132 ECHINODERMATA PELMATOZOA SUB-KINGDOM m 



the form of a small ring-like rosette. Finally, the button-shaped centrodorsal, 

 which is now beset with numerous cirri, detaches itself from the stalk, and the 

 creature becomes capable of independent motion. 



The ontogeny of Antedon reveals the fact that the basals, orals, and stem 

 represent the most primitive skeletal structures, while the radials and brachials 

 are formed at a subsequent period. Similar evidence is afforded by numerous 

 fossil Crinoids, in which the basals and column are very strongly developed, 

 while the radials are mostly of inferior size, and the arms either rudimentary 

 or absent. 1 



Habitat. Existing Crinoids live together in large numbers, and inhabit 

 depths ranging from shallow water to a maximum of about 3000 fathoms. 

 Their distribution, however, is mostly very local. Fossil Crinoids also appear 

 to have been gregarious in habit, and their remains are frequently found com- 

 mingled with those of reef-building corals in Palaeozoic strata. Owing to the 

 extremely delicate constitution of many of the skeletal parts, and the looseness 

 with which the plates and segments are united, the Crinoid organism is by no 

 means favourably adapted for preservation in the fossil state. Perfect crowns 

 are of comparatively rare occurrence ; but, on the other hand, detached joints 

 of the stem and arms are often very abundant, and occasionally form beds of 

 considerable thickness. Crinoidal limestones of greater or lesser extent are 

 met with in numerous formations from the Ordovician to the Jura ; those of the 

 Carboniferous and Muschelkalk (Trochitenkalk) being especially characteristic. 



Classification. The first attempt to construct a classification of the Crinoids 

 was that of J. S. Miller in 1821. Four groups differing in the form and mode 

 of union of the calyx plates were distinguished by Miller, as follows : C. arti- 

 culata, semiarticulata, inarticulate, and coadunata. The classification of Johannes 

 Miiller, in 1841, was based upon a number of differential characters, such as 

 the articular or close suture of the radials, the thickness of the calyx plates, the 

 mobility of the arms, and the plated or coriaceous character of the ventral disk. 

 Two principal groups were recognised : Articulata and Tesselata ; while a third 

 (Costata) was constructed for the reception of the unique genus Saccocoma. 

 T. and T. Austin and F. Roemer adopted the untenable divisions of Stalked 

 and Unstalked Crinoids. 



The importance of Wachsmuth and Springer's investigations on the structure 

 of the calyx, especially of the tegmen, and on the orientation of the stem and 

 its canals in monocyclic and dicyclic forms, cannot be overestimated. Two 

 groups were put forward in their classification of 1879: Palaeocrinoidea and 

 Stomatocrinoidea ( = Neocrinoidea, Carpenter) ; groups which correspond in the 

 main with the Tesselata and Articulata of Johannes Miiller. This classification 

 was subsequently abandoned, and a new one proposed for it in 1888, in 

 which four principal orders were recognised, as follows : Camerata, Inadunata, 

 Articulata, and Canaliculata ; the latter being nearly identical with the 

 Articulata of Miiller. The Inadunata are further divided into the two sub- 

 orders/, larviformia and I. fistulata ; but it is preferable to regard these groups 



1 [Some of the Ichthyocrinidae are almost identical, in fact, with the pedunculate stages of 

 Antedon. Wachsmuth and Springer, from their observations on the orientation of the stem and its 

 canal in fossil monocyJic and dicyclic Crinoids, were led to infer the presence of infrabasals in the 

 nepionic or adult stages of many forms previously supposed to be without them. This prediction 

 was abundantly confirmed by Mr. Bury's discovery of minute infrabasals in the gastrula larva of 

 Antedon. See especially //. Bury, Early Stages in the Development of Antedon rosaceus, Philos 

 Trans, vol. CLXXIX. pp. 257-301, 1889. TUANS.] 



