RIVER OF LIFE 47 



SO it is not surprising to find that as evolution progressed their 

 tips became more and more coiled. Thus they gave rise to a 

 group known as the ammonites^ whose coiled shell reached 

 a diameter of several feet. Altogether they flourished in the 

 oceans for over 250 million years, but today the coiled cephalo- 

 pods are represented only by the pearly nautilus. During 

 the period when ammonites were still very numerous the 

 belemnites also appeared. These voracious predators, unlike 

 the others, possessed an internal shell. In their present-day 

 successors, the squids, this is reduced to a mere rudiment. Like 

 the squids, the belemnites swam by jet propulsion, squirting 

 the water from their respiratory chamber. With ammonites the 

 belemnites disappeared by the end of the Cretaceous, 400 mil- 

 lion years ago, and today their representatives, the squids, sur- 

 vive as comparatively minor creatures of the sea. 



It is true of life in the seas, as of men and nations, that those 

 who over-specialize and thereby lose flexibility to meet chang- 

 ing conditions have within themselves the seeds of their 

 eventual downfall. The lampshells or brachiopods — shelled 

 animals with a superficial resemblance to bivalves — were once 

 enormously successful. The excessive conservatism of this group 

 of sea creatures was reason enough for their decline. Today 

 they are represented by a few localized species. On the other 

 hand, one of them, lingula, well adapted to its life in the mud 

 and subjected to little competition, has remained unaltered 

 for over 400 million years since Ordovician time. It is probably 

 the oldest creature alive today which has survived so long un- 

 changed — well called living fossils. 



The starfish, sea urchins, and sea cucumbers are well distrib- 

 uted in the seas and oceans today, both in shallow and in deep 

 water. This is far from true of their flower-like relatives, the 

 stalked crinoids or sea lilies. They are unquestionably animals 

 but well merit their name, for they are attached by stalks to 

 the muddy sea floor, and from their uppermost part there 



