364 



COMPARATIVE ANATOMY 



CHAP. 



considerably in the different species, and indeed in different individuals of one 

 and the same species. The orals now no longer occupy the whole of the tegmen, 

 but are supposed by some writers to be represented by certain plates which occur 

 at its centre, and vary in number and regularity, often being replaced by small 

 irregularly arranged perisomatic plates. The mouth is always hidden beneath 

 them. From these supposed orals the five ambulacral furrows run over the tegmen 

 to the bases of the five much branched arms. Each furrow is covered or bordered 

 by two or four rows of alternating covering plates. Interradially there are 5 plates 

 (deltoids), the edges of which meet beneath the ambulacrals and form the floors of 

 the furrows. They sometimes appear at the surface for a certain distance between 

 the ambulacrals ; in other cases, they are even here covered by more or less numerous 

 interambulacrals. 



In some species of Cyathocrinus, and in many related Inadunata the posterior or 

 anal interradial area bulges out to form a ventral or anal sac, which is sometimes 

 cylindrical, sometimes club-shaped or bladder-like (Fig. 315). This anal sac, besides 



the hind-gut, probably contained a large part of 

 the body cavity. It is covered with numerous 

 plates arranged in vertical rows. The plates 

 of the neighbouring rows alternate in some 

 species. The anus lies sometimes near the tip 

 of the sac, sometimes on its anterior side, and is 

 often encircled by special plates. The anal sac 

 may attain such dimensions that it is as long 

 as, or even longer than the arms. The first 

 tendency to the formation of such an anal sac is 

 met with in Hijbocrinus, in which the posterior 

 interradial region of the tegmen is somewhat 

 though still only slightly bulged. 



The Inadunata so far mentioned are palaeo- 

 zoic forms. From them certain more recent 

 types may be derived. In Encrinus (Trias) the 

 anal sac has again become a short cone. In 

 forms closely related to this genus, and in 

 Marsupitcs (Chalk), the anal pieces as well 

 have disappeared, so that, while the base is 

 dicyclic, the regularly radial dorsal cup consists 

 only of the plates of the apical system, periso- 



FIG. 310. Cyathocrinus longimanus, 

 after Angelin, frum the anal side, after 

 removal of the greater part of the arms. 

 pr, Ventral sac ; x. 

 In, basals. 



anal plate; r.radials; matic P ieces bein S> in tllis system, altogether 

 wanting. 



The same is the case in the dorsal cup of the 



family Holopidce (Lias to present time), Hyocrinida' (Lias to present time), Botliit- 

 crinidce (present time). In the tegmen calycis of these forms we first notice that the 

 large anal sac of the Cyathocrinidce is reduced to a small anal tube. In Holujms, 

 between the base of the open oral pyramid and the edge of the calyx, there is only a 

 very narrow zone beset with irregular perisomatic plates. This zone is still wider in 

 ff>/ocrinus (cf. Fig. 298, p. 335), and is thickly covered with numerous small plates. 

 Between the ambulacra! furrows lie the interambulacral plates ; the furrows, im- 

 mediately on emerging from between the oral plates, are bordered and covered by 

 lateral and covering plates. In the posterior interambulacral area, near the edge of 

 the tegmen, sometimes excentrically, there rises the short conical plated anal tube, 

 with the anus. In Bathycrinus, where the orals are either wanting or reduced, the 

 interradial region is either naked or plated with small pieces. The ambulacral 

 furrows have lateral plates only. The anus lies on a very short papilla-like anal cone. 



