xiv] 



CRINOIDEA 



361 



present and they have shield-shaped ends. They have long ampullae 

 which project into a thin web or swimming membrane which sur- 

 rounds the front part of the body like a collar. No gill- trees. The 

 stone-canal opens on the external surface. 



Class V. CRINOIDEA. 



The last group of the Echinoderms is termed the Crinoidea (Gr. 

 a lily), animals long familiar to collectors of fossils under the 

 name of Lily-encrinites. They differ from other Echinoderms in 

 that from the centre of what corresponds to the upper or aboral 

 surface of other orders, there springs a 

 jointed stalk by which the animal is 

 moored to the substratum. 



Animals of this type were much 

 more common in past times than now. 

 Large masses of limestone are actu- 

 ally made up of their skeletons. The 

 modern order of Crinoidea includes a 

 few species surviving at great depths 

 in the ocean, and about the mode of 

 life of these we know little. There 

 are, however, besides these a number 

 of species not sharply marked off from 

 each other assigned to a family, the 

 Comatulidae, containing two genera, 

 Antedon and Actinometra, which live 

 at moderate depths in the ocean and 

 which have been thoroughly studied. 

 These however are exceptional in that 

 they break off from the stalk when 

 they are mature and swim about by 

 muscular movements of the long arms. 

 The stump of the lost stalk forms a 

 knob called the centro-dorsal ossi- 

 cle, which is provided with grasping 

 processes called cirri, by means of which the animal can temporarily 

 attach itself. 



We may select for our type the common Antedon rosacea, 

 which can easily be captured by the dredge at moderate depths. 

 This animal has a small disc and ten extremely long arms. It 



FIG. 169. Antedon acoela, Car. 

 A young individual x 1J. After 

 Carpenter. 



