116 Prof. J. Miiller on the Structure of the Echinoderms. 



are formed at the point of the arms, but this can only hold 

 good of those which reach the extremity of the arm. In most 

 Asterida many series of plates do not attain the end of the 

 arms, as in Ophidiaster, Asteriscus, Astrogonium. He supposes 

 further, that the plate in question has the same relations in the 

 Asterida as in the Sea-urchins, and that the eye is in the Aste- 

 ridce also seated in it (Ann. d. Sc. Nat. t. vi. 1846, pp. 309, 311). 

 It is here presupposed that the radial nerve has an internal 

 course underneath the ambulacral plates in the Asteridae as in 

 the Echinidce, which, however, is true only of the latter and not 

 of the former. Neither is there any aperture in the azygos plate 

 which lies at the end of the ambulacrum and at the beginning 

 of the dorsal part of the arm. The analogy of the plates in the 

 Sea-urchins and Asterida, however, is not weakened, but is 

 strengthened, by this circumstance. It is in all cases the ter- 

 minal plate, the outermost of the radius. In the Asterida, which 

 have large marginal plates developed in their peripheral border, 

 it is the marginal plates, viz. the upper marginal plates, which 

 have the same shape as this apical radial plate and form one series 

 with it. The marginal plates are, therefore, in a manner repe- 

 titions of the apical plate of the radius, which unite the radii in 

 festoons and separate the abdominal or inter-ambulacral side 

 from the dorsal or antambulacral side. The terminal plate is 

 smooth when the marginal plates are smooth {Astrogonium) ; in 

 other cases, when all the inter-ambulacral and ambulacral plates 

 are granulated {Scr/taster, Ophidiaster), it is covered with gra- 

 nules. As the terminal piece of the arm, this plate has relations 

 to the antambulacral as well as to the ambulacral and inter- 

 ambulacral sides, and it is equally true for the antambulacral side 

 that new plates are formed in its vicinity to extend those series 

 which reach the end of the arm. 



In the Ophiurida the terminal portion of the radius is a pecu- 

 liarly formed articulation without either spines or suckers, be- 

 tween which and the next all the new articulations of the radius 

 arise, as has been shown by the history of the development of 

 the Ophiurida. This articulation is obviously the analogue of 

 the terminal piece of the arm of the Starfishes. The mode in 

 which the terminal articulation of the arm of the young Ophiurid 

 is penetrated by the ambulacral canal, whose csecal extremity is 

 visible for a long time projecting from the end of the articula- 

 tion, is described in the memoir upon the Ophiurida of the 

 Adriatic. 



In the Asterid-larva described by Busch, there is a promi- 

 nent azygos process of the ambulacral canal ; not however at the 

 point of the arm, but on its ventral surface. This would appear 

 to result fi'om the position of the ambulacral canal of the As- 



