n 4 EVOLUTION OF THE EARTH 



observation. That the invertebrates also have advanced since 

 the Proterozoic is certainly true, but their evolution is largely 

 one of detail and does not represent the establishment of any 

 new principle or type, that is, if we except the insects. With 

 the vertebrates, however, this is not true, for some of the 

 most momentous advances in the evolution of higher forms 

 lie within the range of their fossil record. 



Origin of vertebrates. The distinction between the verte- 

 brates and invertebrates is largely dynamic, for the former are 

 principally motor types, the latter largely quiescent, sluggish 

 forms, often actually sedentary, that is, fixed in their mode of 

 life. A review of invertebrates, especially the aquatic forms, 

 will serve to emphasize this point. The sponges are entirely 

 sedentary, while the coelenterates are either fixed forms like 

 the corals or capable of very slight creeping locomotion like 

 the sea-anemones; or are feeble swimmers like the jelly-fishes, 

 subject to the whim of tidal and other marine currents, or to 

 the turmoil of a wind-swept sea. Of the worms, using the term 

 in its old collective sense, some again are fixed, some crawling, 

 some feeble swimmers, and the same is true of echinoderms 

 and many molluscs. The arthropods are perhaps the most 

 venturesome of marine invertebrates except the cephalopod 

 molluscs, but even to them do the same three conditions of 

 fixedness, crawling, or none too effective swimming apply. 

 Such as are pelagic are, like the jelly-fish, so non-resistant as 

 to be largely the victims of circumstance. 



Of all aquatic invertebrates, the cephalopods alone have 

 developed locomotive powers of marked significance, but their 

 locomotion is strikingly different from that of a vertebrate in 

 that it is the perfection of a method of jet propulsion which 

 other invertebrates have also developed, although none have 

 carried it so far. Briefly, the perfected cephalopod, such as 

 the squid (see Fig. 15), has an elongated, spindle-shaped 

 body, at the hinder pointed end of which are two horizontal 



