ECHINODERMATA. 151 



observed 1 . Between these five genital plates, lie five smaller (the 

 ocellar plates), also pentagonal but with the point turned inwards, so 

 that the plates are wedged into the spaces between the points of 

 the first five. There are still other smaller plates, variable in 

 number, that lie within the ring formed by these ten, and immedi- 

 ately surround the anus. 



The chemical composition of the calcareous shell is remarkable 

 on account of the extremely small proportion of organic matter ; it 

 consists almost entirely of carbonate of lime. The growth of the 

 shell is effected by enlargement of the plates, and by addition to 

 their number upwards ; hence younger individuals are flatter than 

 older ones, and the form of itself alone becomes a deceptive criterion 

 in determining species. 



The moveable spines, of which the number increases with the 

 age, have at their base a small cavity, by which, as by an articular 

 surface, they are connected with the tubercle of the shell. These 

 tubercles are placed in rows, like the apertures of the ambulacra, 

 and are found both on the ambulacral and on the interambulacral 

 fields. 



The mouth is provided with five teeth, whose points are sharp 

 and hard, and meet in a pentagon at the inferior aperture. These 

 teeth are long rods, which become soft and transparent inwards. 

 They perforate five triangular pyramids, which by their mutual 

 arrangement form a conical apparatus with the broad base facing 

 inwards, and to which base still other small calcareous pieces are 

 united. This very complicated apparatus, with its provision of 

 muscles, bears the singular name of ARISTOTLE'S lantern*. 



Sea-urchins cannot swim, but only creep along the bottom of the 

 sea. There are species that sometimes live in cavities which they 

 have formed in the rocks 3 . 



The food of the Sea-urchin consists, according to the microscopic 

 investigation of the excretions in JEchinus lividus by VALENTIN, 



1 NatuurTc. Uitsp. i. bl. 132. 



2 This apparatus is described in detail by CUVIER Lemons d'Anat. comparee, Paris 

 1805, in. pp. 329335, and 2e e"dit. Paris 1837, VL PP- 377 382 ; comp. also the 

 works of TIEDEMANN and VALENTIN already cited, and R. JONES, Outline of the Anim. 

 Kingd. pp. 166 169. 



3 Echinus lithophagm of LEACH, which does not appear to differ from Echinus 

 lividus, is often thus found on the western coast of Ireland ; E. T. BENNETT Linn. 

 Transactions, xv. 1827. pp. 74 77. 



