ECHINODERMATA. 17 



an important character for the subdivision of the order into two primary groups. The 

 intestine winds rounds the shell, attached by a mesentery, the surface of which, as well as 

 the membrane lining the shell, is covered with vibratile cilia. 



Type. The common Sea-urchin, Echinus splicera, Midler. 



The Echinoidea are represented by one family in the Palaeozoic rocks, and by 

 numerous families in the Mesozoic and Tertiary groups, several of which characterize 

 these great periods of geological time. They likewise abound in our present seas. 



Order iv. Asteroidea. — Body stelliform, depressed, with five or more lobes or 

 hollow arms, forming a continuation thereof, and containing prolongations of the viscera; 

 the mouth is always inferior and central, and the intestine often terminates in a vent 

 opening at the upper surface; in some genera the vent is absent; rows of retractile 

 tubular suckers occupy the ambulacral areas in the centre of the under surface of the 

 rays. Skeleton complicated, composed of numerous solid calcareous pieces, variable as to 

 number, size, and disposition ; skin coriaceous, studded with calcareous spines of various 

 forms ; a madreporiform plate on the upper surface, near the angle between two rays ; 

 eyes placed at the extremity of the rays ; reptation performed by tubular suckers. 



Type. The common Star-fish, Vr aster rubens, Linn. 



This order is represented in the Silurian rocks by several genera. The Oolitic, Cre- 

 taceous, and Tertiary rocks contain many extinct forms. The existing species are very 

 abundant in all our present seas. 



Order v. Ophiuroidea. — Body discoidal, distinct, depressed, provided with long, 

 slender arms, in which there is no excavation nor prolongation of the viscera; they 

 are special organs of locomotion, independent of the visceral cavity, and have spines, 

 and membranous tentacula developed from their sides ; mouth always below and central, 

 serving at the same time as the vent. Skeleton complicated, composed of calcareous 

 pieces, of which the size and number varies in different genera. Their long, slender rays 

 are supported internally on a framework of central vertebra-like pieces ; they form special 

 organs of locomotion, independent of the visceral cavity, and numerous plates and spines 

 are regularly disposed along their sides to assist in reptation. 



Type. The common Sand-star, Op/dura texturata, Lamarck. 



This order is represented by one genus in the Silurian, and several genera are 

 found in the Oolitic, Cretaceous, and Tertiary rocks, as well as in our present seas. 



Order vi. Blastoidea. — Body in the form of an oval calyx, composed of solid, 

 calcareous plates, provided with five inter-ambulacra and five ambulacra, the latter united 

 superiorly, striated transversely, and having a deep furrow down the middle ; ten ovarial 

 holes, opening into five at the summit, with a central oral aperture, a short, slender 

 stem, and the body destitute of arms. 



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