CARADOCIAN CYSTIDEA FROM GIRVAN. 417 



undesirable gyration, but a drag was provided by the toe-spine and the stem in their 

 positions of greater mechanical advantage. Here, then, we find the explanation of the 

 inward curve of the toe-spine towards the leg, and of its downward bend towards the 

 sea-floor. Here, also, we see the reason for the retention of the stem and for the relative 

 rigidity of its distal region ; and the power of lateral movement observed in the 

 proximal region was, we may suppose, preserved and developed so that the animal 

 could vary its resistance to the normal rotatory tendency, just as the rudder of a boat 

 has to be shifted perpetually in accordance with every variation of wind and current ; 

 or, if at any time the bellows-action was reversed, then the animal would put its 

 helm over and thus meet efl"ectively the force changed in direction and lessened 

 in amount. 



The mechanism of the stem-movement is easy to understand. The flexibility of 

 the proximal region, due to the structure of the dimeres, has already been emphasised 

 (§§ 201, 220). The fusion of some four following pairs of dimeres to form the reducing 

 piece must have been due to the need for a relatively rigid structure in that position ; and 

 this need is manifest if we suppose muscles to have passed from the downturned edges 

 of marginals 4 and 5, through the wide lumen, to the inner wall of the reducing piece. 

 The ends of these muscles would naturally have been attached to the ridges on the 

 fluted surface of those walls, and were probably the original cause of those structures, 

 although the fluting would also enhance the rigidity of the median and distal regions 

 of the stem. 



§ 241. The question of the Internal Anatomy is an interesting one. Assuming 

 the correctness of our interpretation of the thecal structures, then we must suppose 

 that the fifteen or more mouths opened into a common pharynx. One can hardly avoid 

 a passing comparison of these elliptical openings to pharyngeal clefts in Chordata, if 

 only for the sake of warning the speculative morphologist against taking this as a sign 

 of atiinity. 



§ 242. From the pharynx a gut, of course, led to the anus. It would be unreason- 

 able to suppose that it left the pharynx at the toe-end, furthest from its goal, and in a 

 situation that would involve a strangulating bend ; moreover, the natural supposition 

 that it followed the direction of the pharynx from the toe towards the heel is confirmed 

 by the presence of a distinct groove or channel at the base of the strut in marginals 

 5 and 4 as may be seen in Gil, G26, and G51. 



§ 243. Did the gut, then, pass in a simple curve round the frame into the rectum, 

 or did it return in a coil as is the wont of the Echinoderm gut ? The compression of 

 the theca is perhaps opposed to the idea of a coil, but, if there was one, then it is 

 probable that the ascending curve passed up the strut and bent round in the space 

 above the pharynx. But without further evidence further discussion is vain. 



§ 244. The large lumen of the proximal region of the stem suggests the presence 

 therein of a nerve-centre, possibly also a blood-plexus, and a strand of reproductive 

 tissue, A further use of the strut may have been to protect extensions from these, and 



