364 



COMPARATIVE ANATOMY 



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

 and the same species. The orals now no longer occujiy 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 month is always hidden beneatli 

 them. From these supposed orals the five and)ulacral furrows run over the tegmen 

 to the bases of the five mucli 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 andiulacrals and form the floors of 

 the furrows. '1 hey 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 

 cylindiical, 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 neighliouring rows alternate in some 

 species. The anus lies sometimes near the tij) 

 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 tirst 

 tendency to the formation of such an anal sac is 

 met with in Hijbocrinus, in which the })Osterior 

 interradial region of the tegmen is somewhat 

 though still only slightly bulged. 



The Inadunata so far mentioned are paleo- 

 zoic forms. From them certain more recent 

 types may be derived. In Encrinxhs (Trias) the 

 anal sac has again become a short cone. In 

 forms closely related to this genus, anil in 

 Marsupites (Chalk), the anal pieces as well 

 have disappeared, so that, while the base is 

 dioyclic, the regularly radial dorsal cup consists 

 only of the plates of the apical system, periso- 

 matic pieces being, in this system, altogether 

 wanting. 



The same is the case in the dorsal cup of the 

 family Holoptdtc (Lias to present time), Ilyocrinida: (Lias to present time), Bathy- 

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

 large anal sac of the Cyathocrinidcc is reduced to a small anal tube. In Holo2nis, 

 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 jilates. This zone is still wider in 

 Ilyocrinus {cf. Fig. 298, p. 335), and is thickly covered with numerous small plates. 

 Between the ambulacral furrows lie the interambulacral plates ; the furrows, im- 

 mediately on emerging from between the oral i)lates, 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 pr 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. 



Fici. 315.— Cyathocrinus longimanus, 

 after Angelin, iVidn the un;il side, after 

 removal of the greater part of the arms. 

 pr, Ventral .sac ; x, anal plate ; r, radials ; 

 h(i, basals. 



