292 FISHES 



diatoms and other vegetable substances in the mud. The 

 Cyprinodonts are divided into two groups, carnivorous forms in 

 which the intestine is short, and those which feed on vegetable 

 substances and mud and have long convoluted intestines. The 

 latter form the sub-family Poeciliinae including Poecilia, Mol- 

 lienisia, and two other genera. Some of the Labridae and their 

 allies the fresh-water Cichlidae are herbivorous, and also some 

 of the Sparidae or sea-breams. 



With the comparatively few exceptions mentioned above the 

 fishes feed on other animals ; the larger and more voracious 

 forms capture and devour smaller and weaker fishes, these 

 weaker or more peaceful forms feed on invertebrates of various 

 kinds, molluscs, crustaceans, and annelids chiefly. The majority 

 of fishes therefore depend on the vegetable kingdom only in- 

 directly, the invertebrates feeding on plants. In the sea, with 

 the exception of the sea-grass, Zostera, the only conspicuous 

 plants are the algae or sea-weeds which are attached to the rocks 

 round the shores. As no light penetrates beyond two hundred 

 fathoms there are no plants living beyond that depth. The 

 number of invertebrates which feed on the larger sea-weeds is 

 small, the principal being the herbivorous gastropods such as 

 the periwinkle. The great class of Lamellibranchia or bivalve 

 molluscs such as the oyster and mussel are nourished by micro- 

 scopic particles which they filter from the current of water which 

 is continually passing through the branchial cavity ; these 

 particles are chiefly the microscopic unicellular plants called 

 diatoms. Diatoms are abundant everywhere around the coasts 

 and form a sediment on the surface of mud deposits, so that 

 they contribute largely to the nourishment of mud-eating in- 

 vertebrates such as annelids and Holothurians or sea- 

 cucumbers. In the vast extent of open ocean beyond the 200 

 fathoms line the role played by diatoms is still more impor- 

 tant. In the upper stratum of the ocean from the surface to 

 200 fathoms there is a teeming life of animals and plants ; the 

 latter are all miscroscopic and consist chiefly of diatoms, the 

 animals are of various sizes and various invertebrate classes, but 

 the most abundant are small crustaceans of the kind called 

 Copepoda. This world of floating life is called the plankton, 

 and is the basis of oceanic fish-life. The Copepods and other 

 animals are all dependent directly or indirectly on the diatoms. 



