ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 373 



" This ring, which is subsequently developed into the 

 permanent centrodorsal plate, gradually thickens and becomes 

 more regular in form, maintaining its position at the top of 

 the stem, the lower edges of the basal plates resting on its 

 upper surface. During the earlier stages of the growth of 

 the Pentacrinoid it is simply a circular band of the ordinary 

 calcified areolar tissue enclosing a shaft of the peculiar 

 fasciculated tissue of the stem, gradually enlarging, with a 

 central aperture continuous with the bore of the tube-like 

 stem joints/' 



According to Dr. Carpenter,^ the centrodorsal piece " first 

 presents itself in a form which nowise differentiates it from 

 the other joints of the cylindrical stem." The diameter of this 

 original annular disc gradually increases, while the centre 

 becomes filled up by an inward extension of the calcareous 

 trelliswork from the first formed portion. A narrow axial 

 cavity, the remains of the original right peritoneal sac, per- 

 sists, however, for a long time. This lodges the vascular 

 axis of the stem, and in the young Comatula just set free 

 from the stem a minute perforation may be seen in the centre 

 of the lower surface of the centrodorsal piece, which thus 

 preserves its annular character until the commencement of 

 adult life. This perforation thus forms the communica- 

 tion between the cavity of the centrodorsal piece and the 

 central canal that remains in the upper segments, at any 

 rate, of the discarded stem. In the recent Comatula it 

 is soon closed up by an extension of the calcareous net- 

 work, so that no trace of it remains visible, either internally 

 or externally. But in the cretaceous Comatula, Glenotre- 

 mites, this was not the case, and the centrodorsal piece 

 seems to have preserved its primitive annular character 

 throughout life. 



Sars' researches" on the Pentacrinoid stage of Antedon 

 Sarsii show in the same way that although the centrodorsal 

 piece of the adult Comatula may appear externally to be a 

 solid plate or hemisphere, yet it is really hollow, enclosing 

 a portion of the aboral ccelom, and that its imperforate con- 

 dition is altogether a secondary character. 



This appears to me to be a serious objection to our regard- 

 ing the centrodorsal piece of Comatula as homologous with 

 the central abactinal plate of the other Echinoderms. Simi- 

 lar and similarly placed as they are in the adult animal, the 

 relative positions of the two with regard to the right peri- 

 toneal tube of the larva are quite different. The centrodorsal 



' ' Phil. Trans.,' vol. clvi, p. 706. 



\ • Criuoides Yivauts, ii. Du Peutacrinoide de V Antedon Sarsii,' p. 53. 



