692 



DEPTHS OF THE OCEAN 



Specific 

 surface. 



Numerous forms absorb water to such an extent that their 

 water-contents may amount to 90 per cent of the whole organism, 

 as in the medusae, ctenophores, and many fish eggs. In fish 

 eggs chemical analysis shows how the amount of water decreases 

 during development, and how this decrease continues as the 

 larvae seek deeper water and finally settle on the bottom. 

 Salpse and Pyrosomidae with large soft integuments also contain 

 a high percentage of water. 



All the forms living in the surface waters of the sea, which 

 have developed special floating devices in the shape of air- 

 bladders or bells, may also — at all events in order to avoid a 

 too formal classification — be ranged into this group. These 

 remarkable devices are specially noticeable in the wonderful 

 group of the siphonophores. The air-filled lungs of whales and 

 seals and the air-bladders found in most fishes are also instru- 

 mental in diminishing the specific gravity of these animals. 



(2) A reduction of the specific gravity of the kind mentioned 

 above must necessarily reduce or abolish the surplus gravity, 

 which tends to make the animals sink. But even if a surplus 

 gravity is present they will float, if they can offer a sufficient 

 amount of surface resistance, which may be effected either 

 actively by swimming, or passively as a consequence of the 

 shape of the body. 



In order to understand the various and complicated adapta- 

 tions within this field, we should have .to compare the various 

 types of shape found in pelagic animals. I will at present 

 limit myself to pointing out the main laws as laid down by 

 Ostwald and Chun. In considering surface resistance two 

 points are essential : (i) the size of the organism, and {2) the 

 shape of the organism. 



If we take two bodies, for instance two balls, consisting of 

 the same substance but with different diameters, and let them 

 sink in the same fluid, the larger one, that is, the ball in which 

 the relation between surface and volume is smallest, will sink 

 the faster ; thus the smaller the body the slower will it sink. 

 Ostwald terms the relation between surface and volume the 

 " specific surface," and gives the above-mentioned fact in the 

 following words : "small bodies sink slower than similar large 

 bodies which have the same surplus gravity, because their 

 specific surface is greater." 



Next it is important to take into account the diameter of 

 organisms transverse to the direction in which they sink. A 

 thin plate sinks much faster in a vertical than in a horizontal 



