^Ol. 55.] ECHINOIDEA AND OPHIUKOIDEA. 711 



cases) are distinguished by a like difference in position : so that it 

 would seem necessary to rotate Aristotle's lantern through an angle of 



^ to bring it into homologous relations with the buccal armature of 



star-fishes. 



The structure of the Asteroid armature has been correctly 

 analysed by Ludwig, who has illustrated by a diagram the manner 

 in which the jaws have arisen by modification of the first 

 pairs of ambulacral and adambulacral plates : this diagram might 

 almost stand for a drawing of the mouth-parts in Palceocoma 

 Marstoni, a fossil Asteroid of Lower Ludlow age. From this we 

 learn that the first pair of ambulacral plates, losing the parallelism 

 which prevails in the arm, diverge from each other to overlap near 

 their termination the enlarged first adambulacral plates, which bear 

 the teeth or, rather, spines. In urchins, on the contrary, there 

 appears to be no splaying-out of the enlarged ossicles at the com- 

 mencement of an ambulacrum, and the rauscularly-attached sides of 

 the alveoli continue in the radial direction. The alveoli are, how- 

 ever, generally regarded as interradial in nature ; and from this it 

 seems to follow that while in star-fishes the ambulacral ossicles have 

 diverged to meet in an interradius, in urchins on the contrary it is 

 the adambulacral ossicles which have diverged to meet along the 

 radii. 



At this stage of our enquiry we receive important aid from 

 ontogeny. The diagram on the following page is taken from Loven's 

 faithful drawing of the mouth-parts in a young specimen of Gonio- 

 cidaris canaliculata, A. Ag., 1*45 mm. in diameter (fig. 15). The 

 hemipyramids are seen to be interradial in position, and on our 

 hypothesis represent the first pair of adambulacral plates: the 

 epiphyses are as truly radial, and may be regarded as the first pair 

 of ambulacral plates. Immediately over the interradial line, where 

 the two hemipyramids meet, lies a plate which subsequently becomes 

 the tooth ; it occupies a position precisely corresponding to that of 

 the odontophore in Asteroids. In a second illustration, the buccal 

 ossicles of the same specimen are represented as seen from the side 

 (fig. 16, p. 712). This is an extremely instructive view : the epi- 

 physes or ambulacral elements are shown lying above the pyramids, 

 as they should do if of ambulacral origin ; and, further, they are 

 obviously divergent from the radial plane, or convergent towards the 

 interradius, thus recalling the splaying-out of the first pair of ambu- 

 lacral ossicles in the jaws of star-fishes. 



So far the resemblance between the buccal armature of a young 

 urchin and that of a star-fish would appear to be complete, and it 

 would be difficult not to see here a case of precise homology, were it 

 not for the difference in the relations of the two sets of ossicles to 

 the water-vascular system ; yet, since this difference does certainly 

 exist, we are driven to conclude that the resemblance is but another 

 instance of that far from uncommon phenomenon which Eay 

 Lankester has designated ' homoplasy.' The young specimen just 

 mentioned presents rudiments of both rotula and radius, which, 



