256 The Study of Animal Life PART m 



The males are often different from the females smaller, brighter, 

 and less numerous. In some cases they court their mates, and 

 fight with their rivals. Most of the females lay eggs, but a few 

 bony fishes and many sharks bring forth living young. In two 

 sharks there is a prophecy of that connection between mother and 

 offspring which is characteristic of mammals. The fish's egg is 

 usually a small thing, but those of Elasmobranchs are large, being 

 rich in yolk and often surrounded by a mermaid's purse. This 

 egg-case has long tendril-like prolongations at the corners, these 

 twine automatically around seaweed, and the embiyos may be 

 rocked by the waves until the time of hatching. When the egg is 

 enclosed in a sheath, or when the young are hatched within the 

 body of the mother, fertilisation must take place internally, but in 

 most cases the male accompanies the female as she spawns, and 

 with his milt fertilises the eggs in the water or on the gravelly 

 spawning-ground. As love for offspring varies inversely with their 

 number, there is little parental care among the prolific fishes. 



Most fishes live either wholly in the sea or wholly in fresh 

 water, but some are indifferent, and pass, at spawning time espe- 

 cially, from one to the other. A few, such as the climbing 

 perch,* venture ashore, while the mud -fishes and a few others 

 can survive drought for a season. In caves several blind fishes 

 live, and species of Fierasfer find more or less habitual lodging 

 inside sea-cucumbers and some other animals. 



The fishes which live in deep water are interesting in many 

 ways. Giinther has shown that from 80 to 200 fathoms the eyes 

 are rather larger than usual, as if to make the most of the dim 

 light. Beyond 200 fathoms "small-eyed fishes as well as large-eyed 

 occur, the former having their want of vision compensated for by 

 tentacular organs of touch, whilst the latter have no such accessory 

 organs," and can see only by the fitful light of phosphorescence. 

 " In the greatest depths blind fishes occur, with rudimentary eyes, 

 and without special organs of touch." The phosphorescence is pro- 

 duced by numerous marine animals and by the fishes themselves. 



6. Amphibians. The Amphibians which now live are neither 

 numerous nor large. Giant Amphibians or Labyrinthodonts began 

 to appear in the Carboniferous period, but most of the modern 

 frogs and toads, newts and salamanders, are relatively pigmies. 



Young Amphibians always breathe by gills, as Fishes do, and in 

 some cases these gills persist in adult life. But whether they do 

 or not, the full-grown Amphibians have lungs and use them. The 

 skin is characteristically soft, naked, and clammy. Amphibians 

 are the first Vertebrates with hands and feet, with fingers and toes. 

 Unpaired fringes are sometimes present on the back and tail as in 

 Fishes, but are never supported by fin-rays. 



