ORGANIC FORM AND ARCHITECTURE 669 



angles to the former, which will leave one siphonoglyph for each 

 of the halves. Similarly, though a sea-urchin or a starfish is super- 

 ficially radial, and has some radiate arrangement of internal organs, 

 there is only one cut that will yield perfectly mirroring halves, 

 namely, the cut that passes through the centre of the madreporic 

 plate by which water enters the hydrauliq locomotor system. In the 

 same way, while a typical two-tentacled Ctenophore,such as Hormi- 

 phora or Cydippe, is in a general way radial — a singularly beautiful, 

 transparent, free-swimming globe, there is a hint of bilaterality in 

 the position of the two tentacles and the sheaths into which they 

 can be retracted. This hint of bilateral symmetry is more pro- 

 nounced in some other Ctenophores, such as Venus's Girdle {Cestus 

 veneris), and this is interesting in relation to the plausible theory 

 that the bilateral Turbellarian worms evolved from creeping 

 Ctenophores. 



Radial symmetry is suited for a sedentary mode of life, and the 

 capture of food by means of tentacles surrounding the mouth ; and 

 also for an easygoing life in the Open Sea, where the movements 

 are not energetic and tend to be somewhat aimless. It is rarely of 

 survival value to an animal like a Medusa to be able to move rapidly 

 in a definite horizontal direction, but it is interesting to notice that 

 it is often important for the Ctenophores that most of them are 

 able to move rapidly downwards, for this saves their extremely 

 delicate body from being broken on the surface in stormy weather. 

 Most of them descend into quiet water before the waves begin to 

 break. 



(b) In bilateral symmetry, such as that of an earthworm or a 

 mussel, a fish or a bird, there is only one plane that divides the 

 body into two mirroring halves — the vertical plane passing through 

 the middle of the dorsal and ventral surfaces. It is characteristic 

 of most animals above the level of Coelentera. It is obviously better 

 suited than radial symmetry for vigorous locomotion, as in pursuing 

 food, avoiding enemies, and chasing mates. With bilaterality is 

 associated the evolution of a head-brain, which probably arose 

 racially, as it arises individually, from the local differentiation and 

 insinking of neurons at the anterior end of the body, where the 

 maximum number of stimuli will be received. We do not mean that 

 the stimulation produced nerve-cells; differentiation cannot be 

 accounted for as simply as that. We mean that, given the nerve- 

 cells, those bilateral animals that varied in the direction of having 

 them in the head would have a marked advantage. It should be 

 noted that among the Turbellarian worms, in which a head-brain 

 first appears, the chief ganglion often lies near the middle of the 

 ventral surface. 



(c) Among many animals that are on the whole bilateral there 

 is a departure from perfect symmetry. Thus the upturned right 



