502 DR F. A. BATHER. 



of growth is termed by Hyatt "geomalic" (1881, p. 541). Geomalic growth may of 

 course occur without symmetry. When symmetry is as obvious as in Dendrocystis, 

 we recognise in it the simplest means of attaining equilibrium. But other circum- 

 stances may render some amount of asymmetry more conducive to the same end, as 

 in a catamaran, a motor-bicycle with a side-car, or a Cothurnocystis. 



§ 583. Before inquiring why Dendrocystis fell over on its side, or what it was 

 doing in that position, we must carry the analysis further. There is, combined with 

 the bilateral geomalic growth, an extension which, though bilateral, is in a sense 

 axial. I refer to the brachial and antibrachial processes at one end, to the anal and 

 antanal lobes at the other end. In these cases the fundamental structures are the 

 brachiole and the anal lobe, the intake and the vent ; these existed first, and we trace 

 the gradual evolution of the antibrachial process and the antanal lobe as mere 

 attempts to restore balance, ultimately attained in some individuals of D. scotica. 

 What, then, has brought about the positions of the intake and vent ? In the normal 

 pelmatozoic position, both openings would have been near the upper pole. In the 

 prone position two principles may have operated. Assuming that this position was 

 connected with locomotion, the intake would naturally be at the forward end and the 

 vent would gradually shift to the rear. But even without locomotion, there would 

 come into play the natural wholesome tendency to emit the excrement as far from the 

 food as possible ; and this would have the same effect. I incline to the opinion that 

 the latter principle was the effective one in this case. Had the brachiole persisted at 

 all with locomotion, it would be likely to have assumed a median axial position ; the 

 antibrachial process would have been an obstruction to progress, and would not have 

 been developed. The vent also might have become more axial. Dr Kirk (1911, 

 p. 18) believes that the brachiole "doubtless served in the propulsion of the animal." 

 I am at a loss to understand the possible mechanism ; but assuming the possibility, 

 for the sake of argument, the result of either a pull or a push would be mainly to 

 turn the animal round. 



§ 584. The dimerism of the stem of Dendrocystis, as of all the Heterostelea, 

 appears to be a modification of that polymerism which is found in most of the early 

 Pelmatozoa, and is, in those of more normal habit, usually fixed in the form of penta- 

 merism. The substitution of dimerism, bilaterally symmetrical in the same sense as 

 the theca, indicates a corresponding change in the relation to gravity. The structure 

 of the proximal region implies considerable flexibility, and the absence of dimerism in 

 this region shows that movement could take place in any direction. The stem was not 

 fixed by any root ; but its considerable length, which increases rather than diminishes 

 in proportion to the size of the theca, indicates that it retained its function of attach- 

 ment, possibly, as Dr Kirk suggests, "through the looping of its distal portion about 

 some stationary object," though I see no sign of any such curvature. 



§ 585. I picture to myself the tapering end of the stem inserted into either the 

 sand of the sea-floor or some crevice aff"orded by another organism ; and the flatter 



