172 ECHINODEEMATA. 



that possess them ; but, from their small size and general appearance, 

 they seem but ill adapted to such an office. 



(450.) The skeleton or calcareous framework imbedded in the skin of 

 the Asteridae is by no means the least remarkable part of their structure : 

 this consists of several hundred pieces, variously disposed, and for the 

 most part fitted together with great accuracy, being either firmly sol- 

 dered to each other, as we have seen them to be in the formation of the 

 calcareous box that constitutes the central portion of Ophiura, or united 

 by ligaments, so as to allow of a considerable degree of motion to take 

 place between them, as in the rays of Ophiura, Gorgonocephalus, and 

 other Asteroid forms. 



(451.) In the generality of Star-fishes, the arrangement, and indeed 

 the entire character of the calcareous plates, differs materially in dif- 

 ferent parts of the body ; and even in the same species considerable 

 modifications are observable. In the coriaceous integument forming 

 the dorsal parietes of the animal, the pieces in many cases seem rather 

 to be represented by calcareous granules disseminated through the in- 

 terior of the skin ; in other cases they are arranged in lines anasto- 

 mosing with each other in all directions, so as to represent, when the 

 skin is dried, a rude network of solid particles, upon the exterior of 

 which the various cutaneous appendages already noticed are sustained. 



(452.) It is, however, upon the ventral aspect of the Asterias that 

 the skeleton assumes its most perfect development ; the floor of every 

 ray is made up of a continuous series of detached pieces, or vertebrae, 

 as they are generally called, fitted to each other and united by a strong 

 ligamentous substance, so as to form a succession of joints, upon which 

 the flexibility of the ray depends. The pieces around the mouth con- 

 stitute a strong circular framework enclosing the oral aperture, from 

 which, as from the centre, the rest of the skeleton radiates. The joints 

 forming the floor of the ray succeed to this ; these are partially repre- 

 sented in fig. 90, where, the soft parts having been removed from the 

 ray marked 6, their general arrangement is displayed. 



(453.) The vertebrae, as they are called, when thus exposed are found 

 to be individually composed of several pieces, and each is articulated by 

 oblique facets to those which precede and follow it, a kind of union 

 that admits of considerable motion, and provides for the flexibility of 

 the ray, so as to render it capable of executing movements requisite for 

 the purpose of progression, or of seizing prey. The connexion of the 

 vertebrae is effected in such a manner, that between each pair of calca- 

 reous plates minute orifices are left, which in the entire state of the ray 

 are seen to be arranged in a quadruple series ; these holes give passage 

 to the locomotive suckers, and from this circumstance have been named 

 the ambulacral holes, while the furrows, seen upon the ventral surface, 

 into which they open are designated the ambulacral grooves. 



(454.) The suckers, which at the will of the animal are protruded 



