868 LIFE : OUTLINES OF GENERAL BIOLOGY 



disc must always be ventral or posterior to the direction in which 

 the creature is swimming, one cannot distinguish head and tail ends, 

 nor right and left sides. Bilateral symmetry implied that one end 

 of the body always took the lead in locomotion, and this was doubt- 

 less advantageous in pursuing food, avoiding enemies, and chasing 

 mates. Its acquisition had far-reaching consequences, which include 

 our knowing our right hand from our left. 



BEGINNINGS OF BRAINS —Very careful investigation of the 

 minute structure of sponges has not furnished any trace of nerve- 

 cells, and this defect is enough of itself to account for the fact that 

 sponges do not seem to lead on to any other type of animal, but 

 represent a blind alley in organic evolution — albeit often a very 

 beautiful one. 



Among Coelentera or Stinging Animals, such as sea-anemones 

 and jellyfishes, there are numerous nerve-cells more or less diffusely 

 distributed. A common state of affairs is that found in sea-anemones, 

 where there are superficial sensory nerve-cells connected by fibres 

 with a loose network of ganglion-cells. These lie beneath the skin 

 and give off motor fibres to muscle-cells. The essence of a nervous 

 system is here, and if the tentacles on one side of the sea-anemone 

 are stimulated repeatedly by faked food they cease to respond, 

 having learned a simple lesson. Yet the tentacles on the other side 

 of the sea-anemone may then be cheated, as Prof. G. H. Parker has 

 shown. In other words, the experience enregistered in the neurons 

 of the one side of the body does not affect the behaviour of the 

 neurons on the other side. This illustrates in a simple way one of the 

 advantages of having nerve-centres or ganglia. 



Beginnings of ganglia are found in some Coelentera, but it is to 

 simple worms that we must look for the first appearance of "head- 

 brains", which may be associated with the acquisition of bilateral 

 symmetry. If a polarity had been established so that one end of the 

 body always took the lead in locomotion, it seems reasonable to 

 suppose that nerve-cells would be most developed in that much- 

 stimulated, and otherwise over-educated, head region. Variants that 

 wasted an abundance of nerve-cells on less strategic points would 

 tend to be ehminated. Moreover, bilateral locomotion in early stages, 

 with consequent frequency of stimulation at the head end, wouldJ 

 tend in the individual lifetime to evoke the nervous initiatives or] 

 factors in these anterior cells. Thus, as a brain always develops from 

 the sinking in of ectodermic cells from the surface of the embryo,] 

 its evolutionary beginning in the head-ganglia of simple worms is' 

 in part explained. A head-ganglion became worthy of the name ofj 

 brain when it was not only the chief sensory centre (receiving tidingsj 

 from without and from within) and the chief motor centre (issuing] 

 orders to muscles and glands), but also the chief co-ordinating] 



