72 I'HE COMING OF MAN 



worms, the annelids. Here well marked muscular fibrils line 

 the body-wall, sheathe the intestine and enclose the perivisceral 

 cavity. The digestive system is fast attaining its definite 

 general form, though it will improve greatly in details. The 

 reproductive system receives a smaller surplus and is simpler. 

 Circulatory, excretory and respiratory organs have arisen in 

 service of muscle. Sensory organs pay, and are continually 

 stimulated as the animal glides through the water. They and 

 the chief ganglion are improving rapidly. We have been 

 speaking of nerves and muscles, we ought to speak of one 

 neuro-muscular system. Every added muscular fiber demands 

 the addition of a nerve-fibril, and of one or more nerve-cells 

 in the nervous arc. Every improvement of the sense-organs 

 demands and stimulates the rise of new cells in what is fast 

 becoming a brain. Muscle is the leading partner and drags 

 or pushes all the organs to a higher stage. 



In some or many annelids we find two or more claw-like 

 horny teeth at the mouth. These are very efficient weapons. 

 The period of universal peace between leading forms is past, 

 the gladiatorial struggle has begun, a quite new phase of Hfe. 

 Survival can continue only through exercise of the muscles, 

 strength and agility, a keen lookout of the sense-organs and 

 guidance of the developing brain. The annelid represents the 

 culmination of millions of years of unceasing experiments of 

 our progressive vermian ancestors; the aberrant forms mark 

 a few of the partial successes. The failures, and they seem 

 almost innumerable, have disappeared and left no trace. 



Certain points of attachment of muscular fibrils, or the 

 whole skin, secrete a solid support or defence; a skeleton ap- 

 pears. The external protective skeleton of mollusks was 

 vastly productive of ease, comfort, safety, placid enjoyment 

 survival and success, of all that makes life worth living with 

 stagnation '^ thrown in." The external, chiefly locomotive, 

 skeleton of arthropods was simple, easy to build, admirably 

 suited to small animals, light, resistent, useful in a variety of 

 ways. Arthropods were precocious and their development was 

 rapid. 



