Flotation 



247 



r~j 



quate powers of flotation. The strokes of its swim- 

 ming antennae are, like the beating of our own hearts, 

 intermittent but unceasing, and when these fail it falls 

 to its grave on the lake bottom. 



Flotation devices usually impede free swimming, 

 especially do such expansions of the body as greatly 

 increase surface contact with the water. It is in the 

 resting stages of animals, therefore, that we find the 

 best development of floats: such, for example, as the 

 overwintering statoblasts of the Bryo- 

 zoan, Pectinatella, shown in the accom- 

 panying figure. Here an encysted mass 

 of living but inactive cells is sur- 

 rounded by a buoyant, air-filled an- 

 nular cushion, as with a life preserver, 

 and floats freely upon the surface of 

 the water, and is driven about by the 

 waves. 



Too great buoyancy is, however, as 

 much a peril to the active micro-organ- 

 isms of the water as too little. Contact 

 with the air at the surface brings to soft 

 protoplasmic bodies, the peril of evap- 

 oration. Entanglement in the surface 

 film is virtual imprisonment to certain 

 of the w^ater-fleas, as w^e shall see in 

 the next chapter. It is desirable that 

 they should live not on but near the 

 surface. A specific gravity about that 

 of water would seem to be the optimum 

 for organisms that drift passively about : a little greater 

 than that of water for those that sustain themselves in 

 part by swimming. 



Terrestrial creatures like ourselves, w^ho live on the 

 bottom in a sea of air with solid ground beneath our 

 feet, have at first some difficulty in realizing the nicety 



Fig. 150, The over- 

 wintering stage 

 of the bryozoan, 

 Pectinatella; a 

 stato blast or 

 gemmule. The 

 central portion 

 contains the liv- 

 ing cells. The 

 dark ring of min- 

 ute air-filled cells 

 is the float. The 

 peripheral an- 

 chor-like pro- 

 cesses are attach- 

 ment hooks for 

 securing distribu- 

 tion bv animals. 



