412 DR F. A. BATHER. 



§ 222. Comparison of G. curuata with G. Elizae. — The essential differentia of this 

 species is the narrowness of the subvective grooves. One might, however, have 

 hesitated to base a distinct species on this alone, when only some half-dozen specimens 

 out of six dozen were found to present it. It might conceivably have been explained 

 as due to post-mortem changes, to a difference of preservation, or to some temporary 

 physiological condition connected perhaps with breeding. But when these specimens 

 also agree, so far as can be seen, in the peculiar outline of the theca, in its curvature, 

 in the position of the subvective tract, and in the roughness of the integumentary 

 plating, then we must conclude that all these slight differences of structure are not 

 chance variations, but correlated with one another and with a slightly different habit 

 of life. The environment, however, seems to have been the same for this as for 

 C. Elizae ; and there is no evidence of any difference in geological age. We are 

 justified therefore in regarding C. curvata as an independent contemporary species. 



Organisation and Mode of Life of Gothurnocystis. 



§ 223. This creature, though plainly a Pelmatozoon, is so different in structure and 

 outward form from any other Pelmatozoon as yet known, that it is by no means easy 

 to discover its true affinities. The very fact of this difference points to the conclusion 

 that the animal was modified for some unusual habit of life ; and our first task must 

 be to analyse its modifications, and remembering that life consists in the mutual 

 reactions of organism and environment, to reconstruct from the dry bones the 

 living organism. Not till some workable conception of the animal's physiology and 

 bionomics has been formulated, can we distinguish the characters that are adaptive 

 and secondary from those that are primary in the sense of being derived from a less 

 modified ancestor. 



§ 224. As starting-point let us take a structure as to which no doubt can exist. 

 This is the Vent (§ 199), situated at the top of the thecal extension described as the 

 leg of the boot. The arrangement of the small plates in that region is quite character- 

 istic, and may be compared with the similar appearances in Dendrocystis (§§ 73, 141) 

 and Pleurocystis (§ 382) ; it indicates an anus closed by a sphincter muscle. The 

 position of the vent at the very summit of the long narrow leg shows that it opened 

 at the end of a distinct rectum, and its varying relations to the thecal frame can 

 best be explained on the assumption that the anal end of the rectum was capable of 

 slight extension beyond the limits of the frame, though always between the tag and 

 tongue processes. This extension was probably due to the pressure of fluid in the 

 rectum, and the relaxation of the anal muscles. It is possible that those muscles 

 comprised, not merely a sphincter developed in the loose integument, but also some 

 retractors attached to the expanded marginals 11 and 12. 



§ 225. Accepting this description of the anal structures, but postponing further 

 discussion of their function, we recognise that a vent of this nature implies a gut and 



