MODES OF EESPIEATION. 167 



And since, according to tHs hypothesis, the amount of the 

 substance absorbable in a given time depends on the volume of 

 the water, and increases or diminishes with it, growth would 

 cease entirely if the body of water were so small that it had a 

 stronger affinity to the unknown matter than the skin of the 

 animal has. On the other hand, the attainment of the full size 

 within the corresponding period would only be possible if the 

 volume of water were so great that the Lymnaea could at all 

 times absorb this unknown stimulant from the water.^' 



IV, Influence of oxygen or air in the water. — We have 

 seen in the foregoing sections that the effect of the substances 

 held in solution in the water, and also apparently that of the 

 volume of the water, depend t*on the osmotic action of the 

 animal's skin. Another substance held in solution in the 

 water must take effect in a precisely similar manner — namely, 

 the air used up in breathing by a number of animals. It is 

 known that every animal, even the simplest Infusorium that 

 lives in water, requires air, or rather oxygen, for respiration ; and 

 as most aquatic animals have no special organs for breathing 

 in the air itself — like most of the Yertebrata, as well as 

 Insects, Arachnoidse, Myriapoda, and many other creatures — 

 their efficient respiration depends exclusively on the absorption 

 of the air (oxygen) contained in solution in the water through 

 the skin, or through the membranes of some internal organs 

 through which water flows in and out in a constant stream. 



It is self-evident that every growing or young cell must be 

 capable of breathing, i.e. of taking in oxygen and disengaging 

 carbonic acid, if this respiration is a function of protoplasm 

 itself. All growing parts which are in contact with media rich 

 in oxygen, such as air and water, must consequently breathe,''' 

 taking it for granted that their surface offers no resistance to 

 the absorption of oxygen. But tbe disposition to absorb it 

 may be very different in different parts of the body ; and we are 

 accustomed to call such parts as seem better fitted to absorb 

 oxygen, as compared with the others, simply ' orga,ns of 

 respiration.' 



Such specialised organs of most of the animals that live in 

 water and breathe water ^° are, in the first place, the skin, then 



