154 CIDARITES. 



though by some they are considered as respiratory tubes. 

 Neither they nor the tubular tentacula are ciliated. There 

 is usually an irregular circle of slender spines developed on 

 the oral membrane near the mouth ; they do not differ in 

 structure from those on the surface of the body-plates. 



The five polished pointed teeth which project from the 

 mouth form part of a remarkable dental apparatus, known 

 by the fanciful appellation of Aristotle's lantern. The 

 lantern consists of five long three-sided triangular sockets 

 or jaws united together, with their apices pointing down- 

 wards, so as to form a sort of pyramid. Each of these 

 jaws is broadly keeled on its outer side, and bordered by 

 raised margins ; towards the upper and broader part, 

 which forms part of the base of the pyramid, the keel 

 bifurcates and the space between the bifurcations is tri- 

 angular and perforate, being filled up with membrane in- 

 stead of bony matter. The shape of the keel and of the 

 perforation varies in different species. In the Echinus livi- 

 dus the keel is grooved, and the apex of the perforate space 

 broad, obtuse, and bidentate. In Echinus miliaris the base 

 of the triangular perforation is much more acutely angular 

 than in Echinus sphara, and the keel broader in proportion. 

 The other two or inner sides of each jaw are quite flat and 

 finely striated across, their edges being pectinated like a 

 comb. Each of these jaws is separated from the others 

 above and in a manner united with them, as well by five 

 oblong horizontal plates, with their outer ends bent down- 

 wards and widened, so as greatly to strengthen the ap- 

 paratus. The margins of these plates are not straight but 

 somewhat fiddle-shaped. From the inner ends of these 

 oblong plates, and springing seemingly from the centre of 

 the lantern immediately around the gullet, rise five long 

 slender processes bent outwards, and having mace-shaped 



