18 



at a suitable distance from each other, extend the whole skin like a sheltering roof above 

 the said internal organs. 



These very strong calcareous transverse beams, which undoubtedly, in spite of their 

 very different appearance, are to be referred to the same category as the isolated calcareous 

 particles in the dorsal skin of the disc (both formations corresponding to the calcareous net 

 of the normal star-fisk) are also (see Tab. I, fig. 12, Tab. II, fig. 1, 2 & 4) very conspicuous 

 on the exterior as more or less strongly projecting transverse ribs. Like the calcareous 

 particles in the skin of the disc, these transverse ribs give also issue to spines, which are al- 

 ways arranged in a single transverse row along the rib. With respect to the extent, number 

 and form of these transverse ribs, there is considerable variation. Usually they are distinct 

 on all the baSal fourth part of the arm. and from 10 to 14 in number. They are always placed 

 at some distance from each other; so that generally there are 2 vertebrae between each of 

 them and its neighbor. As to their form, they are often more or less sinuous, more rarely 

 forked on one of the sides. Sometimes they are incomplete or interrupted at parts; and 

 this seems to be the rule with those furthest from the disc; the sides only being developed, 

 and one of the above-named soft transverse ridges forming the continuation '^n the middle. 

 They are, as noticed, firmly attached on each side by a ligament to the ambulacral skeleton; 

 and their somewhat enlarged extremity gives at this point of connexion issue to an unusu- 

 ally long spine directed outwards and sidewise, which we will call the marginal spine, as it 

 proceeds from the margin of the arm. These marginal spines are however, as will be seen, 

 not confined to the basal part of the arm, but are continued also along the edges of the 

 whole remaining part of the arm, even to its extreme point; and on closer examination we 

 shall find that they are also here not attached to the ambulacral skeleton itself, but to 

 small calcareous pieces, connected with it at regular intervals, which may therefore be con- 

 sidered as a sort of rudiments of the so strongly developed calcareous ribs on the basal part. 



[1 The spines. 



The various spines projecting over the surface of the body, issue partly from the cal- 

 careous particles deposited in the interior stratum of the skin, partly direct from the ambu- 

 lacral skeleton itself; andihey are always more or less movably connected with the same. Their 

 form and length vary in the different parts of the body; but the proper calcareous spine is 

 always enveloped as in a sheath by the exterior stratum of skin, which usually forms a bag 

 projecting far beyond the point of the spine, and, like the rest of the cuticular sheath, 

 covered with numerous pedicellaria?. 



All the spines articulated with the ambulacral skeleton, issue from the ventral side, 

 and are here attached to the adambulacral plates which form the lateral boundaries of the 

 ventral furrows, for which reason also the corresponding spines in other Asterida3 are usu- 

 ally called furrow-papillae or furrow-spines. They are situated in the Brisinga alternately, 



