458 DR r. A. BATHER. 



even greater in the British forms hereinafter described ; in some of them, too, the down- 

 ward extension is also greater, and is brought about by the bending of plate 3 far below 

 the stem-attachment, so as to form a sort of rectal gutter. This is well developed in 

 P. quadrata and especially P. fo7'iolus (PI. VI. figs. 72, 76), and is also visible to a 

 less extent in some specimens of P. Rugeri, though not figured hitherto. There is no 

 obvious reason why the periproct should not be further extended in this manner, did 

 any necessity arise. Indeed, the power of skeletal modification, by deposition and 

 absorption of stereom, is so extraordinary in Echinoderms, that it is never safe to say 

 of any of them that it has reached the bounds of possibility. This is pre-eminently 

 one of those matters " dont la preuve consiste en exp6riences, et non en demonstrations." 



§ 384. Dr Jaekel (1899, pp. 131, 133, 188, 233) has pointed out, as a consequence 

 of the downward movement of the anus, that the rectum must be directed downwards 

 from left to right, i.e. in a contrasolar direction, while the periproct is extended 

 upwards in a solar direction. In March 1897 (p. 34) he used this with other evidence 

 to prove that in Cystidea and Cladocrinoidea the gut had a contrasolar coil ; but in 

 1899 (pp. 134-137), partly in consequence of my private criticism, he gave up this 

 extreme view. In Pleurocystis, as in all Glyptocystidea, the periproct occupies the 

 right posterior interradius, and, morphologically considered, the extension of the 

 periproct is along this interradius: upwards through plate 13, and downwards into 

 the compound basal, 3. There is no concrete evidence as to the course of the parietal 

 septum, but, on the homology of Echinoencrinus as figured by Jaekel (1899, p. 126), 

 we may suppose that it passed from plate 23, through plates 12 and 6, to basal 2 ; in 

 other words, down the left edge of the theca. In the archetype of the Glyptocystidea 

 the gut may have accompanied the mesentery in this direction for a short space, and 

 then probably coiled as usual in a solar direction, down to the anus on the side of 

 the theca (cf text-fig. 45). Already in Cheirocrinus the periproctal border has 

 moved a little lower, possibly drawing the rectum with it, and thus pulling out the 

 coil ; but the position of the vent itself is still uncertain. In Pleurocystis it may be 

 that the coil was almost entirely lost, and that an enlarged mid-gut or stomach 

 occupied the space covered by the periproctal integument, to the right or solar side of 

 the parietal septum. There is, therefore, no need to suppose any departure from the 

 fundamental structure of the Echinoderma, and all the peculiarities may be explained 

 as modifications in accordance with altered habits of life. 



§ 385. The Stem consists of a wide proximal and a narrower distal region, not 

 sharply defined. 



Dr Kirk (1911, p. 21, pi. 3, f 3) gives a drawing of the stem of P. Jilitexta, and 

 says : " It will be noted that the structure of the column is quite diff"erent from that 

 given by either Bather or Jaekel." His figure is no doubt correct for the specimen 

 studied by him, but it is the case that the appearance of the stem, at least in its 

 proximal region, varies greatly according to its position in the fossil and its state of 

 preservation. After careful re-examination of a large number of specimens, represent- 



