CONDITIONS OF LIFE 293 



and the diatoms owe their life, like other plants, to the sunlight. 

 These microscopic plant-cells supply the place in the surface 

 waters of the ocean of the grass and other vegetation on the 

 land-surface, and it has been truly said that as all flesh is grass 

 so all fish is diatom ; for the smaller gregarious fishes such as 

 the Clupeidae feed chiefly on the small Copepods, and the Cope- 

 pods feed on diatoms, while the gregarious fishes are preyed on 

 by the larger predatory forms. The fishes which feed on 

 plankton have an efficient filtering mechanism formed by the 

 gill-rakers, a double series of cartilaginous or bony rods which 

 project forwards from each branchial arch ; these rods form a 

 sieve completely covering each of the gill-clefts, for they extend 

 across the interval between each gill-bar and the next. The 

 water therefore which enters the mouth is strained as it passes 

 out between the gill-rakers, and any solid particles it may con- 

 tain are left in the pharynx. The sufficiency of the plankton 

 as a supply of nutriment is shown by the fact that the largest 

 sharks, Selacke, the basking shark, and Rhinodon, feed entirely 

 by the method of filtration through the gill-rakers. There are, 

 however, some tracts of the ocean which have a floating vegeta- 

 tion of large algag, as mentioned above, namely the region of 

 the sargasso in the Atlantic and a similar region in the Kara 

 Shiwo or Black Current in the Pacific. The sargasso has a 

 fauna of its own, but how far the weed supplies food as well as 

 shelter or concealment is a matter which has not been minutely 

 investigated. Abyssal fishes, since plant life is entirely absent 

 in the great depths, are ultimately dependent for nutrition on 

 the organisms which fall within their reach from the surface 

 waters ; many of them, as mentioned in the general description 

 of them already given, are predatory, and live upon other fishes 

 of the same habitat, but the latter must feed on invertebrates 

 such as Crustacea and these again upon the organic matter de- 

 rived from surface life. 



The range of temperature under which different fishes live is 

 very great. In fresh waters fishes extend northwards until they 

 are stopped by the freezing of the water, and even exist in 

 waters which are only liquid for a few months of the year ; 

 Dallia pectoralis, a species allied to the pike, lives in the north 

 of Alaska and seems to be accustomed to be frozen in the ice for 

 considerable periods in winter ; one which was swallowed in the 



