96 



scopic spines — quite in contrast to what is the case in the Br. coronata — whereby the sur- 

 face of the arms acquires a peculiar rough, or, as it were, finely chagrined quality (see fig. 11). 

 In the interior, at the base of the arms, these spines stand most closely; further out on the 

 arms they are arranged in larger irregular spaces, mostly in a transverse direction and with 

 bare intervals. All these minute spines, of which there is no trace to be seen in the Br. 

 coronata, rise (see fig. 12) from small thin circular perforated calcareous plates in the skin; 

 and among these spiniferous plates there are besides a great number of still smaller disc- 

 like plates, which are without spines, but from which spines are no doubt subsequently 

 gradually developed. By these innumerable small cuticular plates in connexion with the 

 numerous calcareous ribs, the cuticle of the arm in the Br. endecacnemos acquires, on the 

 whole, a considerably firmer consistency than in the Br. coronata; a fact which stands in the 

 closest connexion with the very different arrangement of certain internal parts (the organs 

 of generation) in the latter species, as will be further noticed in the sequel. With regard 

 to the proper arm-spines, they are nearly like those of the Br. coronata. They are pro- 

 portionally even somewhat longer than in this species, especially the so-called exterior 

 furrow-spines which are attached in the middle of the adambulacral plates (comp. fig. 11). 

 The number of the interior furrow-spines is moreover also rather variable in different indi- 

 viduals, and seems usually to increase with age. The marginal spines are in the basal part 

 of the arm very small; but outwards they increase rapidly in length; so that in the middle 

 of the arm they become 3 times as long as its transverse diameter. They are also movably 

 articulated to the outer extremities of the calcareous ribs, which form very distinct plate- 

 shaped enlargements furnished with an evident joint (see fig. 13); in the exterior part of 

 the arm, where the calcareous ribs are wanting, they are attached to special small plates 

 (dorsal marginal plates). At the point of the arm there is an organ of sense quite like 

 that in the Br. coronata, and likewise arched over by a peculiar calcareous plate furnished 

 with long marginal spines (see fig. 17). 



In quite young specimens the disc (see fig. 10) is very thin and transparent; 

 and the exterior set of vertebrae come forward here in the same manner as in equally 

 small specimens of Br. coronata. Of the dorsal spines, a great many have still the 

 peculiar bristle-like form which is described in the young of the Br. coronata, and which 

 in all probability is a remnant from the embryonic or larva-state. On the other hand the 

 madreporic body already shews completely the form and arrangements which are charac- 

 teristic of the species. The arms are, in proportion to the diameter of the disc, conside- 

 rably shorter than in adult specimens, evenly tapering from the very base, and as yet 

 without any evident calcareous ribs. 



In the structure of the ambulacral skeleton there is certainly a great resemblance 

 to the Br. coronata, nevertheless, on a more minute investigation, it will be easy to dis- 

 cover, even here, several specific differences. 



The oral ring (tig. 7) is, in conformity with the greater height of the disc, of a 



