingdom. The series of feet 

 which protrude along certain fixed lines from the body of 

 an Echinoderm have received the name of " ambulacra ; " 

 and hence, says Mr. Huxley, " we may distinguish their 

 system of vessels as the ambulacral vascular system. The 

 existence of an ambulacral vascular system has as yet been 

 demonstrated only in the following orders : JEchinidea, 

 Ophiuridea, Crinoidea, Asteridea, and Holotkuridea, with 

 which the fossil Cystidea and Blastoidea are inseparably 

 connected. I therefore limit the E chinodermata to the 

 very natural group formed by these orders. A more or 

 less complete calcareous skeleton is always developed 

 within the Echinoderms, resembling that of the Actinozoa, 

 not only in this respect, but also in consisting of detached 

 spicula. In this form the skeleton remains in the Holo- 

 thuridea, but in the other JEchinoderms, the spicula coalesce 

 into networks, which may become consolidated into dense 

 plates by additional deposits. It is by the different shape 

 and arrangement of these plates that the diversity ex- 

 hibited by the skeletons of different Echinodermata is 

 produced." 



(1) Derived from echinos, a spine, and derma, skin. 



