ME. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 71 



more peripherally placed cords proceeding from the upper leaflets, which enter the later- 

 developed cirrhi. Ludwig 1 regarded them as rudimentary structures in Ant. rosacea, 

 because he found them in Ant. Eschrichtii to enter the more centrally placed cirrhi, which 

 are not removed, hut persist throughout life, as already mentioned. It will be evident, 

 however, from the facts stated above, that these cirrhus-cords, which end on the dorsal 

 surface of the plate, are not rudimentary structures, but the proximal ends of more 

 complete cords that have undergone a retrogressive metamorphosis, which in Ant. 

 Eschrichtii is not carried so far as in Ant. rosacea. 



These facts all tend to strengthen the view first expressed by Wyville Thomson, that 

 the centrodorsal piece represents a coalesced series of the nodal stem-joints of the stalked 

 Crinoids. In Ant. Eschrichtii six or even more rows of cirrhus-sockets may be traced on 

 the hemispherical surface of the " Knopf," each row corresponding to a node in the 

 stem of Pentacrinus. Even in those Actmometrce in which only one row of sockets is 

 visible externally, the composite character of the centrodorsal piece is indicated by the 

 verticils of degenerate cirrhus- vessels at the base of the chambered organ (PI. VIII. 

 figs. 3, 7), and by the partially obliterated openings on the central part of the floor of 

 the centrodorsal cavity (PI. V. figs. 2, 7, II; PI. VI. figs. 3, S, 10, 15, 17, it). 



Sometimes, indeed, the " Knopf " may actually assume a more or less columnar 

 form, as in the specimen of Ant. macrocnema mentioned in sect. II, and in the genus 

 Solanocrinus ; in both of which three or four alternating rows of cirrhi are visible. 

 In these forms we may reasonably suppose that the columnar centrodorsal was de- 

 veloped by the enlargement of the uppermost stem-segment on which alternating whorls 

 of cirrhi successively appeared, just as in Ant. celtica (PI. IV. figs. 1, 6, 8), Ant. rosacea 

 (fig. II), and Act. robusta (PL V. fig. 15), but not in such numbers as to obscure the 

 alternate arrangement (p. 69). 



(§ 19) Gotte-, to whom we owe a series of most beautiful observations on the de- 

 velopment of the water-vascular system and perivisceral cavity of Comatula, has recently 

 questioned the accuracy of those observations of Wyville Thomson and Dr. Carpenter, 

 according to which the uppermost of the embryonic stem-segments developes into the 

 centrodorsal piece, and lias also attacked the view that it may possibly in some 

 cases arise from the fusion of two or more stem-segments as represented in Goldfuss's 

 figure. 



His description of its origin is as follows : — " Die Anlagen der Centrodorsalplatte sind 

 schmale, aber doch netzformige Skeletstreifen, welche gleichzeitig mit den Basalia an 

 deren unteren Biindern entstehen und die obersten, noch eng zusammengedriingten 

 Stielgliederanlagen umgeben (fig. 13). Es ist daher spater, wenn diese Stelle sich 

 verschmachtigt, nicht immer ganz leicht, jene Anlagen der Centrodorsalplatte von den 

 obersten Stielgliedern zu unterscheiden. Beachtet man jedoch, dass sie anfangs das 5.-8. 

 Stielglied, und nachdem diese abwarts gcriickt sind, das 9., 10., 11., 12. u. s. w. umschlies- 

 sen, was Thomson iiberhaupt nicht erwahnt, so kann man sich der Ueberzeugung 



1 Beitriige, loc. cit. p. 69. 



2 " Vergleioh. Entwickelungsgesoli. d. Comatula mediterranca." Archiv, f. rnikrosk. Anat. Bd. xii. 1876, p. ij'.tl. 



