416 DR F. A. BATHER. 



§ 235. In C. Elizae the weight of theca and stem pressing on the rather thin knobs 

 might have forced them too deeply into the sand. This was checked by the stem, by 

 the toe-spine, and by the tag and tongue ; the latter processes were broadened in the 

 extension-plane, since it was particularly necessary for them to keep the vent above the 

 sand. In C. curvata, where those processes were absent, the same result was attained 

 by the lateral shifting of the vent. 



§ 236. In the case of certain broadened (geomalic) Pelmatozoa with tapering stem, 

 it has been suggested that they had some power of locomotion. Such a suggestion, 

 however, is not applicable to Cothurnocystis with its extreme asymmetry, to say nothing 

 of the knobs on its under surface. 



§ 237. The animal, then, though unattached, was statozoic, and obtained all its food 

 from currents of water driven along the ciliated grooves. The combined length of those 

 grooves for which there is direct evidence in any individual is very short in proportion 

 to the size of the theca, as compared with the subvective system of most Pelmatozoa. 

 It is this fact which lends some probability to the hypothesis of exothecal extensions 

 from the subvective system. But since there is no concrete evidence for anything of 

 the kind, one seeks other means by which the food-current may have been increased. 

 Possibly this was brought about by such a bellows-like action of the pliant theca-wall 

 as has been observed in flexible sea-urchins. What muscular mechanism may have 

 effected this, we do not know ; but the cavity could be enlarged only by bringing the 

 sides of the frame nearer together, and it is only in the leg region that this could have 

 been accomplished. That some such movement could take place in C. curvata is indi- 

 cated by the displacement and crossing of the sides of the leg in some of the fossils. 

 In C. Elizae the flat extensions of marginals 11 and 12 may have served as sliding 

 bearings. Let us assume that some mechanism existed, then the expansion of the 

 thecal cavity would necessarily draw in water through the openings, and its contraction 

 would expel the water. If the anal sphincter were closed and the cover-plates of the 

 subvective grooves opened, the water would be drawn rapidly into the gullet, and by 

 the reversal of the process would be driven out through the anus. Here the value of 

 the strut in stiff"ening the frame is very obvious ; but it may have served yet a further 

 purpose, in checking the regurgitation of water into the gullet. 



§ 238. Conversely, with closed subvective grooves and open anus, rectal respiration, 

 such as is known in holothurians, might have been eff'ected by similar bellows-action. 



§ 239. In connection with the out-current from the vent, the tag and tongue 

 processes probably had a further use, since they could serve to guide the stream, and, 

 above all, to keep it away from the intake. 



§ 240. But the forcible expulsion of water, especially if confined in the manner 

 suggested, must have reacted on the theca ; and the force would be further multiplied 

 by the leverage exerted at the end of the leg when the fulcrum was furnished by the 

 knobs in the lower half of the theca. Thus the whole animal would tend to pirouette 

 in a solar (dextral) direction. The knobs alone would scarcely be enough to check this 



