346 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Fixation by the centrale results in a great strain being exerted, either by the 

 constant motion of the arms or by the motion caused by waves or by other organisms, 

 along the sutures between the centrale and the first circlet of plates, and between 

 the individual plates of that circlet. This is met in such genera as Holopus (figs. 

 514, 517, pi. 1) by a solid welding together of all the calyx plates, resulting in a 

 solid calcareous mass with no possibility of motion except in the tegmen or in the 

 arms. But most commonly the strain is relieved by a combination of two proc- 

 esses, the fixed base elongating into a column with many joints, giving flexibility, 

 and the plates of the lowest circlet slipping inward over the ventral (upper) surface 

 of the topmost columnal (the primitive centrale), so that they are supported by a 

 considerable portion of their outer surfaces instead of by their edges only, and 

 the weak vertical suture between the centrale and the plates of the lowest circlet 

 is eliminated. The horizontal sutures, by which the plates meet end to end while 

 lying parallel to the axis of the stem, are perfectly capable of supporting a reason- 

 able weight by a mere thickening of the adjacent plates, and thus are not altered. 



This arrangement is satisfactory for a crinoid with comparatively short arms 

 on a semirigid column, but if the column becomes very rigid, or if the arms become 

 very loug, it is evident that a great strain will be brought upon the sutures between 

 the plates of the lowest circlet (now horizontal or nearly so) and those of the circlet 

 just above; this is met by a change in the second circlet of plates by which they 

 become braced on the first, just as the first became braced on the topmost columnal, 

 and thus cease to form a part of the calyx wall. This has happened in the penta- 

 crinites. In the comatulids fixation is by means of very numerous cirri all arising 

 from a single ossicle, which act collectively as grappling hooks (figs. 306, 307, p. 265), 

 and is much more firm than in the case of the pentacrmites, the crowns of which 

 sway at the summit of a long, broadly spiral flexible stern. The comatuhds, there- 

 fore, must solidify the calyx still further to meet the conditions of life under which 

 they live, and they have done this by reducing all the calyx plates to a horizontal 

 position and welding them solidly together by close suture or by synostosis. 



Atelecrinus typically does not cling to foreign objects as do most of the coma- 

 tulids, but rests upon the ooze on a circular disk formed by the long, nearly straight 

 cirri. It is thus not subject to any great calyx strain, and has retained its basals 

 in the condition in which we find them in the pentacrinites. 



The purely mechanical origin of the reduction of the calyx plates must be con- 

 stantly borne in mind, as it may easily be seen that a comparatively small change 

 in habit may result in an enormous change in the form and in the proportions of 

 the calyx plates which is of but minor systematic significance. An excellent 

 example of this is seen in the genus Marsupites (fig. 565, pi. 7), which superficially 

 does not in any way resemble the recent comatulids, though in reality it is very 

 closely related to them. 



With this reduction circlet by circlet of the calyx, it naturally follows that, as 

 can be seen in the young developing Antedon, the internal organs are progressively 

 extruded more and more from the calyx, until they come to lie on and to be 

 protected by, the lower segments of the postradial series (fig. 74, p. 127). The 



