58 



very pale reddish color, and when held up against the light so transparent that several of 

 the interior organs may be distinctly seen through the dorsal skin. 



The dorsal spines are proportionally smaller and more dispersed; and only a few of 

 those attached nearer to the centre may sometimes be distinguished by a peculiar strongly 

 elongated bristle-like form, evidently a relict from the embryonic state; the madreporic 

 body occupies the same characteristic place as in the adult specimens, but is always very 

 small and only slightly convex. The spines attached on the lower side of the disc (to the 

 adambulacral plates) are relatively much shorter than in full grown specimens, and fewer in 

 number; and of the oral spines directed towards the mouth, there are only 2 or 4 developed 

 for each interradial space. 



c. Development of the arms. 



In the little juvenile Brisinga previously mentioned, the extreme points of all the 

 arms were, as already stated, broken off; so that their exact form and length could not be 

 accurately ascertained. I have noticed as characteristic the less distinct demarcation between 

 them and the disc; the entire absence of calcareous ribs and transverse ridges, and the 

 presence of only a single row of spines on each side (the exterior furrow-spines). 



We have however -even in fully developed specimens of the Brisinga the best oppor- 

 tunity for studying the development of the arms, and for tracing their changes to a yet 

 much earlier stage of development than in the young one mentioned. There is always 

 taking place in the Brisinga, as before stated, a continually repeated reproduction of new 

 arms, in place of those previously detached from the disc. But it is here to be remarked 

 that these arms, formed by a sort of germination, develope themselves apparently in a 

 manner somewhat different from that in which the arms originally belonging to the young 

 animals are developed. 



In the earliest stage of development these new arms sprouting from the disc 

 (see Tab. VI, fig. 1 x) have only the appearance of inconsiderable conical processes, issuing 

 nearer to the ventral side from the middle of the radial spaces. But on closer examination 

 the rudiments of the most important parts may already be distinguished. The extremity 

 always exhibits a distinct button-like enlargement, from the lower side of which there pro- 

 jects in a horisontal direction a relatively large cylindrical appendage (b) which is the 

 previously mentioned peculiar terminal apparatus, the first evidently developed organ that 

 appears. On the upper side, and especially on the terminal enlargement of the arms, there 

 appear (see fig. 11, 12, 13) already several pedicellarise developing themselves, but still quite 

 in a cellular form, and distributed without any distinct order over the skin. Below there 

 may be seen very distinctly the rudiments of a limited number (12-14) water-feet (w) in 

 the form of small button-like warts arranged in 2 longitudinal rows, and rapidly increasing 

 in size towards the base of the arm, with a distinctly marked raised line (n) running between 



