20 



surfaces are in the periphery connected by tendinous muscular ligaments, whereby the spines 

 can be moved in various directions. The thick sheath formed by the exterior stratum of 

 the skin (cuticle) which envelopes the spine, terminates always in a somewhat enlarged bag, 

 often prolonged far beyond the extremity of the spine, and not unfrequently curved in various 

 ways, sometimes nearly at right angles (fig. 18). 



The very long spines, attached along the sides of the arms, which with reference to 

 their position we have called marginal spines, are also of exactly the same structure. These 

 spines are however, as already noticed, not attached to the ambulacral skeleton, like the fur- 

 row spines, but, in the basal part of the arm, to the extremities of the calcareous trans- 

 verse ribs, and further out, to peculiar small calcareous plates connected by ligaments with 

 the adambulacral plates. They form (see Tab. II, fig. 1 & 2) on each side a single row 

 from the base of the arms to their extreme point. 



If we consider these marginal spines together with the furrow-spines, the arms (see 

 Tab. I, fig. 14) will appear furnished on each side with 4 rows of spines, of which those in 

 the exterior row (the marginal spines) (1) are the longest. As the marginal spines do not, 

 like the furrow-spines, correspond in number with the adambulacral plates, but most fre- 

 quently are to the latter only in proportion of 1 to 3, their number is also always smaller 

 than that of the furrow-spines belonging to a single row. At the base of the arms they 

 are only slightly developed, but increase rapidly in length outwards until towards the middle 

 of the length of the arm, where they are 34 times as long as the arm is broad. Further 

 outwards they then gradually decrease in length, until they become rudimentary again to- 

 wards the point (see Tab. II, fig. 6). 



The dorsal spines of the arms, are, as noticed, only attached along the calcareous 

 transverse ribs, and thus confined to the basal part alone. 



They form here always only single rows, and usually increase in length outwards, 

 but without attaining to nearly the length of the marginal spines and of the exterior furrow- 

 spines. They seem to be of somewhat stouter structure than the other spines of the arms, 

 with a less developed cuticular sheath and fewer pedicellarise. 



The spines attached to the dorsal skin of the disc are however rather different. 

 These are quite short, situated vertically, all about of the same length and close together, 

 whereby the whole of the dorsal side acquires, as it were, a hairy appearance, (see Tab. I, 

 fig. 4, 6). They are of cylindrical form with a bluntly rounded extremity. This form is 

 however chiefly attributable to the thick cuticular sheath in which they are enveloped. On 

 applying a solution of potass, which renders the exterior cuticle transparent, the interior 

 proper calcareous spine (see fig. 8, 9 b) will appear attached by a somewhat enlarged base 

 to one of the calcareous pieces (tig. 9 a) imbedded in the interior stratum of the skin. It is 

 finely reticulated, narrowest in the middle, while the extremity is somewhat enlarged, termi- 

 nating in a greater or less number of fine points (fig. 9, 10, 11). 



