Fishes . es the lung, we canhardly doubt that, when it is in water of normal compositionand sufficiently pure to yield the necessary supply of oxygen,these organs are sufficient for the purpose of breathing, andthat the respiratory function rests with them alone. Butwhen the fish is compelled to sojourn in thick muddy watercharged with gases, which are theproducts of decomposing organicmatter (and this must be the casevery frequently during the droughtswhich annually exhaust the creeksof tropical Australia), it commencesto breathe air with its limg in theway indicated above. If the mediumin wh
Fishes . es the lung, we canhardly doubt that, when it is in water of normal compositionand sufficiently pure to yield the necessary supply of oxygen,these organs are sufficient for the purpose of breathing, andthat the respiratory function rests with them alone. Butwhen the fish is compelled to sojourn in thick muddy watercharged with gases, which are theproducts of decomposing organicmatter (and this must be the casevery frequently during the droughtswhich annually exhaust the creeksof tropical Australia), it commencesto breathe air with its limg in theway indicated above. If the mediumin which it happens to be is perfectlyunfit for breathing, the gills cease tohave any function; if only in a lessdegree, the gills may still continueto assist in respiration. The Barra-munda, in fact, can breathe by eithergills or lung alone or by both simul-taneously. It is not probable thatit lives freely out of water, its limbs being much too flexiblefor supporting the heavy and unwieldy body and too feeble. Fig. 170—Upper jaw of Neocera-todtisforsteri Giinther. (AfterZittel.) 242 Subclass Dipneusti, or Lung-fishes generally to be of much use in locomotion on land. How-ever, it is quite possible that it is occasionally compelled toleave the water, although we cannot believe that it can existwithout it in a lively condition for any length of time. Of its propagation or development we know nothing exceptthat it deposits a great number of eggs of the size of those ofa newt, and enveloped in a gelatinous case. We may inferthat the young are provided with external gills, as in Pro-topteriis and Polyptcnis. The discovery of Ccratodns does not date farther backthan the year 1870, and proved to be ofthe greatest interest, not only on accountof the relation of this creature to the otherliving Dipneusti and Ganoidei, but alsobecause it threw fresh light on thosesingular fossil teeth which are found instrata of Triassic and Jurassic formations176—Lower jaw of i various parts of Europe,
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