198 P. HERBERT CARPENTER. 



as to enclose the water-system. The Ophiurids and Urchins 

 remain permanently in this condition, but in the Asterids 

 the superambulacral plating is resorbed along the central line, 

 its edges sending out small spurs to form the vertebral ossi- 

 cles or subambulacral system, which makes up the principal 

 element in the Starfish skeleton. In the Crinoids, however, no 

 extensive changes of this kind take place, and in this respect, 

 as well as in the condition of their actinal skeleton, they are 

 in a far more embryonic condition than are the other Echi- 

 noderms, so that we have another strong piece of evidence in 

 favour of the view that they are phylogenetically the oldest 

 members of the group. 



It will have been gathered from the foregoing pages that 

 the general views Avhich I hold respecting Echinoderm mor- 

 phology are essentially those of Johannes MiiUer, as modified 

 and extended by A. Agassiz, Gotte,^ however, has recently 

 put forward some considerations which have led him to 

 adopt precisely the opposite of Miiller's views, namely, that 

 the apex of a Starfish represents the Avhole convex part of 

 an Echinid shell, instead of the apex of the Urchin corre- 

 sponding to the whole antiambulacral surface of the Starfish. 

 Further, Gotte considers the arms of the Crinoids to be 

 homologous with the oral tentacles of the Holothurians, A 

 view similar to this was put forward some time ago by 

 AVyville Thomson,^ and also later by Huxley,^ who seems 

 to have subsequently abandoned it, as there is no mention 

 of it in his ' Anatomy of the Invertebrata.' 



Gotte regards the Echinoderm skeleton as, so to speak, 

 the result of a combination in varying degrees of two modes 

 of radiation which are essentially opposed to one another. 

 One of these systems corresponds in position with the water- 

 vascular trunks, and is thus radial as regards the general 

 symmetry of the Echinoderm type. In the brachiate orders 

 (Starfishes and Crinoids) it forms the skeleton of the arm 

 bases. The other skeletal system, as seen in the Crinoid, is 

 that of the first formed part of the calyx, viz. the basis, on 

 the abactinal surface of the body, together with the oral 

 system of the actinal surface. This system alternates in 

 position with the tentacle-bearing arm bases, and is, there- 

 fore, interradial. It is the more prominent of the two in the 

 young Comatula, in which the basals and orals attain a 

 considerable size before the appearance of the radials. 



1 Loc. cit.,' pp. 627—630. 

 - 'Edinburgh New Phil. Journal,' loc. cit., p. 115. 

 ^ "Notes ou the Invertebrata," 'Medical Times and Gazette,' Aug. 14, 

 1875, pp. 173, 174. 



