450 AUSTIN HOBART CLARK, 



the posterior arms may not reach to one third the length attained by the 

 anterior. The sexual organs are .primarily equal and equivalent on every 

 arm, and they are physiologically (though not anatomically) confined to the 

 arms with no connection with a circumoral ring; the water vascular and 

 other radial systems are also primarily equivalent and equal on every arm, 

 but they are of necessity in intimate connection with the central circumoral 

 structures; they are thus on the posterior arms handicapped in the proper 

 performance of their functions by an increase in length equal to one half 

 of the periphery of the disk over that of the same structures on the an- 

 terior arms. There is no such handicap in the case of the genital organs 

 of the posterior arms which thus, under the stimulation of heat, are easily 

 able to outstrip the other structures in their development and to inhibit 

 their growth through a monopolization of most of the available body- 

 building material. 



When the mouth is central and the ambulacral grooves converge to 

 it in five furrows of equal length all the structures on every arm are 

 equally balanced and there is no opportunity for the genital organs to 

 develop abnormally at the expense of the others. In those species of the 

 Comasteridae in which the mouth is central, and in the eudocyclic forms 

 living in warm water, all the arms are therefore equal and equivalent. 



Cold water appears to hinder the development of the genital struc- 

 tures at least as much as it does the other structures, so that exocyclic 

 species found in cold water have all the arms, though short, equal or 

 nearly so. 



In a paper on the composition of the crinoid skeleton (Proc. U. S. 

 Nat, Mus., XXXIX, p. 487) I stated that "the crinoids of the deep seas 

 and from the colder regions have more delicate and more open skeletons 

 than those from comparatively shallow water in the tropics, and it there- 

 fore seems most probable that cold has the effect of retarding the dis- 

 position of inorganic matter by the animals to a far greater degree than 

 it retards the general body development". At first sight this would appear 

 to be directly contradictory to the view that the short stout arms of 

 tropical forms living in cold water were acquired as a result of the 

 coldness of their habitat. We are dealing, however, with two entirely 

 different things. The arms of a criuoid may be short and stout as a 

 result of a retardation of the elongation of the arm which forces the 

 brachials to acquire an undue lateral extension, and yet the skeleton laid 

 down may be more delicate and open than usual. This is the case, for 

 instance, in Antedon pehisus. The two conditions are entirely independent, 

 though investigation will probably show that they often occur together. 



The coraatulids of the deeper water and of the arctic and antarctic 



