130 MARKETABLE BRITISH MARINE FISHES CHAP. 



and soil and water with the help of the sunlight. So it is in 

 the sea. Creatures may prey upon other creatures, but in the 

 last result they depend upon the vegetable kingdom. The 

 elucidation in detail of the mode in which this is effected still 

 offers an interesting field of study. We know that large numbers 

 of univalve shell-fish, such as the periwinkle, graze on the fields 

 of sea-weed which fringe the shores, and that these shell-fish are 

 eaten by fishes. But it is certain that the chief supply of plant 

 food in the sea consists of very minute microscopic plants. These 

 exist constantly in a detached suspended form in the water, and 

 also on the bottom. Then there are exceedingly minute micro- 

 scopic forms of animal nature that live upon the minute plant- 

 forms. All the bivalve molluscs, which form no inconsiderable 

 portion of the food of fishes, live entirely by the "suction" of 

 these exceedingly minute animals and plants ; they are able to 

 draw a current of water constantly through the cavity within the 

 shells, and there this food-dust is strained out and passed into 

 their stomachs. In the upper waters the microscopic plants form 

 the food-basis of all the population. 



The variety of form in these plants is remarkable, and their 

 abundance in individuals often so great as to render the water 

 turbid and to colour large areas of the sea. One common form 

 in British seas is a small sphere of a bright green colour. The 

 diatoms are distinguished by their yellowish brown colour, and 

 their indestructible siliceous (flinty) coating. There is another 

 tribe of organisms which appear to live like plants, but have often 

 been placed in the animal kingdom. One of the most characteristic 

 of these is shown in Fig. 81. Unlike those already mentioned, 

 these forms are provided with long vibratile processes or flagella, 

 although these are only seen under favourable conditions. 

 Scarcely any gathering of floating minute organisms can be 

 obtained off our coast without including some specimens of the 

 organism figured (Ceratiiiin tripos]. 



Every kind of fish has its own special way of obtaining its 

 food, to which its habits, senses, and structure are adapted, and 

 the adaptation is often most curious and interesting. The herring, 

 sprat, and pilchard, depending chiefly on minute creatures in 

 small numbers, do not have much need for teeth, and accordingly 

 in them the teeth are small or absent. In these fishes, and in 

 certain others, e.g. the basking shark, as mentioned in Chapter II., 



