744 DE. W. B. CARPENTEE ON THE STEUCTTJRE, PHYSIOLOGY, AND 



exogenous additions made to the calcareous reticulation, whilst the radiating sarcodic 

 twigs for whose passage they are destined are prevented from gaining access to them by 

 the iraperforateness of the inner wall. And looking to the numerous examples which 

 we have in the development of Antedon, not only of the entire absorption of some parts 

 of the skeleton, but of the complete remodelling of others, — as shown in the growth of 

 this very plate, and still more remarkably in the formation of the "rosette" from the 

 original "basals" (^§ 89, 90), — I cannot but regard it as more probable that these 

 cirrhal twigs, as they are successively put forth in the cavity of the basin and impinge 

 upon its inner wall, have the power of forcing (as to speak) a passage for themselves, 

 by inducing an absorbent action in that part of its substance that lies in their course 

 towards the external surface. 



89. Basal Plates. — The mode in which the "rosette" (§ 35) is formed by the remo- 

 delling and subsequent coalescence of the five Basals, and in which the sarcodic exten- 

 sions of the central axis which are transmitted through the Eadial plates to the Arms and 

 pinnules, come to lie on the dorsal or external face of the " rosette," whilst they origi- 

 nally lay along the ventral or internal face of the basals, is certainly the most curious 

 feature in the developmental history of the skeleton of Antedon. — It has been already 

 shown (§ 76) that the cribriform plate of which each basal at first entirely consisted, is 

 so much thickened by endogenous growth during the later stages of Pentacrinoid life, 

 that the radial sarcodic cords come to be entirely invested by calcareous reticulation ; 

 and the floor of the ventral cavity shows no inequality as we pass from the central por- 

 tion formed by the basals to the peripheral formed by the I'adials (Plate XLI. fig. 5). 

 Very soon after the detachment of the young Antedon, however, a remarkable change 

 begins to show itself in the basal pentagon, which is now entirely concealed externally 

 by the extension of the centro-dorsal basin over its dorsal surface ; for the cribriform 

 film of which each basal plate was originally composed, and which still fonns its ex- 

 ternal layer, now undergoes absorption, especially where it covers in the radial prolon- 

 gation of the axis, so that the central space left by the incomplete meeting of the valves 

 of the basal pentagon, is extended on its external aspect into five broad rays (Plate XLII. 

 fig. 7, b), though on its internal or ventral aspect, where it is bounded by the last-formed 

 portion of the endogenous reticulation, it shows no corresponding increase (Plate XLII. 

 figs. 2, 6, b). 



90. This removal of the older and outer part of each basal plate by absorption, and the 

 consolidation of the newer and inner by additional calcareous deposit, go on at a rapid 

 rate ; so that in specimens whose size and general development show but little advance 

 upon the earliest Antedon type, we find the basals already modelled into such a form 

 that their coalescence will produce a somewhat unshapely " rosette." In Plate XLI. 

 fig. 3, a, is shown the dorsal aspect of one of the basal plates in which the removal of 

 the external layer has been carried so much further, that what is now left of it consti- 

 tutes only a kind of thickened margin along those sides of the plate which are received 

 between the First Eadials; and by an extension of the same process along the median 



