THE CRINOIDEA 



then the first on the left, and so on, may be smaller than the other 

 branch. Thus there arises a main trunk giving off smaller branches 



FIG. XX. 



Specialisation of arm-branching. 1, a non-pinnulate, regularly dichotomous arm (isotomous) ; 

 2, a less regular dichotomous arm, a type common in Cyathocrinoidea ; 3, 4, two stages in 

 the evolution of unilateral heterotomy ; 5, 6, 7, three stages in the evolution of bilateral 

 heterotomy, culminating in pinnulation. For other types, see Fig. CX. 



right and left alternately. These smaller branches may themselves 

 undergo a similar process, and so form armlets (ramuli) borne by 

 the main arm -branch 

 (minus). When the 

 ramules cease to branch 

 themselves, and are reg- 

 ularly placed on alter- 

 nate sides of successive 

 brachials of the main 

 branch, they are called 

 piunulae, and the arm is 

 " pinnulate." This pro- 

 cess of evolution has 

 been phylogenetically 

 traced in Botryocrinus 

 (Fig. XXL), while the 

 primitively dichotomous 

 origin of the pinnules 

 may be seen in the 

 developing Antedon. 

 The pinnule, as P. H. 

 Carpenter said, is an 

 arm in miniature ; it 



rliflfovo in nrktViinrr Vmt bear ramuli, of which 'all except the proximal one are un- 



QinerS in notnmg \ >Ut branched) and are almost reg ularly disposed. In B. pin- 



Position from the Small nulatus the ramuli have become regular unbranched pin- 



' . . , nules. (After Bather, 1891.) 



end-branches of a simple 



dichotomous arm ; but, in a pinnulate arm, it differs from 

 the ramus by the restriction to it of the fertile portions 



8 



FIQ. XXI. 



Evolution of pinnules in Botryocrinus. 1, B. ramosus, 

 a species in which each arm has two rami bearing branched 

 ramuli ; 2, B. decadactylus, a species in which the two rami 



