GRAVITY, PRESSURE, AND SOUND 



133 



1. Reduction in skeleton or shell as com- 

 pared with bottom-dwelling relatives— pelagic 

 Foraminifera, for example. 



2. The incorporation of large amounts of 

 water in jelly-like matter, as in jelly fish. The 

 excess weight of living protoplasm is thus 

 spread and made to displace a larger amount 

 of water. 



3. The storage of light materials: 



(a) Water of lower specific gravity, as in 

 the ctenophore, Beroe 



(b) The accumulation of fat; for example, 

 the sunfish, Mola mola 



(c) The storage of oil droplets, as in the 

 radiolarian protozoa or in the floating 

 eggs of many fish 



(d) The inclusion of air in the float of 

 siphonophores or in the air bladder 

 of bony fish or in the air cells of the 

 eggs of anopheline mosquitoes 



(e) Carbon dioxide secretion as a tissue- 



enclosed gas 



4. Flattening of the body, as in most jelly- 

 fish, in which the oral aboral axis is usually 

 shorter than the radial axis. 



5. Suspensory projections. These are efiFec- 

 tive only among small or very small organisms; 

 for example, the foraminiferan Globigerina; also 

 many copepods, the tropical forms of which 

 have a greater development of plumose ex- 

 tensions than do those of colder waters. This 

 is in keeping with the reduced viscosity of warm 

 water. 



6. Suspensory projections increase the sur- 

 face area. This is closely related to a general 

 principle; the rate of sinking; of a body heavier 

 than water is directly correlated with the ratio 

 between friction and the difference between 

 the specific gravity of the body in comparison 

 with that of the surrounding water. A relatively 

 large surface area is associated with small size, 

 and this is probably one of the important 

 reasons why plankton organisms are usually 

 small. 



Animals with structures that aid in float- 

 ing also frequently show behavior patterns 

 that serve the same end. Swimming is one 

 of the most common of these reactions. This 

 may be either directly against the pull of 

 gravity or a component of the normal for- 

 ward movement. Daphnia, for example, 

 show more or less rhythmic alternations of 

 quiet, passive sinking and active, vertical 

 swimming. The shark is an excellent ex- 

 ample of an animal that avoids sinking bv 

 forward swimming, a component of which 

 is devoted to maintaining level in the 

 water. 



The problem of keeping afloat is greater 

 for animals that live in fresh water than 

 for marine organisms. Similar antisinking 



mechanisms are employed. The relative 

 wealth of the minute nannoplankton in 

 lakes, as compared with marine habitats, is 

 in part related to the greater difficulty of 

 the larger, coarser animals in keeping afloat. 

 (Nannoplankton is too small to be caught 

 in a fine-meshed plankton net.) Often the 

 nannoplankton has five times the biomass of 

 the net plankton for a given body of fresh 

 water. Further, the practical absence in 

 lakes and rivers of large plankton organisms, 

 such as are fairly common in the sea, is a 

 testimony to the lesser support offered by 

 the less dense and less viscous medium 

 (see also p. 165). 



Structures Produced by Animals 



Constructed nests, unlike excavated ones, 

 need adequate support, as do the roofs of 

 excavated burrows or tunnels. Nests placed 

 flat on the ground present a minimal prob- 

 lem in this connection; those built above 

 ground, where, incidentally, they are safer 

 from predation, must be constructed on, or 

 against, or suspended from some solid sup- 

 port. The frequent nicety of the instinctive 

 solution of the engineering problems should 

 focus attention upon, rather than divert it 

 from, the fact that it is the steady pull of 

 gravity, as well as stresses from wind and 

 rain, which is being built against. Covered 

 and bridging roadways of ants or termites 

 illustrate the same point. Even in subter- 

 ranean nests, the frequent construction of 

 a water drain is, indirectly to be sure, a 

 response to gravitv. It is a matter of some 

 interest that termites may excavate wood 

 until a paper-thin shell is left, but they do 

 not excavate to the point of immediate col- 

 lapse of the wood under its own weight. 



Orientation to Gravity 



In their reactions to gravity, animals mav 

 either orient and move in response to gravi- 

 tational force, or thev may merely maintain 

 position or body equilibrium. Thev are geo- 

 negative if the orientation is awav from the 

 earth's center of gravitv, geopositive if to- 

 ward that center, and transverse if at right 

 aneles to the pull of gravity. 



The transverse reaction to gravitv has 

 resemblance to equilibrium responses, and 

 both may be related to what is called the 

 "ventral-earth reaction." In the ventral-earth 

 reaction, the animal keeps an accustomed 

 part of its body, usually the ventral side, 

 oriented toward the ground or other effec- 



