DEVELOPIMEXT OF ANTEDON (COMATTTLA, LAJMK.) ROSACETJS. 705 



that the cord wliich traverses the length of the Arms between the canal just mentioned 

 and another canal that overlies it, and which was regarded by Professor Mi'ller 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 

 MuLLER 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 tlie function of Nerves, though 

 not exhibiting their characteristic structure. 



2. Of the Component Pieces of the Skeleton. 



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

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

 (2) to the CalijA\ and (3) to the Arms. 



21. Stem. — As the body oi ylnteclon, 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 Comatulm exists a pentagonal unperforated 

 plate, slightly convex externally, and concave on the inside. It is analocjous 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 without central perforation"'. The true homology of this central plate w^as also dis- 



' The above description must be taken in connexion with that of the " Pel%-is" 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-like cavity, which appears to occupy the place of the pelvis of the Pentacrinite. At the 

 upper edge this' pelvis-like plate is pentagonal, ha\ang between each of the angles a horseshoe-like impression 

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

 Lamakcs; 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 

 marguial circvunference of the plate." It would hence appear that Miller was led by his idea of the homo- 

 logies of the ccntro-dorsal plate to regard it as composed of two jjieces, 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 imity 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. MtLLKR to be quite untrue as regards 

 the recent Pentacrinus Caput-Medusa; ; and I shall hereafter show that nothing of the Idnd obtains in the 

 Pentacrinoid stage of ComatuJa. At a time when so little was known of the Anatomy of the Echiuodcrm type, 



5d2 



