24 "fHE COMING OF MAN 



so far as was possible to him. He failed through no fault of 

 his own. He was doomed to failure by his earliest molluscan 

 ancestors. He could rid himself of his shell, but the short, 

 clumsy unsegmented body was fastened indelibly upon him 

 by heredity. From this he could never escape. This is the 

 tragic chapter of molluscan history. It seems to me worthy 

 of that name. 



Arthropoda. The external skeleton of the arthropods has 

 the double advantage that it can be used either for protection 

 or locomotion. In barnacles it is as completely protective as 

 in the clam; in most crabs, and in many beetles, it serves very 

 largely the same purpose; in spiders and many insects, like 

 flies and wasps, it is almost purely locomotive. The value of 

 so wide a range of adaptability is evident. 



The arthropods form a series extending and completing the 

 annelid line of development. The body consists of the two 

 concentric tubes, the intestine and. body wall, with a peri- 

 visceral cavity between them; the nervous system has the 

 same type, a ganglion in every ring of the well segmented 

 body. Both have on some or most of the segments of the 

 body a pair of appendages; fins in annelid, jointed legs or 

 their modifications in the arthropod. The insects have added 

 ^o pairs of wings in connection with their terrestrial or 



^ g^rial life. 



2 "o The insects have grouped the segments of the annelids in 



SSliree body-regions: head, thorax and abdomen, and have 

 •^folded each region for a special function or purpose. The 

 c§bdomen is usually composed of from nine to eleven segments, 

 P^d is the chief seat of digestion, excretion, circulation, respi- 

 i;^tion and reproduction, the organs of vegetative life. To the 

 ^iree segments of the thorax, two pairs of wings and three 

 pairs of legs are attached. These segments are very closely 

 united, and in bees and flies are fused in one globular mass. 

 The head consists of six, or possibly more, completely fused 

 segments. The appendages of three of these behind the mouth 

 have been changed into as many pairs of jaws, or still further 



