ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 201 



represents the peristomial area of the Starfish ; while the arms 

 of the latter are structures peculiar to it, and therefore not 

 comparable to any parts of an Urchin. This is of course a 

 complete reversal of Miiller^s idea that the apex of the 

 Urchins represents the whole antiambulacral dorsum of the 

 Starfish. So far as the actual origin of the ambulacral 

 plates is concerned, there is some ground for this hypothesis 

 of Gotte's ; for Agassiz has shown that the external limestone 

 plates forming the test of a Sea-urchin are all directly 

 derived from the simple system of limestone plates on the 

 abactinal surface of the embryo. But he has also shown 

 that the reticulated network of the actinal and abactinal 

 surface of a Starfish, together with the ambulacral and inter- 

 ambulacral plates, have the same origin, which tells strongly 

 against the truth of Gotte's hypothesis. 



Other considerations too, demonstrate the general correct- 

 ness of Mliller's views. The corona of the Urchins is the 

 result of an extreme vertical elongation of that portion of 

 the equatorial zone of the larva that lies between the peris- 

 tome and the radials (=oculars), which last remain in close 

 contact with the basals ( = genitals). It may, therefore, be 

 termed extra-radial. In the Crinoids the radials also remain 

 in close proximity to the basals, as in the Urchins, but the 

 equatorial zone is very much extended horizontally. It is 

 supported by a dorsal skeleton, Avhich is built up gradually 

 upon the radial circlet, and is also therefore extra-radial, or 

 heiier , supra-radial. In the Starfishes, on the other hand, there 

 is a similar lateral extension of the equatorial zone which 

 forms the ventral surface of the arras, but their dorsal 

 surface is altogether unrepresented in the other Echino- 

 derms, and may be called intra-radial. For it is the result 

 of a separation of the radials from the rest of the calyx 

 by a constant formation of new spines at the base of each 

 ray, so that instead of their resting directly on the basals 

 there is a long interval between the two rings of plates. 

 The dorsal surface of a Starfish is therefore strictly compara- 

 ble to the apex of an Urchin or the calyx of a Crinoid (as 

 far as the first radials), as was supposed by Miiller. But be- 

 tween the arms of a Starfish or Crinoid and the ambulacra 

 of an Urchin, there is only a general homology, not one 

 which can be followed in much detail.^. 



The views advanced above may perhaps be better under- 

 stood by the help of the accompanying simple diagrams. If 

 the basal, radial, and oral circlets of the young Crinoid be 



» Compare Agassiz, 'Revision/ pp. 758—760, and 'North American 

 Starfishes,' pp. 87, 88. 



