14 "fHE COMING OF MAN 



markedly depressed, that is flattened in a dorso-ventral direc- 

 tion. We might, perhaps, venture to imagine that some very 

 primitive coelenterate ancestor, akin to a jelly-iish (or a 

 ctenophore) swam or crawled at the sea-bottom and moved by 

 the musclar tissue, orginally fibers growing out of and be- 

 tween the cells of the two layers of the body. This muscular 

 tissue increased and formed a thick mass and in time layers 

 whose fibers run in all directions through the body. These 

 muscles serve two main and distinct purposes. The inner 

 layers are attached to and control the digestive sack. The 

 outer layers are used for the locomotion of the body. These 

 two sets' of layers are used independently and become more 

 and more distinct. The space between the two is occupied 

 by a spongy mass of parenchym, perhaps orginally musclar, 

 perhaps connectile. 



In this spongy mass the tubular reproductive organs spread 

 and branch. Here they are safely protected. In our modern 

 flat worms they are exceedingly highly developed and very 

 complex; more so than in any later or higher animal. The 

 thickened skin or integument makes more and more difficult 

 the discharge c^f the steadily increasing nitrogenous waste 

 products of cjombustion in the muscles and elsewhere. Hence 

 we find two tubes, often with* many branches, serving as ex- 

 cretory organs. There is as yet no circulatory system, for 

 the fluid in the spongy mass of parenchym allows of easy 

 diffusion of nutriment and oxygen to all parts of the body 

 and oi waste products from them to the excretory tubes, es- 

 pecially in a small animal. The exchange of carbon-dioxide 

 for oxygen can still take place through the skin; there are 

 no special gills. 



The mouth is sometimes in the middle of the under surface 

 of the body, sometimes near the front end. Over the mouth 

 or oesophagus is a nerve-center or ganglion and from this, 

 chords containing cells and fibers extend backward. The 

 sense-organs consist of sensory hairs distributed over the body 

 and clustered in sacklike depressions, otolith vesicles. We find 

 pigment eyes, of various stages of development in different 



