508 DR F. A. BATHER. 



§ 604, The preceding conclusion lays on us the necessity of explaining the peri- 

 proctal enlargement of Cheirocrinus as an independent phenomenon. Although that 

 enlargement may have facilitated the ultimate adoption of a prostrate habit, it can 

 scarcely have been a consequence of, or an adaptation to, such habit. Tn itself, of 

 course, an enlarged periproct has no necessary connection with a prostrate habit. 

 Essentially this modification is adopted by a form whose theca consists of a definite 

 number of relatively large plates, as a means of obtaining an area of flexible integument. 

 In such a form as Dendrocystis the enlargement of the periproct is inconceivable, since 

 the whole theca already consists of a finely-plated flexible integument. In Cothurno- 

 cystis, also, the whole obverse face subserves the function of an enlarged periproct, 

 whether it arose as such, or directly by the multiplication of ordinary thecal plates. 

 In the Glyptocystidea, however, with their rigid and hereditarily flxed thecal plan, the 

 enlargement of the periproct was the only means of attaining some flexible area. 



§ 605. The object of a flexible region is primarily to permit of expansion and con- 

 traction, according as some internal system is filled and emptied. The system in con- 

 nection with the periproct is more likely to be the alimentary than any other. The 

 gut, and in particular the rectum, of a Pelmatozoon is distended by water, which has 

 entered either through the mouth for nutrition, accompanied in part by respiration, or 

 through the anus for respiration alone. In the Heterostelea it seems probable that 

 anal respiration was frequent and an eff"ective factor in modification. In the Glypto- 

 cystidea, however, we believe that respiration was performed by the pectinirhombs ; 

 and, if these organs were ever eflicient, they must surely have been so in Cheirocrinus. 

 Therefore an expansion of the rectum by anal respiration is not what one would expect 

 in such a form ; or if it were admitted, then one would look for it all the more in the 

 Callocystidae with only three pectinirhombs. 



§ 606. Some light may be thrown on the subject by that hypothesis of a conflict 

 between pectinirhombs and gut, on which I ventured in 1900 (p. 58). Comparing 

 Cheirocrinus with the ancestor there imagined, we note two important changes : the 

 increase of the subvective area, and the specialisation of ordinary pore-rhombs into 

 pectinirhombs. Increase of food-supply involved increased size of gut, but that could 

 not be attained without pressure on the inner wall of the theca and interference with 

 certain pectinirhombs. For the pectinirhombs to act in their turn, their inner faces 

 had to be bathed by freely circulating fluid ; but this was hindered by the distension 

 of the gut. Many ways out of the difliculty can be imagined. One way was the 

 concentration of the respiratory area into three pectinirhombs, and the high specialisa- 

 tion of those three, as in Callocystidae. The mere enlargement of the theca may have 

 been tried, but since it involved a corresponding increase of both nutritive and 

 respiratory functions, it was not a course to be recommended in this world of com- 

 petition. Some of the Echinoencrinidae therefore built out a sort of rectal chimney, 

 lost their balance, and subsided, it would seem, on to the sea-floor. Cheirocrinus 

 attained the desired end by enlarging its periproct, a method which had the advantage 



