THE DEGENERATION OF ARMOUR IN ANIMALS 131 



Mollusca, the tendency to extinction, owing to increased rigidity 

 and excessive accumulation of armour, is even more marked ; 

 the efforts of the race towards attaining flexibility are more 

 than once evidenced by forms which evolved in response to 

 new and altered conditions of environment. 



The accompanying genealogical diagram (after Bather l ) 

 shows graphically how, in his opinion, the two great sections 

 of Crinoids, the Monocyclica and the Dicyclica, both produced 

 forms in the Palaeozoic era which displayed an increasing 

 tendency towards rigidity of structure. Although exception has 

 been taken to his separating into monocyclic and dicyclic groups 

 the Inadunata (those forms in which the arms are quite free 

 from the calyx, which are regarded by Wachsmuth and Springer 

 as a single, homogeneous group) and to his treating the 

 Camerata as having a double genetic origin from each of these 

 groups, yet for the present purpose this difference of opinion 

 among experts is immaterial. In fact, in discussing the evolution 

 of Crinoids and their tendency to greater rigidity (Camerata) 

 or flexibility (Flexibilia), the differences become even more 

 greatly emphasized by taking in turn the monocyclic and 

 dicyclic divisions. 



Thus, in the Camerate section of the Monocyclica and of the 

 Dicyclica the brachial plates of the arms and their first branches 

 became rigidly incorporated in the calyx and the ambulacrals 



1 " Echinoderma," vol. iii. of Lankester's Treatise on Zoology, 1900, from 

 which many of the following facts have been taken. 



