LIFE HISTORIES. 541 



compared with what Hertwig has observed in regard to Echinoderms, 

 that ova which are retained beyond the normal period become over-ripe 

 and pathological. Except in Elasmobranchs, the ova are relatively 

 small, and large numbers are usually laid at once. In Elasmobranchs the 

 egg is large, and in the oviparous genera it is enclosed in a " mermaid's 

 purse." 



Most sharks and a few Teleosteans are viviparous, the eggs being 

 hatched within the body of the mother, in the lower part of the 

 oviduct in sharks, in the ovary in Teleosteans. In two of the viviparous 

 sharks (Mustelus ILCVIS and Carcharias glaucus} there is an interesting 

 union between the yolk-sac and the wall of the oviduct, which should 

 be compared with a similar occurrence in two lizards, and with the 

 yolk-sac placenta of some Mammals. 



As to fertilisation, the usual process is that the male deposits 

 spermatozoa or "milt" upon the laid eggs or "spawn," but fertilisation 

 is of course internal when the eggs are enveloped in a firm sheath, or 

 when they are hatched within the mother. 



Most Fishes have a great number of offspring, and parental care is 

 proportionately little. Moreover, the conditions of their life are not 

 suited for the development of that _ virtue. When it is exhibited, it is 

 usually by the males, e.g. by the sea-horse (Hippocampus} and the 

 pipe-fish (Syngiiathus), which hatch the eggs in external pouches, and 

 "the male of some species of An' us, \\\\o carries the ova about with 

 him in his capacious pharynx." The female oiAspredo carries the eggs 

 on the under surface of the body until they are hatched, much in the 

 same way as the Surinam toad bears her progeny on her back ; while 

 in Solenostoma a pouch for the eggs is formed by the ventral fins and 

 skin. At least a dozen kinds of fishes make nests, of which the most 

 familiar illustration is that of the male stickleback, who twines grass 

 stems and water-weeds together, glueing them by mucus threads exuded 

 as semi-pathological products from the kidneys, which are compressed 

 by the enlarged male organs. 



Fishes have a less definite limit of growth than most other Vertebrates, 

 and it is rare for a fish to exhibit any of the senile changes associated 

 with old age in other Vertebrates. But surroundings and nutrition 

 affect their size and colour very markedly. Some, such as the flounder, 

 seem almost equally at home in fresh or salt water, but many are 

 sensitive to changes of medium. Many can endure prolonged fasting, 

 and some may survive being frozen stiff. Lowered temperature may 

 induce torpor, as seen in the winter sleep of the pike, while in the dry 

 season of hot countries the mud-fishes, the Siluroids, and others, encyst 

 themselves in the mud, and remain for a long time in a state of "latent 

 life.;' 



Life histories. The life histories of fishes form the subject of an 

 endless chapter, of which w r e can only give a few illustrations. We 

 know how the lusty salmon return from the sea to the possibly safer 

 rivers, and after a period of fasting deposit their eggs and milt on the 

 gravelly bed of the stream. A similar migration is true of the sturgeon. 



In great contrast to these cases is the life history of the eel, the 

 mystery of which has been at least partially removed. From the in- 

 land ponds and river-stretches the female eels migrate on autumn nights 



