68 ME. P. H. CAEPENTEE ON THE GENES ACTINOMETEA. 



dorsal cirrki, he described this species as having a column of three segments, and gave 

 a sectional figure in support of his statements L , which shows three segments below the 

 circlet of first radials, each bearing a row of cirrhi. It is doubtful how far this figure can 

 be relied on as accurate, though I have occasionally met with somewhat similar appear- 

 ances myself. Goldfuss, like Miller, was unacquainted with the remarkable condition 

 of the basals in this type; and as the "pelvis" described by Miller in Comatula was 

 rightly regarded by him as representing apart of the stem of Penlacrinus, he was led 

 to believe in the absence of basals in Com. mediterranea, though he found them in the 

 Com. multiradiata [Comaster), in which he described the rudimentary column as con- 

 sisting of only a single segment. Miiller was led, by his comparison of the component 

 pieces of the calyx of Comatula with those of the calyx of Pentacrinus aster/a (Captd- 

 medasce), to recognize the very close general correspondence between tbem ; and he 

 pointed out 2 that the presence of cirrhi at the upper end of the stem of the Penta- 

 crinoid larva on the one hand, and on the centrodorsal plate bearing the first radials 

 of the young Comatula on the other, indicate that the latter is comparable to the stem 

 of PentacriuHs, which bears the cirrhi in verticils separated by longer or shorter in- 

 tervals. This view of Midler's was pretty generally recognized as the true one, and it 

 was adopted and greatly strengthened by Wyville Thomson and Dr. Carpenter, who 

 came to precisely the same conclusions upon developmental grounds. The former 

 defined it as representing a " coalesced scries of the nodal stem-joints in the stalked 

 Crinoids," namely, of those joints which bear whorls of cirrhi, so that " the centro- 

 dorsal plate with its dorsal cirrhi in Antedon is the homologuc of the stem with its 

 cirrhi in the stalked Crinoids." Ludwig 3 also, while referring to the development of 

 the centrodorsal as the enlarged uppermost stem-segment, speaks of it as " ein zusarn- 

 mengedrangter, oberer Stengelabschnitt, in welchem das verkalkte Gewebe keine Sonde- 

 rung in untereinander^eleo'ene Glieder erfahren hat." 



(§47) The first rudiment of the stem of the Pentacrinoid larva as described by 

 Wyville Thomson 4 consists of a series of delicate calcareous rings forming a curved 

 line, which passes backwards from beneath the centre of the lower ring of plates, the 

 embryonic basals. Within each of these is formed a hollow sheaf of parallel calcareous 

 rods, united together by short anastomosing lateral branches ; the upper one of these, on 

 which the lower edges of the basal plates rest, soon becomes considerably wider and 

 thicker than the rest. " Daring the earlier stages of the growth of the Pentacrinoid it 

 is simply a circular band of the ordinary calcified areolar tissue, enclosing a sheaf of the 

 peculiar fasciculated tissue of the stem, gradually enlarging, with a central aperture con- 

 tinuous with the bore of the tube-like stem-joints." 



This ring is subsccjuently developed into the permanent centrodorsal piece ; but the rudi- 

 ments of the first dorsal cirrhi do not appear around its lower contour until very much 

 later. New rings are developed immediately beneath it, until there are fifteen or sixteen 



1 Petref. German, p. - " Lau des Pentacrinus, p. 10. 



3 "Zur Anatomic ties BJiizo rinus lofot, nsis, "Zuitschr. fiir wiss. Zool. Bd. xxix. p. 127. 



4 '• On the Embryogcny of Antedon rosaceus (Lin ck) (Comatula rosacea of Lamarck)," Phil. Trans. 1805, pp. 53G, 

 5o7. 



