59 



the two rows, which line is the ambulacra! nerve that forms a well defined club-shaped 

 enlargement '(b) at the base of the organ of sense (a). Of spines no trace is yet to be 

 seen. Only after that the nascent arm has increased somewhat more in length, there may 

 be observed on each side of the water-feet a single row of regularly arranged sinuosities in 

 the skin, wherein the calcareous particles begin to develop themselves (fig. 13 p). These 

 are the growing furrow-spines. Nearer to the dorsal side there may be seen a similar row 

 of bag-like enlargements in the skin, inclosing calcareous particles in a state of progressive 

 development, but more irregularly arranged. They represent the growing marginal spines, 

 and on those situated nearest to the base a few pedicellariaB have already begun to develop 

 themselves (see fig. 15 p), The pedicellarise which appear in the dorsal skin have in the 

 mean-time developed themselves further, and now begin to be arranged in distinct raised 

 transverse stripes or bands. The rudiments of water-feet have increased considerably in 

 number; and those placed nearest the base have already acquired a cylindrical form and 

 commenced their activity (fig. 13w). The terminal organ of sense has during the progress 

 of this development retained approximately its original form and size, but is now partially 

 overhung by a distinct protecting calcareous plate with accompanying spines and pedicel- 

 larise (see fig. 14 & 15). 



The further development of the arms consists now essentially in a successive in- 

 crease of joints or metamera proceeding from the base of the terminal enlargement, with a 

 corresponding increase in the numner of water-feet and spines; while the arm grows rapidly 

 in length (see Tab. I, fig. 4, 5. 6 e, Tab: II, fig. 1 & 2). 



The transverse bands over the dorsal skin, which are covered with pedicellarise, 

 appear now both in greater number and with greater distinctness and regularity; and the 

 marginal spines are more and more elongated, especially at the base. Even after that the 

 arm has attained a length of 30 40 mni there is still no trace to be seen externally of the 

 spined calcareous ribs so characteristic of the full grown animal. Only in a much later stage 

 namely after that the organs of generation have begun to develop themselves, these ribs 

 begin to appear as quite narrow slightly raised calcareous stripes, on which the spines are 

 still only short and indistinct (see Tab. I, fig. 1). The marginal spines have already long 

 ago attained their full development, as also the exterior furrow spines; but the interior 

 furrow spines do not begin to develop themselves until much later; nay, it appears that 

 even after the animal has attained its full development, a constant increase takes place in 

 the number of these small spines ranged nearest to the ventral furrow. The arm which in 

 the beginning was thickest at the base and thence rapidly tapering towards the end, begins 

 now to b*e somewhat instricted at the base and thereby to be more sharply distinguished 

 from the disc; as the dorsal skin lies here close on the subjacent ambulacral skeleton; 

 while at some distance from the base, this skin begins to be more and more elevated above 

 it and stretched to the sides. The characteristic almost fusiform appearance, which the 

 basal part of the arm often exhibits in adult specimens (see Tab. I, fig. 12, Tab. II, fig. 1) 



8* 



