22 TH£ COMING OF MAN 



relations. Pteropods swarm in the northern parts of the ocean, 

 and form quite a large part of the food of whales as well as 

 of fish. Thus the clams possess an immense territory of shore 

 and the pteropods the almost boundless surface of tlie colder 

 oceans, an unequalled distribution which they hold almost 

 without competitors. 



Both clam and pteropod live on microscopic food, which is 

 constant in supply and easily obtained. They hold a some- 

 what humble and unostentatious, but most important place in 

 Nature's vast household, corresponding in a way to the in- 

 sects and smaller vertebrates on land. 



It was left to the cephalopods, the third great class of mol- 

 lusks, to attempt a more ambitious role. They also forsook 

 the crawling life of their more primitive ancestors for some 

 reason unknown to us. Perhaps they gained larger and bet- 

 ter food in this way; possibly in the severe competition on the 

 bottom they were forced to the change of habit by stronger 

 and better adapted opponents. 



They invented or stumbled upon a very ingenious mode of 

 locomotion by taking water into the so-called mantle-cavity 

 and then spurting it out between the lobes of the foot in a 

 powerful jet. Modern squids and cuttlefish have united these 

 lobes in a tube, the siphon. By this peculiar method they 

 move with marvellous rapidity, darting through the water like 

 the swiftest fishes. At the front end of the body around the 

 mouth they have developed a circle of long arms bearing rows 

 of suckers so that they can grasp and hold firmly anything 

 which comes within their reach. Many modern forms have 

 a beak like that of a parrot with which they can bite directly 

 through the body of an ordinary fish. 



The swift locomotion developed the muscular and nervous 

 systems to an extent unknown among other mollusks. They 

 have large eyes and organs like ears, and a well developed 

 and distinct head. Their brain is large and complex and sur- 

 rounded by a sort of cranium. Their shells grew continually 

 lighter. In the squid the external protective armor has been 

 completely cast off. Theirs is no defensive campaign. Only 



