UINTACRINUS: ITS STRUCTUEE AND RELATIONS. 57 



most remarkable modification. It is connected with an arrangement of the 

 gnt which is different from that of all other recent Crinoids, and which has 

 not hitherto been known to have a parallel among fossil forms. The latest 

 description of this is by Mr. -Bather in the Lankester Zoology, Pt. III., 

 p. 1.36, as follows: "In most recent crinoids this [the gut] makes a simple 

 dextral coil around the thecal cavity, from central mouth to excentric anus. 

 The mouth may be slightly shifted anteriorly by increase in size of the 

 anus, or by the anal tube coming to occupy the centre of the tegmen, as in 

 Batocrinus, or even to pass beyond it towards the anterior margin, as in 

 Siphoiiocrinus. But the mouth remains in the axis of the coil, and such 

 forms are called ' endocyclic' In Ac'Jnomdra the gut winds in the same 

 way, but instead of issuing immediately the first coil is completed, it con- 

 tinues to coil, not however around the axis of the mouth but around the 

 axis of the anus. The mouth, with its annular accompaniments, therefore 

 lies between the outer coil and the next one, and not in the axis of the 

 coil ; such a form is called ' exocyclic' " 



A fact in phylogeny which might have brought about a change like this 

 is the great specialization of the ventral sac among the Inadunata Fistulata, 

 already referred to, especially those of the family Poteriocrinido3 (W. & Sp.). 

 This was accomplished through an enormous upward expansion of the 

 posterior interradius, which must have been accompanied by some func- 

 tional change, for the anus is not carried upward with it, but remains at 

 some point below the summit, either near the base of the sac,* or half 

 way up,t or discharging out of a lateral branch or spout.^ In all these cases 

 the anus remains on the anterior side of this protuberant sac, towards the 

 mouth. While thus bringing the part of the gut leading to the anus to a 

 vertical position towards the central part of the tegmen, it seems probable 

 that the mouth was to some extent pushed from the centre towards the mar- 

 gin. The precise effect upon the mouth is not known, for no one has ever 

 been able to identify its position in these forms, either from internal 

 structure or the disposition of the plates externally ; although we have 

 sacrificed a number of fine specimens in the effort to do so. 



In the EncrinidfB, which are the successors of the PoteriocrinidiB, the 

 extreme fistulate character was lost, and the ventral sac became reduced 

 again to an insignificant protuberance. What its exact position was, is not 



* Mou. Criii. Can]., I'l. VII., Fig. a. f Ibid., PI. VII., Figs, ia, 3, 4, 7. 



+ Ibid., Pb VII., Fig. 9. 



