290 PHYLUM ECHINODERMATA. 



of the stem of Pentacrinus (Fig. 208) is attached to the sub- 

 stratum, but this attachment may be lost by the breaking 

 of the stem or other cause, and the animal may move freely, 

 anchoring itself by its stalk cirri like an Antedon. Locomotion 

 in the case of Antedon is effected by the movement of the arms. 

 It is possible that some of the extinct forms, e.g. Marsupites, 

 and Uintacrinus were free. 



Food is brought to the mouth by ciliary currents along the 

 arm grooves in the form of floating organisms. The principal 

 external parasite is Myzostoma (vol. I, p. 492), which infests the 

 disc, stalk and arms often in great numbers ; it may be free or 

 enclosed in a cyst in the body wall. 



The class Crinoidea is divided into five orders, Larviformia, 

 Fistulata, Camerata, Flexibilia, and Articulata. The living 

 forms are all contained in the order Articulata. 



The grouping of Crinoids into Palaeocrinoids comprising the Larvi- 

 formia, Camerata, Flexibilia and Fistulata, and Neocrinoids (Articulata) 

 must be given up. As in the case of Echinoids the living forms interdigi- 

 tate with the extinct. The most important character of extinct forms 

 not found in the living is the pentameral asymmetry of the calyx brought 

 about by the presence of interradial plates in the posterior interradius, 

 but this is by no means shared by all the Palaeocrinoids (Platycrinidae, 

 Calyptocrinidae, etc.) and indications of it are present in the jointed 

 process of the posterior interradius of the living Thaumatocrinus. As 

 stated above, the forms grouped under our order Articulata are, with 

 the exception of this character, a very fair sample of all Crinoids that are 

 known. 



The following are some of the characters which were considered to mark 

 the ancient forms. 



In the Palaeocrinoids in which some of the arm plates (above the radials 

 of the calyx) enter into the composition of the calyx, the plates which 

 are so absorbed are united by interradially placed plates (interprimi- 

 brachs etc.) ; in Neocrinoids when the lower arm joints are so taken into 

 the disc, there are usually no interradial plates between them, the arm 

 plates being in contact or the intervening body wall flexible ; though this 

 does not hold in all cases, e.g. Apiocrinus, Guettardicrinus, Uintacrinus. 

 The perforation of the radials by the axial cord is also said to be a char- 

 acteristic of the Neocrinoids. In most Neocrinoids with divided arms, the 

 axillary is the second primibrach (Metacrinus, Plicatocrinus excepted), 

 while in Palaeocrinoids the axillary varies from the radial to the Oth 

 primibrach this is the only important character which separates the 

 Palaeocrinoids Erisocrinus, Phialocrinus and Stemmatocrinus from 

 Encrinus. 



Affinities. The Crinoidea stand far apart from the other 

 classes of living Echinodermata. Their important distinctive 



