xii PHYLUM CHORDATA 405 



They are rarely protected by 

 an egg-shell. They are pro- 

 duced in immense numbers, 

 a single female sometimes lay- 

 ing several millions. In such 

 cases the mortality among 

 the unprotected embryos and 

 young is immense. The eggs 

 may be pelagic, i.e., so light 

 as to float when laid, as in the 

 cod, haddock, turbot, sole, 

 etc., or demersal, i.e., so heavy 

 as to sink to the bottom, as in 

 the herring, salmon, trout, etc. 



Sub-class IV. Dipnoi 



The Dipnoi or lung-fishes, 

 comprising as their living rep- 

 resentatives only the Queens- 

 land Ceratodus (Fig. 244), 

 or Burnet salmon, and the 

 mud-fishes, Protopterus and 

 Lepidosiren, of certain South 

 African and South American 

 rivers respectively, are fishes 

 of such well-marked and spe- 

 cial features that by some 

 zoologists they are separated 

 from the true fishes and re- 

 garded as constituting a sepa- 

 rate class of Vertebrates. One 

 of their peculiar features is 



