63 



and simultaneously with the formation of the ambulacral skeleton. Even in the very earliest 

 foundations of the arms (see Tab. VI, fig. 15) there may be observed in the dorsal cuticle, 

 immediately behind the terminal enlargement, 3 longitudinal rows of calcareous particles (r) 

 forming themselves in the same manner as the ambulacral and adambulacral plates. These 

 appear in the beginning (next to the terminal enlargement of the arms) quite separate, in 

 the form of the usual regularly perforated circular discs; but in the subsequent progressive 

 ramification of the calcareous staves connecting them, the 3 calcareous plates lying in a 

 transverse row approach each other little by little until they coalesce, and thus in connexion 

 form continuous transverse bands. At each end of these transverse bands there is deve- 

 loped already at a very early stage, a spine (p) which is the marginal spine. In the be- 

 ginning, such calcareous particles in the dorsal skin develop themselves constantly from- the 

 basis of the terminal enlargement. There are thus formed more and more of the calcareous 

 transverse bands. But after that the arm has attained a certain length this formation ceases. 

 The arm continues however to grow by a constantly continued formation of new joints, which 

 gradually force the first formed joints nearer to the base of the arm. Hence it is easily 

 explained that these calcareous particles, which afterwards form the transversal spined cross- 

 ribs, although they originally proceeded from the extremity of the arm, at last become far 

 removed from it and confined only to the basal section. 



It appears that besides the above-noticed calcareous particles specially imbedded in 

 the skin, there are also a number of calcareous plates which belong to the cuticular skele- 

 ton, and which subsequently enter into intimate connexion with the ambulacral skeleton, 

 namely the wedge-plates and dorsal marginal plates standing in connexion with the skeleton 

 of the disc, also the 2 pairs of marginal plates which are found at the base of the arms, 

 and finally the calcareous plate which covers the terminal organ of sense. This last (fig. 

 13 15, pi) is formed already very early, simultaneously with the ambulacral skeleton, as a 

 net of thin calcareous staves ranged in a half-circle, whence subsequently thin spines take 

 their issue (fig. 25). The dorsal marginal plates of the arms appear on the contrary not to 

 form themselves until considerably later (see fig. 16, r, r). 



/ 



f. Development of the spines. 



The first spines which appear distinctly on the arms are, as before mentioned, the 

 exterior furrow spines. They form (see fig. 13, p) first, close to each side of the ambulacral 

 skeleton, a regular row of short cylindrical cuticular processes, each answering to a separate 

 adambulacral plate. In the interior of these processes there appears first a little angular 

 calcareous granule (fig. 26), which then, in the usual manner, begins to shoot out radiating 

 processes whereby there is formed a simple perforated disc (fig. 27). From the middle 

 of one surface of this disc, there arise a certain number of thin calcareous staves, close 

 together, which rapidly grow in length (fig. 28). After these calcareous staves have attained 



