378 p. HERBERT CARPENTER. 



cannot be a true homology between a single plate developed in 

 the centre of the abactinal system and a group of five others 

 disposed in a ring around that centre ; nor can I admit any 

 homology between either the dorsocentral plate or the under- 

 basals and a cirrus-bearing top stem-joint, viz. the centrodorsal of 

 Comatula, as supposed by Agassiz, Loven, Claus, Neumayr, 

 and Wachsmuth. According to the best observations^ the 

 centrodorsal is developed in a ring which encloses the right 

 peritoneal tube near its proximal end/ while the dorsocentral of 

 the Echinozoa occupies the middle of its distal end ; and since 

 there is a plate developed in the embryo Crinoid in this very 

 position, viz. the terminal plate at the base of the stem (fig. 

 Ill, 1), I have been led to regard it as homologous with the 

 dorsocentral of the Echinozoa, a view which I have been glad 

 to find adopted by my friend Dr. Liitken.^ An additional fact 

 in its favour is as follows : — The dorsocentral of the Asterid 

 larva appears in the immediate neighbourhood of the blastopore,* 

 and it is only removed from this position in the Crinoid larva 

 by the few delicate rings of limestone which represent the rudi- 

 mentary stem-joints. When it first appears it is doubtless even 

 closer to the blastopore, but we have no iaformation respecting 

 its very earliest development. 



1 have therefore seen no reason to modify any of the opinions 

 which I published four years ago respecting the homologies of 

 the Crinoidal calyx in the other Echinoderms. At that time 

 I knew of no instance among the Echinozoa of a proximal ring 

 of radially situated plates around the dorsocentral, and I was 

 therefore led to suppose that the under basals of Marsupiies, 

 Micrinus, Extracrmus, and many Palseocrinoids are unrepre- 

 sented in the other Echinoderms. Now, however, I believe 

 them to have been discovered by Ludwig in the young 

 Ampkiura squamata, together with representatives of the Cri- 

 noidal basals, which gives me another argument against his 

 views respecting the homology of the Ophiurids and Asterids. 



Ludwig's study of the development of the Ophiurid skeleton^ 

 has thrown a curious light on the observations of some of his 



^ E.g., those of Sir Wyv. Thomson, Dr. Carpenter, and M. Sars. 



2 Gotte's attempt to disprove the annular origin of the centrodorsal is 

 not a satisfactory one. Even supposing that he is right, and that the 

 centrodorsal does originate out of five primitively separate elements, it 

 cannot, on the views advocated above, represent the primitively simple 

 dorsocentral of the Echinozoa. 



^ Dyreriget, * En Haand. og Laerebog til brug ved hojere Laerea^stal- 

 ter,' Kj0benhavn, 1882, p. 597. 



^ ' Zeitschr. f. wiss. Zool.,' Bd. xxxvii, p. 60. 



'" " Zur Entwickelungsgeschichte des Ophiureuskeleltes," ' Zeitsch. f. 

 wiss. Zool.,' Ed. xxxvi, pp. lSl-200, Taf. x, xi. 



