CLASSIFICATION 589 



Thaumatocrinus, small in Iiliizocrinvs, vestigial or absent in 

 Bathycrinits, and completely absorbed in Antedon. In addition 

 to these main elements, in many species small accessory plates 

 are developed (a) at the sides of the ambulacral groove, over 

 which they can close down (many species of Antedon, Hyocrinus, 

 Holopus, Ehizocrinidae, some species of Pentacrinidae) ; these 

 are " covering plates," and correspond in function to the adambu- 

 lacrals of Asteroidea ; (b) supporting the sides of the groove 

 and corresponding to the ambulacrals of Asteroidea ; l these are 

 " side-plates," and the covering plates articulate with them ; (c) 

 on the surface of the tegmen ; these are the interradial plates, 

 which in Thaumatocrinus alone among recent forms, but in 

 many fossil forms, are continued into the patina, where they 

 separate the radial plates. 



Mode of Branching of the Arms. All modern Crinoids 

 have pinnules, and this, as has already been explained, is due to 

 a suppressed dichotomy. The extent of the suppression deter- 

 mines the number of arms, which varies within the same genus. 



Alimentary Canal. In Hyocrinus there is no dilatation 

 which could be called a stomach ; in Actinometra the mouth 

 is excentric, and the anal papilla occupies the centre of the 

 tegmen. The intestine is elongated, and describes several turns 

 round the papilla before ending in the anus. 



The classification of Crinoidea cannot properly be considered 

 without taking account of fossil forms, but to do so at all 

 adequately is impossible on account of limitations of space. 

 Less regret may be felt because the three specialists in this 

 branch, viz. Bather in England, Springer in America, and 

 Jaekel in Germany, come to fundamentally different conclusions 

 on the subject. If we confine our attention to living forms we 

 may, with P. H. Carpenter, 2 select the stem as the basis of 

 classification. As the method of gaining food is the same in all 

 cases, the Crinoidea have probably split on the method of attach- 

 ment to the substratum. These families^ it is impossible, in 

 view of the greater range of variety in fossils, to dignify them 

 with the name of orders are as follow : 



1 Bather calls the side-plates " adambulacral." The name is unfortunate, as it 

 suggests that the side-plates correspond to the adambulacrals of Asteroidea ; but 

 when the groove is closed the position and function of adambulacrals and covering 

 plates is the same (Lankester's Treatise on Zoology, iii. " Ecliinodermata," 1900). 



2 Challenger Reports {Stalked Crinoids), vol. xi. pt. xxxii. 1884. 



