FRESH WATER ANIMALS " 203 



leaves embrace the stem and serve to hold the water from the 

 rain. So do sagging gutters and discarded pots and pans, while 

 we must keep in mind other receptacles such as vases with cut 

 flowers. In all such situations fresh water animals are always 

 more or less abundant. 



In the sea the water is permanent, and remains always, 

 except for tidal action, at the same level; there are currents 

 and counter currents, which decrease in velocity with depth. 



Conditions in the sea favor the development of attached 

 and sluggish animals feeding on the creatures passing by and 

 distributed by drifting young, and of more or less inert pelagic 

 drifters. 



In fresh water, drifters, unless of very minute size and capable 

 of very rapid reproduction, would soon disappear through being 

 washed ashore or out to sea and lost, and the rich detritus so 

 important in the sea is either smothered in silt or decomposes 

 rapidly, continuously or at certain seasons. 



Conditions in fresh water favor the development of amphib- 

 ious creatures which are capable of living on dry land during 

 some or all of their life stages, or that regularly or on occasion 

 assume a form in which they may be blown or otherwise 

 carried about from place to place. 



Occurring in the sea alone, because entirely incapable of 

 meeting the conditions in fresh waters, are the cephalochordates, 

 balanoglossids, rhabdopleurids, cephalodiscids, tunicates, pho- 

 ronids, chaetognaths, brachiopods, echinoderms, priapulids 

 and sipunculids. 



These creatures are either relatively large pelagic drifters 

 (as many tunicates, the chaetognaths, and one echinoderm), 

 mud swallowers requiring a large and constant amount of rich 

 nutritive material in the mud (Hke the balanoglossids and many 

 of the echinoderms), or they feed on minute drifting organisms 

 of which they must have a constant and a large supply. Of 

 these last those that are not drifters are fixed or very sluggish. 

 They cannot meet the conditions found on the shores of rivers, 

 lakes and ponds, resulting from the fluctuations in the water 



