THE DEGENERATION OF ARMOUR IN ANIMALS 133 



times, this primitive stock budded off a fresh series of flexible 

 forms with pinnate arms (Apiocrinus, Eugeniacrinus, Bourgueti- 

 crinus, and the existing Holopus among fixed forms ; Thaumato- 

 crinus, Antedon, Atelecrinus and Actinometra among free- 

 swimming forms). In the shallow-water genus Holopus the 

 stem is no longer calcified but is represented by a leathery 

 outgrowth from the calyx. Antedon and Actinometra are world- 

 wide in their distribution and testify also by their numbers 

 to the success attained by the adoption of a free and unattached 

 existence in the adult stage. 



Although in the Distincta section of the Dicyclic Inadunata 

 (Dendrocrinus, Botryocrinus, Scaphiocrinus and the Triassic 

 Encrinus) there does not seem to have been any successful 

 attempt at evolving free-swimming forms, yet in the later, 

 Mesozoic, part of the main Dicyclic stock — the Articulata, fixed 

 forms of which were represented by Pentacrinus and Bat/iycrinus, 

 both with living species — there were several such attempts, 

 e.g. Marsupites and Uintacrinus. In the latter especially the 

 theca was flexible and comparatively large and the arms 

 extremely long. Even in the living Pentacrinus (Isocrinus) the 

 stem ceases to be permanently attached to the substratum in 

 the adult stage, for the stem breaks across, the animal swims 

 about freely with active movements of the arms, just as in 

 Antedon and from time to time it attaches itself temporarily 

 by means of a considerable remnant of its stem and its stem- 

 cirrhi. Here we merely find a repetition of what has occurred 

 again and again from the earliest times ; either the end of the 

 stem became smooth and rounded-off (the Ordovician Calceo- 

 crinus) or modified into an anchoring grapnel (the Devonian 

 Myrtillocrinus) or else it is prehensile and able to curl itself 

 round a point cTappui (Acanthocrinus). 



In the light of modern research and consequently of more 

 natural and exact methods of classification, entire classes of 

 animals, which were formerly held to have become entirely 

 extinct at the close of the Palaeozoic era, have now been shown 

 to have left descendants, which flourished in subsequent periods, 

 even down to the present day. Thus the old classification of 

 Palaeocrinoidea has long since been proved to be inadequate 

 and incorrect, since the Mesozoic Crinoids, as already indicated, 

 were descended from and closely allied to Palaeozoic families 

 which had retained sufficient plasticity of organisation to vary 



