Parts of the Lantern of Aristotle in Echinus miliaris. 479 



odontophors underwent a sinistral or dextral rotation which brought them to 

 a radial position ? MacBride* with reference to the inter-pyramidal muscles 

 says as follows : " These on contraction, approximate the pair of jaws into 

 which they are inserted, and it will easily be seen that by the successive 

 contraction of the five comminator muscles a rotating movement of the teeth 

 would be produced which would cause them to exert an action something like 

 that of an auger." Can this action of the lantern, coupled with the fact that 

 the rotulae alone of all the ossicles of the lantern are free from muscular 

 attachment with the shell, be supposed to have brought about this 

 displacement ? 



From its double origin and radial position each compass may be regarded 

 as corresponding to a pair of ambulacral ossicles. It is generally known that 

 in certain extinct star-fishes, the paired ambulacral plates alternated with 

 each other. If the same condition had prevailed among the ancestors of 

 urchins, one might conceive of a displacement consisting of one member of a 

 pair being pushed in front of his fellow. 



In instituting a comparison between a tooth and other ossicles of the lantern 

 or those of the mouth-frame of star-fishes, one should take a pair of primordial 

 lamellae to represent a tooth on the one hand and paired ossicles on the other. 

 For I hold an urchin-tooth is not, in the strict sense of the term, an ossicle ; 

 it is an aggregate of paired ossicles if the lamellae can be so called. Whether 

 the lamellae themselves are ossicles is open to doubt ; as we saw in their 

 development, they neither pass through a tri-radiate spicule stage nor do they 

 grow into fenestrated plates as ossicles of echinoderms do in general by the 

 branching and anastomosing of calcareous offshoots. On the contrary a tooth- 

 lamella grows by accretion confined in the early stages to one particular side, 

 viz., the base of the minute triangular plate. For purposes of comparison, 

 therefore, a pair of tooth-lamellae may be, with the above-mentioned reserva- 

 tions, taken to represent a pair of ossicles corresponding to a pair of " alveoli " 

 or " epiphyses " rudiments. This being so, a whole tooth does not stand in 

 the same relation to its rudiments as the other component parts of the lantern 

 do ; the latter are, par excellence, echinoderm ossicles whereas the former is an 

 aggregate of paired structures which are not undoubted ossicles. An epiphysis 

 for instance, being the direct outcome of a tri-radiate spicule, is a unit in 

 itself ; but a tooth is an aggregate, being the product of several paired units, 

 the lamellae. A tooth is essentially a double structure like a pair of "alveoli" 

 or " epiphyses," the only indication of this in the adult tooth being the 

 median furrow which runs longitudinally along its outer side. 



The homologues of the urchin-tooth are to be looked for among the bristles 

 * See " Echinodermata," 'Cam. Nat. Hist., ; p. 526. 



