406 • DR ARTHUR T. MASTERMAN ON THE 



we take any longitudinal plane through the main axis of, e.g., Hydra, the factors making 

 for symmetry are the same on each side. Axial factors are therefore merely superposed 

 upon bilateral, the latter not being removed. Bury* takes up a rather peculiar position. 

 His 'ancestor' moves about, mouth downwards, on the sea-floor, apparently intermittently 

 fixed by the five tentacles. The axial symmetry proceeds to develop spontaneously, 

 with apparently no suggested reason. The body then, for some reason unexplained, 

 rolls over to the left, the anatomical result being that the mouth becomes situated on 

 the left side of the larva. Bury, with a remarkably able piece of reasoning, backed by 

 numerous ontogenetic facts, shows the necessity for assuming this migration of the 

 mouth to the left side, but naturally he brings the hydrocoele ring round with it. 



Now, in asterids the hydrocoele is on the left (its plane parallel to that of the median 

 larval axis) from the outset. As Bury remarks — (loc. cit., p. 102) — "the twoplanes being 

 at right angles to one another in ophiurids ; inclined at a lesser angle in Brachiolaria 

 and Crinoids ; and parallel from the first in the Bipinnaria." Bury starts with the 

 anomalous hydrocoele ring encircling the oesophagus and at right angles to the median 

 larval axis in his ancestor (also Semon), and he is then led into the position of regarding 

 the asterid larvae as being in this respect the most specialised, and the ophiurids as being 

 the most primitive. As already remarked, the hydrocoele is essentially a left lateral 

 organ, as shown by Bury himself, even in ophiurids, and there is really no evidence for 

 assuming a primitive annular arrangement on the ventral side of the body. 



I think we may assume, between the period of free-swimming life and that of 

 fixation, a period during which the organism was gradually forsaking its pelagic life and 

 contracting the habit of lying on its right side on the sea-floor. Its diet was unquestion- 

 ably microscopic floating organisms obtained by ciliary action, and the universal 

 distribution of such would make this habit possible. At the same time, such a change 

 of position would bring the body into relationship with the factors of symmetry at right 

 angles to that previously existing. As in the analogous instance of pleuronectids, the 

 heterogeneity would be bilateral rather than dorso-ventral, and the mouth would move, 

 with food from above, into the middle of the upper side, the left side of the larva. The 

 tentacles were evolved round the mouth in relation to the already existing ciliated tracts, 

 as in Tornaria, and connected with the cilio-trophic function. The hydrocoele tentacles 

 do not appear to me to have served as organs of support (Bury), and I hardly see the 

 modus vivendi of a larva with the mouth downwards (Bury, MacBride). We may 

 perhaps allow that the tentacles would tend, for purposes of nutrition, to be arranged 

 around the mouth, the more so as the free life was gradually given up. During this 

 stage the pre-oral lobe became more and more frequently an organ of fixation, but not 

 in a permanent degree till the sinistral asymmetry was permanently instituted. 



Axial symmetry. — Fixation by the pre-oral lobe appears to me to be. one of the 

 most clearly indicated phyletic facts in Echinoderm ontogeny. We know that axial 

 symmetry can be induced only under circumstances in which the surroundings are 



* Bury, H., "The Metamorphosis of Echinoderms," Quart. Joum. Micr. Sci., vol. xxxviii. 95-96. 



