viri EGHINODERMATA—MORPHOIJJGY OF SKELETON 371 



bases of the free arms they pass into the ambulacral grooves or food 

 grooves of the tegmen calycis, which run to the mouth. 



The arms, when divided, as they normally are, usually branch 

 dichotomously ; occasionally, however, they give off alternating 

 branches, which may again branch alternately. In most Crinoids the 

 arms and their branches carry, at the sides of the ambulaci^al grooves, 

 closely crowded and alternating processes ; these are rod-shaped, and end 

 in a point, and are known as pinnules. The skeleton of these pinnules 

 resembles that of the arm, and, like the latter, is jointed. The pinnules 

 may best be regarded as the ultimate branchings of the arms, and it is 

 very probable that in the numerous paheozoic Iinuluiuita that have no 

 pinnules the last branches of the arms fulfilled their functions. 



The bracliial skeletons in the Crinoidea are always direct continuations of the 

 radials of the ajiical cajxsiile. The first plate which follows the apical radial radially 

 must be considered, morphologically, as a brachial or ossicle of the arm, although 

 it is only rarely {e.g. in the Inadunata) a free ossicle. The terms introduced to 

 denote the various orders of brachials are almost as numerous as the writers them- 

 selves. It is the clearest plan to speak of them as brachials of the first, second, 

 third, etc. orders, or as primary, secondary, tertiary, etc. brachials. Such a plan 

 was, however, soon found too cumbrous for practical purposes, and was supplanted 

 by the terms costals, distichals, palmars, and postpalmars. To these terms, how- 

 ever, consideral)le exception may be taken, and it seems simplest to adopt the intel- 

 ligible and congruent terms primibrachs, secuudibmchs, tertibrachs, etc., which are 

 capable of indefinite extension, and are readily symbolised as IBr, IIBr, etc. 



It has already been jjointed out, in the section which treated of the perisomatic 

 plates of the calyx, that brachials are incorporated into the dorsal cup in many, 

 indeed, in the great majority of Crinoids. We can accordingly distinguish free 

 brachials from fixed brachials, the latter being those which have become peri- 

 somatic plates of the dorsal cup. The first brachials to be so incorporated are 

 naturally the primibrachs, the next the secundibrachs, the tertibrachs may also then 

 become fixed. In describing the skeleton in detail, therefore, the terms fixed primi- 

 brachs, fixed secundibrachs, etc. are used, and the number of these plates in each 

 arm is given. The arrangements found in the various divisions of the Crinoidea 

 in this respect have been already briefiy described in the preceding section. That 

 of tlie Inadunata is the simplest, since in them the arms are free from their very 

 bases (hence the name), the first primibrach being a free ossicle of the arm ; the most 

 complicated condition is that of certain Came rata (Actinocrinoidcn, etc.), in which 

 the brachials of several orders are incorporated into the calyx, and being connected 

 liy interradials, interdistichals, etc. lend to the dorsal cup its rich plating. 



In branched arms those joints above which the divisions or branchings take place 

 are called axillary, e.g. we have an axillary costal, axillary distichal, or, as they 

 may more simply be called, primaxil, secundaxil, etc. (lax, llax, etc.). 



With regard to the distril)ution of the pinnuh^, it is the rule, at least in modern 

 Crinoids, that the axillary joints never carry pinnulre, and that where two joints 

 are connected by syzygial sutures or by ligaments, pinnuhu are also wanting on the 

 lower or proximal joint. 



There are three ditterent ways in whiili the free brachials may be arranged. The 

 arms may consist of a single row of joints, the brachials being superimposed in a 

 single series with parallel surfaces of contact (uniserial). Again, the joints may 

 " alternate," if they are M'edge-shaped, and if, in the row, the thick and tlie thin sides 



