PEDICELLATA. 501 



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

 other exactly, and perforated by innumerable holes, for the transmission of 

 the membranous feet, disposed in several very regular ranges. The sur- 

 face 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 pro- 

 vided with five teeth, set in an extremely complex, calcareous framework, 

 resembhng a pentagonal lantern, furnished with various muscles, and sus- 

 pended 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 ves- 

 sels for the feet. Five ovaries 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 are divided into regular and irregular, and form several sub- 

 genera. 



E- esculentus, L. The common Echinus is of the form and size of Sn 

 apple, completely covered with short, radiating, and usually violet spines. 

 Its ovaries, which are reddish, and of an agreeable flavour, are edible in 

 the spring. 



HOLOTHURIA, Lin. 



The Holothurise have an oblong coriaceous body open at each end. At 

 the anterior extremity is the mouth, surrounded with complicated tentacula 

 susceptible of being entirely retracted. At the opposite end is the aper- 

 ture in which the organ of respiration terminates. It is in the form of an ex- 

 tremely ramified hollow tree, which is filled with water, or emptied, at the 

 will of the animal. The mouth is edentate, or merely furnished with a cir- 

 cle of bony pieces; it receives saliva from certain sac-like appendages. 



The Holothurise are divided according to the arrangement of their feet. 



H. tremula, Gm. Blackish, and when completely extended more than a 

 foot long; its back is bristled with soft and conical points, and its mouth 

 provided with twenty ramous tentacula. This species is very common in 

 European seas, the Mediterranean particularly. 



