CHAPTER IX. 



THE ECHINODERMATA. 



The Echinoderms are exclusively marine animals. They 

 are always provided with a skeleton, composed of calcareous 

 spicula, which commonly unite into networks, and give rise 

 to definite skeletal plates. These generally become connect- 

 ed with one another by joints or sutures, but sometimes re- 

 main distinct. A more or less spacious peritoneal cavity 

 separates the walls of the body from those of the alimentary 

 canal, The nervous system, in those Echinoderms in which 

 it has been most satisfactorily made out, presents a ring, 

 which surrounds the gullet, and gives off radiating longitudi- 

 nal cords. A remarkable system of vessels, termed ambula- 

 cra^ which also form a ring around the gullet, is highly char- 

 acteristic of the Eihinodermata. The most conspicuous and 

 familiarly-known Echinoderms — the Star-fishes (Asterided), 

 Brittle-stars (Ophiuridea), Sea-urchins (JBchinidea), and 

 Feather-stars (Crmoidea) — have a marked radial symmetry ; 

 similar parts, usually to the number of five, being arranged 

 around a central axis ; and the body is spheroidal, discoidal, 

 or stellate. The Sea-cucumbers and Trepangs (Holothuridea) 

 are elongated and vermiform ; but the radial symmetry is 

 still traceable in the arrangement of the oral tentacula, the 

 nervous, and the ambulacral systems. It is to be remarked, 

 however, that, in many Echinoderms, the radial symmetry, 

 even in the adult, is more apparent than real ; inasmuch as a 

 median plane can be found, the parts on each side of which 

 are disposed symmetrically in relation to that plane. With 

 a few exceptions, the embryo leaves the egg as a bilaterally 

 symmetrical larva, provided with ciliated bands, and other- 

 wise similar to a worm-larva, which may be termed an Echi- 

 nopmdium. The conversion of the Echinopsedium into an 

 Echinoderm is effected by the development of an enteroccele, 



