ECHINODERMATA. 151 
observed’. 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 ArtsToTLe’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*. 
The food of the Sea-urchin consists, according to the microscopic 
investigation of the excretions in Echinus lividus by VALENTIN, 
1 Natuurk. Vitsp. 1. bl. 132. 
? This apparatus is described in detail by Cuvier Legons d’Anat. comparée, Paris 
1805, Ill. pp. 329—335, and 2e édit. Paris 1837, VI. pp. 377—382; comp. also the 
works of TIEDEMANN and VALENTIN already cited, and R. Jonus, Outline of the Anim. 
Kingd. pp. 166—169. 
3 Echinus lithophagus of LeacH, which does not appear to differ from Echinus 
lividus, is often thus found on the western coast of Ireland ; E. T. Bennert Linn. 
Transactions, XV. 1827. pp. 74—77. 
