134 



THE CRINOIDEA 



In some genera, and especially, as Jaekel has suggested, in 

 those exposed to rough water or currents, the stem shortened con- 

 siderably while its attachment was preserved (e.g. Eugeniacrinidae, 

 Fig.CXX., Cupressocrinus). Cotylederma, Eudesicrinus, and Cyathidium 

 are fossil genera, Holopus (Fig. CXXL), a recent genus, in all which 

 the stem is reduced to a mere mass of stereom cementing the cup 

 to some solid object. 



Although the Crinoidea are the Echinoderms in which the 

 Pelmatozoan habit has had most effect on the anatomy, yet they 

 present a constant tendency to relinquish the attached mode of 

 life and to lose that typical organ, the stem. So early as the 

 Ordovician, stems are found that during the life of the animal 

 were separated from the root, and became attached to other 

 objects, either by the remaining cirri, or 

 by winding around them. Often the 

 distal end of the stem formed a coiled 

 support like a serpent's tail (e.g. Acantho- 

 crinus rex, Jaekel, 1895). In some Silu- 

 rian genera (e.g. Calceocrinus, Mastigo- 

 crinus) stems have been described that 

 were rounded off at the distal end during 

 life. In Herpetocrinus the stem was rarely 

 if ever attached by anything except its 

 cirri ; while in the species described by 

 Hall as Brachiocrinus nodosarius, the stem 

 ends distally in a bulb. In the Devonian 

 Myrtillocrinus the stem ended in a four- 

 fluked grapnel ( = Ancyrocrinus, Fig. LI.). 

 Similar detachment took place in many 

 Carboniferous and Mesozoic crinoids ; the 

 recent Isocrinus ( = Pentacrinus) is known 

 to change its place, probably by swimming 

 with its arms, and the lower surface of 

 the distal columnal is "smoothed and 

 rounded" (Wyville Thomson, 1873). 



Addiction to this habit led to the 

 gradual shortening of the stem ; in 

 stages have been described by P. H. 

 Carpenter (1882), from a stem of seventy columnals over 50 

 mm. long, down to a single ossicle, the proximale (Fig. LIL). 

 Continuance of this process led to the evolution, along many 

 different lines, of crinoids that are generally described as un- 

 stalked, and for which older writers were wont to erect an order, 

 Astylida. These fall into three groups : First, those in which a 

 portion of the stem remains, becoming compressed and fused, with or 

 without the infrabasals, into a cirrus-bearing compound ossicle, to 



Fio. LI. 



Grapnel of Myrtillocrinia (after 

 Hall). 



Millericrinus Pratti all 



