CARADOCIAN CYSTIDEA FROM GIRVAN. 415 



animal brushed against a tentacle, the podia were quickly retracted, all the cover- 

 plates shut down, and the hoods pulled up against them so as to close the mouths. 



^ 231. In the great majority of Pelmatozoa, and certainly in all that were attached 

 by a stem, the intake is at the opposite pole of the theca to the stem-facet, and it will 

 doubtless be admitted that the general principle governing the development of the 

 fixed Pelmatozoa is the raising of the oral pole away from the sea-floor, just as the 

 opposite principle governs the growth of Bleutherozoa. In Cothurnocystis, however, 

 the subvective system is close to the stem. This fact alone is enough to suggest that 

 the animal did not live in the normal erect position, a conclusion already drawn from 

 the structure of the stem (§ 205). 



In what position, then, did it live ? Applying the same principle, we infer that the 

 obverse face of the theca was directed away from the sea-floor, and the reverse face 

 towards it ; and this conclusion is confirmed by many details of structure. 



§ 232. The position of the vent in the Pelmatozoa is governed by two sanitary 

 principles : first, the ejection of the excreta into the more open water away from the 

 sea-floor ; secondly, the greatest possible remoteness from the intake that is compatible 

 with the principle of distance from the sea-floor. In Cothurnocystis the position of 

 both intake and vent on the obverse face, and the limitation of the remoteness between 

 them by the thecal frame, are brought into harmony with these principles only if we 

 suppose the reverse face to have been directed towards the sea-floor, 



§ 233. The extreme flattening and lateral extension of the theca are also features 

 characteristic of living structures that spread out parallel to the surface of the earth, 

 whether in the water or on dry land. They represent the geomalic tendency of growth 

 (§ 582). 



§ 234. If these arguments be admitted, we are impelled to the conclusion that the 

 theca, and indeed the whole skeleton, lay flat on the sea-bottom. Clearly no half-way 

 position was practicable. At once all the peculiar outgrowths of the frame become 

 intelligible. The weight of the body was removed from the floor, rough with sand and 

 shell-fragments, by the knobs on the reverse. These, acting as the legs of a chair, 

 raised the whole frame, from which the flexible integument of the reverse side depended. 

 The strut on that side served to prevent the frame from collapsing and to check too 

 great sagging of the integument. The restriction of the knobs to the lower or ad- 

 columnal half of the theca is due to the simple fact that the weight of the stem required 

 to be counterbalanced by a projection of the upper part of the theca beyond the knobs, 

 further counterweight being added by the outgrowths of spines and processes in the 

 plane of the theca. That was the plan of C. Elizae ; but in C. curvata a similar result 

 was attained by the curvature of the theca and the depression of the stem-facet, so that 

 the animal rested on the proximal region of the stem, the flat knob at the ball of the 

 foot, and the upper outer edges of the thecal frame ; and if in this species the toe-spine 

 and processes of the leg were not, as it appears, so highly developed, it is simply because 

 they were not needed or would even have been in the way. 



TRANS. ROY. SOC. EDIN., VOL. XLIX. PART II. (NO. 6>. 55 



