332 Aquatic Societies 



also by the way in which forms hke Chironomus, that 

 ordinarily remain in hiding in the bottom, come out 

 betimes in the open and take a swim. But there yet 

 remain at least two classes of organisms that belong 

 neither to the top nor to the bottom, nor yet to the 

 free -swimming population. These are forms that are 

 able to sustain themselves above the mud by taking 

 advantage of plant stems or other solid supports. These 

 get their oxygen from the water. They are: 



1. Climbing forms, that hold on by means of claws, 

 as do the scuds and some dragonfly, damselfiy and may- 

 fly larvse, or by a broad adhesive foot as do certain 

 minute mussels. Many members of this group find 

 temporary shelter between the leaves and scales of 

 plants. 



2. Sessile forms that remain more or less per- 

 manently attached, like sponges, bryozoans, hydras, 

 etc. 



Many members of both these groups construct for 

 themselves shelters. Chironomus, for example, while 

 usually living in such tubes as are shown in figtire 134 

 on page 226, is able to creep about freely upon the 

 stem. Cothumia (fig. 73) and Stentor, and many 

 sessile rotifers build themselves shelters. 



Such support may be found on the bottom itself 

 where that is hard ; but the bottom is soft where most 

 seed-plants grow. Furthermore, to ascend and remain 

 above the level of the hordes of voracious bottom 

 dwellers must be a means of safety. It is clear, there- 

 fore, that plants rising from the bottom and branching 

 extensively must add enormously to the biological rich- 

 ness of the shoals, by the support and shelter they 

 afford to such animals as these. 



Size — ^As on land a weed patch is a miniature jtmgle, 

 having a population of little insects roughly correspond- 



