67 



Firstly as regards the ambulacra, the present species is in this respect distinguished 

 in a remarkable degree from the other species of Brisinga. While in this latter the number 

 of arms in all the specimens hitherto examined, has been always without exception 11, an- 

 swering to the specific denomination ..endecacnemos", I have found in the specimens of the 

 present species examined by me a remarkable inconstancy in this respect; so that it is very 

 difficult to say what number should be regarded as the normal number. 



Of the 22 more or less perfect specimens examined by me, 7 were 9-armed ; 9, 10- 

 armed; 5, 11-armed, and 1, 12-armed; hence it appear that 9-armed, 10-armed and 11-armed 

 specimens are about equally numerous; while it seems that specimens with more than 11 

 arms are very rare; since out of 22 specimens, only one was found with 12 arms. I do not 

 however consider it improbable that, on examination of a greater number, it may be found 

 that the inconstancy in the number of antimera ranges within still wider limits; and that 

 specimens may be found with fewer than 9 and more than 12 arms. The specimen deli- 

 neated by W. Thomson (1. c.) has thus, according to the drawing, no less than 13 anus. 

 If this is really correct, then the number of arms in the present species would vary at least 

 from 9 to 13: a very remarkable inconstancy in an animal of radiate construction. This 

 variation in the number" of antimera or ambulacra, does not however appear to be the 

 result, as might be supposed, of a subsequent divergent development. On the contrary, I 

 have reason to presume that the number of arms in each individual is already fixed in the 

 embryo or larva body of the star-fishes, and continues unchanged during the whole life of 

 the animal; for I have never seen on the disc the smallest sign of any new formation of 

 ambulacra. All were, independently of the greater or smaller development of the arms, 

 exactly of the same appearance and uniform development. Neither are the younger indi- 

 viduals distinguished by having fewer arms than the older ones. The little tiny young Bri- 

 singa above described had 10 arms distinctly developed, just as in most adult specimens: 

 and of the 3 young individuals with a disc-diameter of 10 — 12""", which I have had occasion 

 to examine, only one was 9-armed, while the other two were 10-armed. 



As regards the length of the arms in proportion to the diameter of the disc, it is 

 far from being constantly the same in all individuals; nay even in one and the same indi- 

 vidual, we may often find a considerable difference in the length ot the arms, which in 

 great part is caused by the regeneration of new arms taking place at various times, in stead 

 of the older arms previously detached from the disc. 



With respect to the number and arrangement of the calcareous transverse ribs on 

 the basal part of the arm, we find an infinity of variations: and even in one and the same 

 individual there will often be some difference on the different arms. The number in adult 

 specimens seems to vary at least from 9 to 14. The number of the soft transverse bands 

 covered with pedicellariae and arranged in the intervals between these calcareous ribs, is 

 likewise very variable. Sometimes there is only one (see Tab. II, fig. 2) sometimes — and 



9* 



