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BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



tendency toward a flattened disk-like form, giving the maximum resistance to wave 

 motion and to external influences generally; sometimes one, sometimes the other 

 of these tendencies gets the upper hand, depending upon the local conditions of the 

 chosen habitat of the particular type; often the form is modified, as in the so-called 

 irregular urchins, by the assumption of locomotion in a definite direction, which 

 immediately results in the elongation of the body in this direction. 



The mechanical factors involved in the habit of the stalked crinoids necessitate 

 the close cohesion of the basals in the form of a closed 

 ring, so that the infrabasals are permanently divorced 

 from the radials, the following plates in the ambulacral 

 series. Similarlyresponsetourgentmechanical exigency 

 has necessitated the incorporation of the radials (which 

 correspond to the ambulacrals immediately surrounding 

 the peristomal in the echinoids) as a closed ring in the 

 calyx just beyond the basals. 



Purely mechanical considerations therefore require 

 that the dorsal portion of the most primitive crinoid 

 calyx, which entirely encloses the visceral mass, shall be 

 composed of closed rings of five plates each, these rings 

 being two in number, as two rings offer much greater 

 resistance to outside forces than three or any greater 

 number. These two rings will be the circlet of radials 

 upon which the arms are borne, and the circlet of 

 basals, situated between their bases. The infrabasals, 

 which lie on the border line between two (half) rneta- 

 meres and are in a way space fillers serving to increase 

 the area of the apical region, will not appear. 



If by any chance circumstances should arise through 

 which the strict operation of these mechanical laws were 

 obstructed or held in abeyance we should expect that 

 at once there would appear in the crinoid calyx addi- 

 tional plates which, far from being new structures or 

 structures appearing for the first time, in reality would 

 be ancient structures long dormant in the crinoid 

 organization awaiting only the relaxation of the strict 

 and closely circumscribed mechanical limitations to 

 reappear in their ancient fashion. 

 It is not at all inconceivable that a new animal type suddenly called upon to 

 meet entirely new and very stringent mechanical or oecological conditions, to 

 respond to mechanical forces entirely different from any which its ancestors have 

 been called upon to meet in the past, should first appear in a somewhat extreme 

 form, a form characterized by the complete dominance of the response to the 

 mechanical or oecological factors involved, coupled with an equally complete reces- 

 sion of the characters which, through a knowledge of its antecedents, we should 

 expect it to exhibit; and, later, as a result of the gradual adjustment to the new 



FIG. 110. LATERAL VIEW or THE PROXI- 

 MAL PORTION OF A SPECIMEN OF PSA- 



ING THE PROPORTIONATE SIZE OF THE 

 CENTRODOBSAL AND ARMS. 



