PROTRACTILE MOUTHS 



various salts in solution, several of which enter into 

 the constitution of living matter. It also contains 

 small particles of organic substances which are carried 

 down by rivers from the land, or are directly formed on 

 the spot by the decomposition of dead bodies. The 

 automatic absorption of these substances is one source 

 of nourishment, the weak degree of dilution being made 

 up for by the extent and regularity with which they 

 enter the system. This is practically the only means 

 known to those microscopic creatures, unicellular 

 animals and vegetables, which are held suspended in 

 the water: they feed by means of imbibition of this 

 sort. It may indeed be — and the suggestion has been 

 several times put forward — that the same is true of 

 larger animals, including fishes, and that the absorption 

 of organic substances through the skin, even if it does 

 not serve as a means of actual nutrition, acts as a sort of 

 accessory to it. However that may be, the proportion 

 of these substances is insufficient to assure the complete 

 nourishment of animals with a complex structure. 

 More than this is needed. Some form of solid organic 

 matter becomes an indispensable article of diet. Then 

 the business of securing nourishment assumes the 

 importance that is its due. It is effected in very 

 different ways in different creatures, and it is extremely 

 interesting to watch the process going on in a large and 

 well-filled aquarium. 



The aquarium has its feeding times. At regular 

 intervals the keeper distributes to his prisoners the food 

 they need, which the restricted area of the tanks and 

 the relative poverty of the water prevent them from 

 finding for themselves as do their fellows living a natural 

 untrammelled life. Sometimes, in fact fairly often, 

 there is struggle and conflict. Even in this confined 

 space, in this prison of glass, appetite is supreme. 

 There are victors and vanquished in the fight. As is 

 the rule in nature, the former devour the latter. That 

 is only a fleeting episode. If the tank were left to 

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