HOMOLOGY OF THE SKELETON. 18 



plate which is found at their summit, and with the masticatory apparatus which is found 

 at their base. Notwithstanding the great number of the ambulacral plates, they hkewise 

 support this view of their comparative relations. As to the anal disc, it is much more 

 extended ; but this we can easily understand, if we recollect the extension which it presents, 

 in the star-fishes, and the very narrow region circumscribed by the ocular and genital 

 plates in the Echinida?. The analogy of the star-fish and of the urchins is even so com- 

 plete, that we may call the star-fishes urchins opened and flattened backwards, and, vice 

 versa, the urchins star-fishes contracted and inflated to form a sphere. This conformity 

 of the urchins and the star-fishes makes me doubt the exactitude of observations which 

 place the nervous filaments, which proceed to the eyes, at the inferior surface or external 

 part of the ambulacra in the star-fishes, whilst they run along the internal surface of the 

 ambulacral areas in the urchins." 



In comparing the Jsteriadaand Echinida, as Blainville and Agassiz endeavoured to 

 do, we soon perceive that the inter-arabulacral plates, observes Professor Miiller,' instead 

 of being analogous in the two orders, are quite differently arranged, and that on this 

 circumstance in a great measure depends the difference between a sea-urchin and a 

 star-fish. In the Jsteriada, we must distinguish different kinds of inter-ambulacral plates 

 from one another. Those which rest upon the external processes of the ambulacral plates 

 have a certain peculiarity, as marginal plates of the ambulacra or adambulacral plates ; 

 they exactly agree in number with the ambulacral plates (fig. la). To the second kind 

 belong, in Astrogonium (fig. 7 c), the more or less well-marked marginal inter-ambulacral 

 plates at the peripheral edge, which are sometimes in single, sometimes in double 

 series. Between the ambulacral 9,nd marginal there are often intermediate inter- 

 ambulacral plates (fig. 7 b). In Jstropecten this area is exceedingly small, and is reduced to 

 a few easily overlooked plates behind the angles of the mouth ; in the pentagonal forms it 

 is very large. In shape and size these plates often, as in Astrogonium, differ both from 

 the adambulacral and from the marginal inter-ambulacral plates. 



The marginal inter-ambulacral and the adambulacral plates extend to the end of the 

 arms ; the intermediate plates cease, for the most part, earlier. In those Asteriada 

 whose arms are round, and whose margin is not developed, the series of plates which 

 marks off the dorsal pore-area from the ventral surface is the equivalent of the marginal 

 plates. In these forms, also, the number of the series of plates, from the groove of the 

 arm to the pore-area, varies very greatly ; in some there are only two series of plates, the 

 intermediate plates disappearing, as in Echinaster and Scgtaster, whilst in OpJiidiasier 

 there are many series of plates between the groove of the arm and the pore-area, the outer- 

 most of which, as adambulacral plates and marginal plates, extend completely to the ex- 

 tremity of the arm, the others, as intermediate rows of plates, are more or less, and, indeed, 

 gradually, diminished. It is obvious that the inter-ambulacral plates of the sea-urchins 



1 c 



Ueber deu Bau der Echiuodermen,' pp. 40, 42, 43. 



