Prof, J. Miiller on the Structure of the Echinoderms. 113 



an anus, Asiropecfen, Luidia, Ctenodiscus ; or cylindrical with a 

 sucking disc and without a calcareous plate, as in all those 

 genera of Asterida which are provided with an anus. The 

 discrimination of Astropecten from Archaster is therefore easy ; 

 it should be remarked, however, that it is often difi&cult at once 

 to recognise the anus, though it is easy to do so, if in Archaster 

 we i-emove the external cro\A'ns of the paxilla in the middle of 

 the starfish by which it is hidden, as the ground is by the 

 foliage of closely planted trees. In Astropecten Parelii, v. D. et K. 

 (Kongl. Vet. Acad. Handl. f. 1844, p. 247. tab. 7. figs. 14-16), 

 the definiteness of this character was recently well exemplified. 

 M. Sars had informed me that this Astropecten formed an ex- 

 ception to the law which I had enunciated with respect to the 

 feet ; I therefore concluded that this starfish would turn out to 

 be no Astropecten, but an Archaster, a genus hitherto not known 

 to inhabit the Em'opean seas. M. Sars has since that time 

 furnished me with a specimen preserved in spirits, in which I 

 at once discovered the anus, when the external crowns of the 

 paxillae were removed so as to render the skin of the back 

 visible. 



In comparing the Asterida and Echinida as Blainville and 

 Agassiz endeavoured to do, we soon perceive that the inter- 

 ambulacral plates, instead of being analogous in the two families, 

 are quite differently arranged. It is not the marginal plates of 

 the Asterida alone which lie between the radii, and the upper 

 marginal plates are already dorsal. 



In the Asterida 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 pecu- 

 liarity as marginal plates of the ambulacra or adambulacral 

 plates ; they exactly agree in number with the ambulacral plates. 

 To the second kind belong the more or less well-marked mar- 

 ffinal inter-amhulacral plates at the peripheral edge, which are 

 sometimes in single, sometimes in double series. Between the 

 adambulacral and marginal there are often intermediate inter- 

 ambulacral plates. In Astropecten 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 

 form 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 Asterida whose arms are round 

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



Ann. tf Mag. N. Hist. Ser. 2. Vol. xiu. 8 



