Life on the Bottom 251 



legs, or antennae, or gill plates, in infinite variety of 

 length, form, position and design. 



Great is the diversity in aspect and in action of the 

 animals that swim. Yet it is perfectly clear, even on a 

 casual inspection, that the best swimmers of them all are 

 those that combine proper form of body — stream-line 

 form — with caudal propulsion by means of a strong 

 tail-fin. 



LIFE ON THE BOTTOM 



Shoreward, the earth beneath the waters gives 

 aquatic organisms an opportunity to find a resting place, 

 a temporary shelter, or a permanent home. Flotation 

 devices and ability at swimming may yet be of advan- 

 tage to the more free-ranging forms; but the existence 

 of possible shelter and of solid support makes for a line 

 of adaptations of an entirely different sort. Here dwell 

 the aquatic organisms that have acquired heavy armor 

 for defense; heavy shells, as in the mussels; heavy 

 carapaces as in the crustaceans ; heavy chitinous armor 

 as in the insects ; or heavy incrustations of lime as in the 

 stone worts. 



The condition of the bottom varies from soft ooze in 

 still water to bare rocks on wave washed shores. The 

 differences are very great, and they entail significant 

 differences in the structure of corresponding plant and 

 animal associations. These have been little studied 

 hitherto, but a few of the more obvious adaptations to 

 bottom conditions may be pointed out in passing. 



First we will note some adaptations for avoidance of 

 smothering in silt on soft bottoms; then some adapta- 

 tions for finding shelter by burrowing in sandy bottoms 

 and by building artificial defenses: then some adapta- 

 tions for withstanding the wash of die current on hard 

 bottoms. 



. 1 7. i*»0 



