468 ECH1NODERMATA. 



articulated stem. The branches themselves are articulated and dichotomously 

 ramose, bearing ranges of articulated threads, the stem being furnished with 

 smaller ones at different heights ; the mouth is in the centre of the rays, and 

 the anus on one side. 



Fossil Encrinites, however, are very numerous, and so various, that they 

 have been divided into several subgenera, according to the composition of the 

 central body placed on the summit of the stem and from which the large rays 

 proceed. 



The fossil productions, known by the names of Entrochites, are portions of 

 the stem and branches of animals belonging to this genus. 



ECHINUS, Linnceus, 



The Echini, or Sea-Urchins, as they are termed, have the body invested by a 

 shell or calcareous crust, composed of angular pieces which join each other 

 exactly, and perforated by innumerable holes, for the transmission of the mem- 

 branous feet, disposed in several very regular ranges. The surface of this crust 

 is armed with spines, articulated on little tubercles, that move at the will of the 

 animal, whose motions, conjointly with the feet situated between them, they 

 effect. Other membranous tubes, much finer and frequently divided at the 

 extremity, probably serve to convey water into the interior of their shell, and 

 then to remove it. The mouth is provided with five teeth, set in an extremely 

 complex, calcareous framework, resembling a pentagonal lantern, furnished 

 with various muscles, and suspended in a large aperture of the shell. These 

 teeth, which resemble long ribands, become indented inferiorly as fast as they 

 are worn away at the point. The intestine is very long, and attached, spirally, 

 to the interior parietes of the shell by a mesentery. A double vascular system 

 extends along this canal, and partly on the mesentery ; there are also particular 

 vessels for feet Five ovaries, situated round the anus, empty themselves by 

 separate orifices; they form the edible portion of these animals. 



The Echini chiefly feed on small shell-fish, which they seize with their 

 feet. Their motions are very slow. Shells of Echini are very abundant in 

 the ancient strata, principally those of chalk, where they are usually filled with 

 silex. 



The Echini should be divided into regular and irregular. 



In the first, 



ECHINUS, Lamouroux. CIDARIS, Klein. 



Or Echinus properly so called, the shell is generally spheroidal, the mouth 

 in the middle of the inferior surface, and the anus diametrically opposite. The 

 little foramina are arranged in ten bands, approximated by pairs, that extend 

 regularly from the mouth to the anus, like the meridian lines of a globe. 



Certain species are furnished with large and stout spines of various forms, 

 placed on large tubercles on their shell, the bases of which are surrounded by 

 other but smaller spines. 



It is among these species, as ascertained by M. Deluc, that we must place 



