Plancton Animals 325 



lighter shells less prone to sink, live in the deeper zone 

 of mixed marl and mud, and so are able to forage farther 

 out on the bottom. On account of their thinner shells 

 they are excluded from residence near the shore line, 

 where the waves would crush them. The heavier 

 shelled Unio requires a more solid bottom for its sup- 

 port, and is uninjured by the beating of heavy waves. 

 Hence, its shoreward distribution. Lampsilis, however, 

 is a more freely ranging form, having a rather light shell. 

 It overspreads the range of all the others, coming in the 

 less exposed places rather close to shore. 



Plancton animals The animals of the shoreward 

 plancton are less transparent than those of the lake. 

 They are also far more numerous. They show more 

 color. The color is often related to situation. In 

 small ponds and marshes they are darker as a rule than 

 in large ponds. They include forms of very diverse 

 habits among which are the following: 



1. Forms that swim freely and continuously in the 

 more open places. These only are common to both 

 littoral and limnetic regions. 



2. Forms that are free swimming, but that rest 

 betimes on plants; Cladocerans with adherent "neck 

 organs" ; Copepods with hooked antennae, etc. 



3. Forms that can and that do swim betimes, but 

 that more habitually creep on plants; many ostracods, 

 copepods and rotifers. 



4. Forms that live on or burrow in the slime that 

 covers stems or other solid supports, and that swim 

 but poorly and but rarely in the open water; Leeches 

 and oligochete worms, rhizopods and midge larvae. 



5. Sessile forms that cannot swim, but that become 

 detached and drift about passively in the open water, 

 at certain seasons; hydras, statoblasts of fresh-water 

 sponges and of bryozoans, resting eggs of rotifers and of 

 cladocerans, etc. 



