414 STRATIGRAPHICAL DISTRIBUTION OF 



lias been nearly quadrupled since the publication of his works,* still the outlines sketched 

 by the hand of our great master remain nearly the same as laid out by him. 



I have already shown that the test of the Echinodermata constitutes an internal and 

 integral part of the body of the animal, participating in its life, intimately connected with 

 the organs of digestion, respiration, and generation, as well as with those of vision and 

 locomotion, and consequently having many of the distinctive characters of the organism 

 indelibly impressed on different parts of the skeleton. The individual plates composing 

 the columns of the test of the Echinoidea, and the ossicula forming the skeletons of the 

 AsTEROiDEA, OpHiUROiDEA, and Crinoidea, are organized after distinct plans ; they are 

 . therefore of great value in determining the species, as the specific characters are often 

 well preserved on even fragmentary portions of the skeleton ; for this reason the remains 

 of these animals are of the highest value in stratigraphical geology, and second in 

 importance to no other class of the animal kingdom. 



In the Echinoidea the body is spheroidal, oval, depressed or discoidal, and enclosed 

 in a calcareous test or shell composed of ten columns of large plates constituting the 

 inter -amhulncral areas; and ten columns of small plates constituting the ambulacral areas, 

 which segments are separated from each other by ten rows of holes constituting the 

 poriferous zones. The external surface of the plates is studded with tubercles of different 

 sizes, in the different families ; to these the spines are articulated, by a kind of ball-and- 

 socket joint, which are of different sizes, forms, and dimensions in the different famihes, 

 and serve to characterise the genera and species. 



At the summit of the test is the apical disc, composed of five genital plates perforated 

 for the passage of the ovarial and seminal canals ; and five ocular plates notched or pei'- 

 forated for lodging the eyes : in one family, the Saleniad^e, an additional or suranal 

 plate, composed of one or many pieces, is introduced within the circle formed by the 

 genital and ocular plates. 



There are two great apertures in the shell, one for the mouth, which is always at the 

 base ; the other for the anus, which occupies different positions on the test ; in one section 

 it is in the centre of the upper surface, directly opposite to the mouth, and surrounded by 

 the genital and ocular plates j in a second section the vent is external to the circle of 

 genital plates, and never opposite to the mouth, but situated in different positions in 

 relation to that opening, being placed on the upper surface, on the sides, the border, the 

 infra-border, or the base, in the different groups. 



The mouth is often armed with a complicated apparatus of jaws and teeth, 

 or it is sometimes edentulous, and provided with lobes formed of the plates of the test 

 itself. 



The Asteroidea have a depressed stelliform body provided with five or more lobes or 

 hollow arms, which are a continuation of the body, and contain prolongations of the 



* 'Strata identified by Organized Fossils,' 4to, 181G.—' Stratigraphical System of Organized Fossils,' 

 4to, 1817. 



