DEVELOPMENT OF ANTEDON (COMATULA, LAMK.) EOSACEUS. 705 



that the cord which traverses the length of the Arras between the canal just mentioned 

 and another canal that overlies it, and which was regarded by Professor MtJLLER as a 

 nerve, really belongs to the Reproductive apparatus. But it will also be shown that a 

 regular system of branching fibres proceeding from the solid cord (described by Professor 

 MilLLEB as a vessel) that traverses the axial canal of each calcareous segment of the Rays 

 nd Arms (§§ 34, 45) is traceable on the extremities of the muscular bundles ; and reasons 

 will be given for regarding these fibres as probably having the function of Nerves, though 

 not exhibiting their characteristic structure. 



2. Of tlie Component Pieces of the Skeleton. 



20. In accordance with the nomenclature now generally adopted in describing the 

 Cbinoidea, I shall distinguish the pieces of the skeleton as belonging (1) to the Stem, 

 (2) to the Calyx, and (3) to the Arms. 



21. Stem. — As the body oiAntedon, although attached by a stalk during the Crinoidal 

 stage of its existence, becomes free by separation from this as it approaches maturity, its 

 skeleton might not be expected to include any representative of the Crinoidal stem. 

 Such a representative, however, unquestionably exists in the central protuberant plate" 

 to the convex surface of which the Dorsal Cirrhi are attached, whilst its ventral surface 

 gives support to the First Radials. The real nature of this plate was discerned by the 

 acute Naturalist to whom we owe our first scientific acquaintance with the Crinoid type ; 

 the following description being given of it by Mr. J. S. Miller (Crinoidea, p. 129): — 

 " At the base of the subglobose body of the Comatulce exists a pentagonal unperforated 

 plate, slightly convex externally, and concave on the inside. It is analogous in situation 

 to the first columnar joint of the Crinoidea; but as it is not required to transmit the 

 passage to the alimentary canal (no prolongation of the column existing in this animal), 

 it is mthout central perforation"'. The true homology of this central plate was also dis- 



' The above description must be taken in connexion with, that of the " Pelvis" in the succeeding paragraph. 

 " On the margin of the pentagonal plate rests an annular plate resembling the rim of a basin, and forming with 

 the former a basin-Hke cavity, which appears to occupy the place of the pelvis of the Pentacrinite. At the 

 upper edge this pelvis-like plate is pentagonal, having between each of the angles a horseshoe-like impression 

 for the insertion of the first costal joint. Extenially numerous auxiliary side arms [the ' dorsal cirrhi ' of 

 Lamarck and most succeeding authors] proceed from the pelvis-like plate, which, when they are broken off or 

 removed, show the exterior surface of the plate marked with concave impressions (the points of their insertion), 

 each surrounded by a hexagonal rim more or less perfect, according as their situation is near the central or the 

 marginal circumference of the plate." It would hence appear that Miller was led by his idea of tlie homo- 

 logies of the centro-dorsal plate to regard it as composed of two pieces, one forming the bottom of the basin, 

 the other its sides and rim. This, however, is certainly not the case ; since not only does the most careful ex- 

 amination of the plate in its mature form show no trace whatever of such separation, but its unity is clearly 

 shown by the history of its development. — The latter part of the above-cited description of the " pentagonal 

 plate" is based on the idea that the canal which passes down the centre of the Crinoidal stem lodges a con- 

 tinuation of the Digestive cavity. This has been shown by Professor J. MfriLER to be quite untrue as regards 

 the recent Fentacrinus Caput- Medusce ; and I shaU hereafter show that nothing of the kind obtains in the 

 Pcntacrinoid stage of Comatuh. At a time when so little was known of the Anatomy of the Echinoderm type, 



5d2 



