14 P. HEEBERT CAEPENTER. 



the left vaso-peritoneal tube of the larva. These are inter- 

 radial in position and correspond to the basals, which are 

 developed in the same manner around the right vaso-peritoneal 

 tube in Urchins, Stellerids, and Crinoids. The oral plates of 

 the Crinoid larva primitively close in the peristome, but 

 eventually separate so as to expose the mouth and tentacles to 

 the exterior. But in the Palseocrinoids the mouth remained 

 subtegminal ; while a central plate corresponding to the dorso- 

 central appeared within the circle of orals (apical dome plates 

 of Wachsmuth). In some genera, e.g. Symbathocrinus and 

 Haplocrinus, the oral or actinal system remained per- 

 manently in this condition ;^ but in more complex types radial 

 and interradial dome plates were developed outside the orals, 

 in many cases corresponding plate for plate with those forming 

 the calyx below. There is, however, even within generic limits, 

 much less regularity in the size and grouping of the plates 

 forming this oral or actinal system than there is in the 

 abactinal calyx plates. This will be readily understood if we 

 remember that the former were not traversed, as the basals and 

 radials were, by motor nerves proceeding outwards from a 

 central organ. Sometimes, indeed, as in Strotocrinus, the 

 oral plates and primary radial dome plates become indis- 

 tinguishable from the numerous other minute plates which 

 form the vault. This condition finds a close parallel in many 

 Ophiurids in which the whole surface of the disc is crowded 

 with minute scales, and the rosette of " primary " plates 

 cannot be made out ; though they are visible enough in young 

 individuals (PI. I, figs. 12, 13) just as the proximal dome 

 plates (orals) are in the vault of the young Strotocrinus or 

 Megistocri nus. 



The extremely Crinoidal appearance of the apical plates of 



> I am greatly indebted to Mr. Wachsmuth both for new information respecting 

 the structure of the summit in these two genera, and also for the opportunity 

 of examining many excellent specimens illustrating this and other points. 

 But his interpretation of the plates is somewhat diiferent from that here given, 

 which is based on general embryological considerations. These will be found 

 detailed at length in this Journal, vol. xix, pp. 180 — 184. 



