64 



a certain size, they shoot out, at right angles and at regular intervals, short lateral processes 

 which grow together with the nearest calcareous staves (fig. 29, 30). Accordingly as the origi- 

 nal number of staves may have been greater or smaller, there is at last formed a more or 

 less complicated latticed process which is the proper calcareous spine; while the basal disc 

 forms the enlarged joint-surface whereby the spine is subsequently articulated with the 

 adambulacral plates. When the spine has attained a certain size, there begin to appear, 

 at the end of the cuticular sheath, some few cell-like bodies projecting more or less from 

 the same, and having small calcareous granules contained in them (fig. 31) ; this is the first 

 development of the pedicellaries. All the other spines are formed precisely in the same 

 manner. The marginal spines on the growing arms and the spines situated on the terminal 

 enlargement of the arm appear nearly at the same time. The last named (fig. 32) are 

 usually remarkable for their particularly slender shape, and for being originally formed of 2 

 or 3 strongly elongated calcareous staves connected with each other at regular intervals by 

 horisontal intermediary bars. Of the spines belonging to the calcareous ribs of the arms, 

 no trace is to be observed before the arm has attained a considerable development. They 

 shew themselves then first as insignificant processes which however grow rapidly, and in 

 adult specimens exhibit the circlets of spines, repeated at certain intervals around the dorsal 

 side of the basal section of the arm, so specially characteristic of this species. 



Among the spines situated on the dorsal side of the disc, we may sometimes ob- 

 serve in very young specimens a spine here and there of a quite unusual form strongly 

 elongated and slender almost like a bristle. In the little young animal above described all 

 the spines seem to have been of this nature; but most of them were already, when the 

 animal was captured, more or less injured and some quite broken oft'. I succeeded however 

 with some certainty in getting an idea of their original appearance. They are, as has been 

 said, very strongly elongated and slender, quite hyaline and finely latticed like the ordinary 

 spines, but very distinguishable by not being smooth, shewing several consecutive distinctly 

 enlarged sections, each of which shoots out in a certain number of sharp points (fig. 34). 

 The whole spine is hereby distinctly echinulated at regular intervals from the very base; 

 while on the ordinary spines only the extreme end exhibits some few microscopic points. 

 These peculiar spines belong, as it appears, only to the embryonic state; they afterwards 

 disappear entirely, and are replaced by the usual spines which form themselves in the same 

 manner as the arm-spines. 



g. Development of the pedicellaries. 



On the dorsal side of the very earliest foundations of the arms, as also on the 

 cuticular sheaths of the spines which afterwards grow out of the arms, there appear, as has 

 already been noticed, a greater or less number of these peculiar small organs, but mostly 

 very far from being developed or in activity. We have thus here occasion to study the first 



