130 EARLY STAGES IN THE 



The explanation of it may, I think, possibly be found, and 

 at all events the suggestion seems to me sufficiently plausible to 

 be worth making, in the fact that in many cases, and probably 

 this applies to the ancestors of the vertebrates, the body cavity 

 was primitively a part of the alimentary. 



Mr Lankester, who has already entered into this line of 

 speculation, even suggests (Q. J. of Micr. Science, April, 1875) 

 that this applies to all higher animals. It might then be 

 supposed that the muscular system of part of the alimentary 

 canal took the place of the primitive muscular system of the 

 body; so that the whole muscular system of higher animals 

 would be primitively part of the muscular system of the di- 

 gestive tract. 



I put this -forward merely as a suggestion, in the truth of 

 which I feel no confidence, but which may perhaps induce em- 

 bryologists to turn their attention to the point. If we accept it 

 for the moment, the supplanting of the body muscular system 

 by that of the digestive tract may hypothetically be supposed to 

 have occurred in the following way. 



When the diverticulum or rather paired diverticula were 

 given off from the alimentary canal they would naturally be- 

 come attached to the body wall, and any contractions of their 

 intrinsic muscles would tend to cause movements in the body 

 wall. So far there is no difficulty, but there is a physiological 

 difficulty in explaining how it can have happened that this 

 secondary muscular system can have supplanted the original 

 muscular system of the body. 



The following suggestions may lessen this difficulty, though 

 perhaps they hardly remove it completely. If we suppose that 

 the animal in which these diverticula appeared had a hard test 

 and was not locomotive, the intrinsic muscular system of the 

 body would naturally completely atrophy. But since the mus- 

 cular system of the diverticula from the stomach would be 

 required to keep up the movement of the nutritive fluid, it 

 would not atrophy, and were the test subsequently to become 

 soft and the animal locomotive, would naturally form the mus- 

 cular system of the body. Or even were the animal locomotive 

 in which the diverticula appeared, it is conceivable that the two 

 systems might at first coexist together; that either (i) subse- 



