218 Dr. W. B. Carpenter on the Structure [Jan. 20, 



clusters of prehensile cirrhi that functionally replace the Crinoidal stem, 

 though homologically they correspond to the nodal verticils of cirrhi borne 

 by the stem of Pentaerinus, which themselves take on the prehensile 

 function when, as not unfreguently happens, this stem breaks off just 

 beneath the nodal segment. And I further showed that in the early 

 Pentacrinoid stage of Antedon the centro-dorsal segment is not distin- 

 guished from the ordinary segments of the stem, and that its enlargement 

 commences with the development of the cirrhi, of which there is at first 

 only a single whorl, but of which two complete verticils at least are present 

 when the young Antedon drops off the stem. Again, I showed that in 

 the early Pentacrinoid stage the Basals, not the first radials, rest upon 

 the centro-dorsal segment ; and I pointed out how, with the metamor- 

 phosis of the circlet of basals into the " rosette," and the progressive 

 enlargement of the centro-dorsal plate, the first radials come to form 

 the central basis of the calyx, resting on the infolded lip of the centro- 

 dorsal. I refer to this now, because it helps greatly in the comprehension 

 of the real import of the curious organ contained within the centro-dorsal 

 basin of the adult Antedon ; for there is no such basin in the Pedunculate 

 Crinoids, whose calyx rests upon the uppermost segment of the stem, 

 which segment, instead of being the largest, is the smallest, being the 

 latest formed, while the basis of the calyx is formed by the thickened 

 and solidified Basals. Hence it seems clear that the extraordinary deve- 

 lopment of the highest segment of the stem into the centro-dorsal basin, 

 which is characteristic of the mature Antedon, is connected with the 

 multiplication of the prehensile cirrhi which extend themselves from its 

 dorsal surface. The number of these cirrhi, as I formerly showed, is 

 very variable ; and I gave reasons for the belief that there is a successional 

 production and exuviation of these appendages, at any rate during the 

 period of growth. 



"When, by the action of dilute acid, the calcareous network of the 

 centro-dorsal plate has been dissolved, so as to give access, by dissecting 

 away its soft animal basis, to the contents of the cavity, we find it to be 

 almost entirely occupied (as seen at g, fig. 3) by a curious hollow, 

 slightly pentangular organ (figs. 4, 5), which was described by Miiller as 

 a single-chambered heart. Prom the angles of the lower or dorsal part 

 of this organ there proceed numerous slender cords (I, fig. 2), which 

 pass through the calcareous wall of the centro-dorsal basin into the 

 cirrhi ; whilst from the upper part of its peripheral wall are given off 

 five large radiating cords (fig. 3, I, m), which pass upwards and outwards 

 through the canals formed by the inflexions of the " rosette," each then 

 divaricating into two branches, which enter the two adjacent orifices on 

 the internal face of each pair of first radials, as described in my former 

 Memoir. All these cords were described by Miiller as vessels; I am 

 satisfied, however, that they are not tubular, but solid. — On making a 

 transverse section of the hollow organ it is found to contain five chambers 



