ME. P. II. CAEPENTEE OX THE GENUS ACTINOMETEA. G9 



ill all, the length and diameter of which are gradually increased by the deposition of new 

 calcareous material at their extremities and upon their outer cylindrical surface. 



Dr. Carpenter 1 has shown that, at or about the period at which the suppression and 

 metamorphosis of the embryonic oral and basal plates begins, " the production of new 

 calcareous segments in the stem appears to cease, and a remarkable change begins to 

 show itself in the one on which the calyx rests. Instead of increasing in length, its 

 original annular disk augments in diameter, becoming convex on its lower surface and 

 concave on its upper, and it extends itself over the bottom of the calyx in such a manner 

 as to receive in its concavity the apices of the basal plates ;" and that portion of its 

 under surface which extends itself beyond the segments whereon it rests begins to be 

 marked by small tubercles, which are the origins of the dorsal cirrhi, while it also 

 " augments not only in absolute but in relative diameter, extending itself over the dorsal 

 or outer surface of the basal plates, which at the time of the detachment of the body 



from the stem are almost entirely concealed by it A second whorl of cirrhi is now 



developed, after the same manner as the first, between the latter (with which it alternates 

 in position) and the base of the calyx (pi. xlii. fig. 3), and a third whorl generally makes 

 its appearance before the detachment of the Pentacrinoid, so that the young Antedon 

 possesses ten cirrhi in different stages of advanced development, and from one to five still 

 rudimentary." 



After the detachment of the young Antedon from its stem a minute five-rayed per- 

 foration is visible for a short time in the somewhat depressed central portion of the 

 inferior surface of the centrodorsal piece. It is the remains of the original " communi- 

 cation between the cavity of the basin-shaped plate and the central canal that is still 

 left in the upper segments (at least) of the stem. This perforation, however, is very 

 soon closed up by an extension of the calcareous network, so that no trace of it remains 

 visible either internally or externally." 



We have thus seen that the centrodorsal piece " first presents itself in a form which 

 nowise differentiates it from the other joints of the cylindrical stem, but begins to take 

 on an extraordinary increase in a peripheral direction at the time when the dorsal cirrhi 

 first sprout forth, aud thenceforward remains in closer connexion with the calyx than 

 with the rest of the stem, from which it separates itself so soon as the dorsal cirrhi are 

 sufficiently developed to serve for the attachment of the animal." Each of these cirrhi 

 receives a " sarcodic thread, which proceeds from the sarcodic axis contained within the 

 cavity of the basin, and runs along the central canal of the cirrhus to its termination." 



New cirrhi gradually appear between those previously formed and the base of the 

 calyx, and each receives a peduncle of sarcodic substance from the central axis ; and 

 " since the arrangement of the whole aggregate of such peduncles is distinctly verti- 

 cillate, the want of a definite plan in the grouping of the cirrhi on the external surface 

 of the centrodorsal plate seems attributable to their very close apposition." 



During the whole period of the growth of the centrodorsal basin there is a " progressive 

 exuviation of the first-formed cirrhi from within outwards, concurrently with the develop- 

 ment of new ones near the margin, those cirrhi which surrounded the summit of the 



1 Phil. Trans, he. clt. p. 732. 



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