.'■.'» 





\<>^>^ 



\N. C. Mcintosh, the nineteenth-century biolo- 

 gist, did these drawings of marine worms. The 

 seamouse (above) grows to a length of about 

 four inches and has an armor of stiff spines 

 and scaly plates. The brightly colored Napthys 

 caeca (right) is about life size. 



i*.i^ 



y /. 



?■ 





Many bristle worms build and live in tubes of sand grains, 

 mucus, or carbonate of lime. These tube dwellers seldom move 

 about much. They wait for their food to drift by and catch it by 

 means of bristles arranged around their heads. Like the sponges, 

 they feed on small particles of animal or vegetable matter, but 

 instead of filtering water through their body they trap the frag- 

 ments directly from the water. Other marine bristle worms, as 

 active as the tube dwellers are static, hunt their prey. They move 

 along quickly, either crawling along the bottom or swimming 

 through the water, and seize their victim with their strong jaws. 

 Although the habits of the various marine worms differ, they all 

 have segmented bodies, longitudinal and transverse muscles that 

 enable them to stretch out and to contract, and a nervous system 

 with the beginnings of a brain as well as a stout nerve cord running 

 the length of the body. Thus the worm is highly specialized com- 

 pared with the radiated invertebrates. 



Once animals had developed a segmented body, a head end, a 

 hind end, organs of locomotion, and a specialized nervous system 

 (including a concentration of nerve cells at one end of the main 

 nerve cord), the way was open to rapid evolutionary progress. So 

 long as they were constructed on a radial plan and had to Uve either 

 fixed to the sea bed or at best crawhng slowly over it, they evolved 

 very slowly. A study of fossils shows that the radiated animals have 

 made relatively few advances since the beginning of the fossil record 

 some 500 million years ago. From the ancestral bristle worms, 

 however, there burgeoned out a great variety of types, most of 

 which are included today in two phyla - Mollusca and Arthropoda. 



Though these two groups developed along radically different 

 lines, we can trace the genealogies of both all the way down to a 

 bristle worm. Within each phylum some classes have made little 

 progress, or have even dropped back in the evolutionary race, 

 while others have forged ahead to become highly developed. Among 

 the molluscs it is the sea-dwelling octopus and squid that have 

 reached the highest point. Among the arthropods it is the land- 

 living insects and spiders ; but their relatives left behind in the seas — 

 the crustaceans - have also been left behind in an evolutionary sense. 



The word "mollusc" can be interpreted to mean soft-bodied, 



