196 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Systematically the primary skeleton is of by far the greatest importance ; the 

 secondary skeleton in certain cases is of very great importance, though usually it 

 is negligible, except for the fact of its non-development; the visceral skeleton has 

 never been employed for systematic purposes, but much more study is needed 

 before we can say that it presents no characters of value. 



The crinoiclal skeleton is formed by a calcareous deposit about or within cer- 

 tain tissues or organs without any material change in the latter, and calcareous 

 deposits are found to a greater or lesser degree throughout the organization of the 

 animals wherever their presence would not be detrimental to the general welfare. 

 The walls of the digestive tube, the mesenteries, and the entire ventral body cover- 

 ing are all more or less calcified, in addition to the large and definite plates included 

 in the cirri, calyx, stem, arms and pinnules. 



This is strikingly illustrated in specimens of recent species where no lime has 

 been deposited in the pinnules or in the distal part of the arm (see fig. 75, p. 128); 

 such individuals appear perfectly able to perform their natural functions, though 

 their appendages are but vaguely divided into segments, and are superficially only 

 comparable to the tentacles of jelly-fish. 



As is well stated by Carpenter, the component pieces of the crinoid skeleton 

 consist of a calcareous reticulation formed by the calcification of an organic proto- 

 plasmic basis hi which numerous nuclei and pigment granules are embedded. This 

 nuclear tissue is in the form of a network around the meshes of which the calcareous 

 material is deposited. The character of the calcareous reticulation varies greatly 

 in different parts of the animal, being much closer at the synostoses and at the 

 syzygies and at the articular surfaces than in the ulterior of the segments. This is 

 at once evident on examination of a longitudinal section of an arm, pinnule or cirrus, 

 the central portion being more or less translucent and the ends chalky white. In 

 many forms the closeness of the calcareous reticulation at the distal ends of the 

 segments results in the more or less complete elimination of pigment from the 

 immediate vicinity of the articulations, so that they stand out white against a dark 

 background and give a banded appearance to the arms, pinnules or cirri. This 

 dense end deposit in the various articulating segments, induced by mechanical con- 

 siderations incident to the exigencies of oscillating motion, does not form a layer 

 of uniform thickness as might be expected, but it takes the form of a cylindrical 

 lens the axis of which is parallel to the fulcra! ridge of the joint face adjacent, 

 beneath which the greatest thickness lies. The fulcral ridges themselves are more 

 dense than any other part of the joint surface, especiaUy the summit, which usually 

 stands out prominently as a vitreous line along an opaque chalky ridge. In the 

 case of synostoses, or of other unions which allow of no specialized motion, the 

 denser layers of the neighboring segments are of uniform thickness and no areas 

 of maximum density occur. Here also the difference between the periphery and 

 the center of the ossicles is usually not so marked, the structure being much more 

 uniform than in the segments between which directive motion takes place. 



In the fully developed Antedon lifida W. B. Carpenter found that the sarcodic 

 base substance of the brachials forms a mere shell, scarcely any trace of it being 



