288 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



Like amphibians, fishes are not dependent on bright illumination 

 and many live in the oceanic abysses far below the maximum depth 

 of penetration of sunlight. A considerable number live in the dark 

 depths of deep lakes, such as Lakes Baikal and Tanganyika, and in 

 the dimly lighted deeps of other lakes. As in the case of amphibians, 

 some fishes are found only in caves and in deep underground waters. 

 Most fresh-water fishes avoid bright light, and many are active 

 mainly at night. Many marine fishes are luminous, usually develop- 

 ing special light organs, but this is not true of any fresh-water fishes, 

 although a tree frog and a lizard are luminous. 



A number of fishes in several widely different groups are provided 

 with electric organs. 



Most fishes produce eggs that are fertilized outside the body, but 

 a number are viviparous with internal fertilization. The eggs, 

 always soft and usually with a gelatinous covering or in a gelatinous 

 matrix, sometimes in a horny capsule, are laid in strings or masses, 

 or scattered individually. Many fresh-water fishes prepare more or 

 less elaborate nests which are usually tended by the males, more rarely 

 by the females or by both sexes. In a number of species in widely 

 different groups the males carry the eggs about in their mouths until 

 they hatch, as do some amphibians. In one South American catfish 

 the eggs are carried about embedded in the skin of the under side 

 of the female, parallel to the case of the Surinam toad (Pipa). In 

 a few fishes the young are tended, but not fed, for some time by the 

 male, more rarely by the female. None of the fresh-water fishes 

 pass through a widely different larval stage such as is characteristic 

 of many marine fishes. The common eel (Anguilla) has such a 

 stage, but it is passed in the sea and the form of the adult is acquired 

 by the little eels before they enter fresh water. 



AMPHIBIANS 



The amphibians were the second group of vertebrates to appear as 

 fossils, in the Carboniferous. At the present time they are represented 

 by three groups, the caecilians, the salamanders, and the frogs and 

 toads. Amphibians live in swamps, marshes, bogs, ditches, ponds, 

 the quiet backwaters of lakes and rivers, and in humid regions usually 

 near permanent or temporary water. Some, including the largest, 

 are wholly aquatic. Others are largely or entirely terrestrial, living 

 on moist land, in damp woods, or under stones or logs in damp areas, 

 being active chiefly at night or during rains when the humidity is 

 highest. Many toads live in relatively dry regions, but these are 

 active mainly at night when the temperature drops and the humidity 

 rises. Some amphibians are chiefly subterranean, appearing at the 

 surface only under certain conditions, often at long intervals, and 



