132 THE bEAS 



The effect is brought about either by greatly increasing 

 the surface area by means of long spines or feather-like 

 projections, or by extreme flattening so that the creature 

 is like a leaf and when lying horizontally in the water is 

 prevented from sinking except at a very low speed. The 

 spiny and hair-like processes on many of the plankton 

 diatoms depicted in Plate 88 serve this purpose in nature. 

 The diatoms themselves are so very minute that the many 

 spines must increase their surface area comparatively to an 

 enormous degree, and they become literally suspended in 

 the water so slow is their sinking speed. The rate at which a 

 diatom will sink through the water has been measured, and, 

 of course, varies for the different species according to their 

 shape. Let us take as an example a species of Chaetoceros 

 (Plate 88, fig. 3) ; this takes on an average about four and 

 three-quarter hours to sink three feet, or eight minutes an 

 inch. This was in still water, but in nature, near the sea 

 surface, owing to wave action there are continual Httle 

 swirls and eddies which will bring it up or down at a much 

 faster rate than it sinks. 



Now, the animals, of course, on account of their larger 

 size and weight will sink considerably faster than plants, 

 but they also possess the power of locomotion, by which they 

 can regain their level. The presence of spines and feathery 

 processes on the animals' bodies, by slowing down the rate 

 at which they sink, prevents them from having to be 

 continually on the move to keep up in the water. If a 

 living plankton animal is watched it will be seen to make a 

 rapid upward movement by swimming and will then rest 

 while it sinks only slowly through the distance that it so 

 rapidly moved up through. 



An extreme example of flattening in an animal is to be 

 found in the case of the larva of the common crawfish or 

 rock lobster. This is, of course, a different species from the 



