402 MANUAL OF ZOOLOGY SECT. 



the soft spongy skin of the belly, and thus carried about by 

 the parent. The ova, although containing a large propor- 

 tional amount of yolk, are always small as compared with 

 those of Elasmobranchs, never exceeding 5 to 10 m.m. in 

 diameter, and being usually much smaller. They are rarely 

 protected by an egg-shell. They are produced in immense 

 numbers, a single female sometimes laying 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 

 representatives only the Queensland Ceratodus (Fig. 228), 

 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 

 special features that by some zoologists they are separate 

 from true fishes and regarded as constituting a separated 

 class of Vertebrates. One of their peculiar features is 

 indicated by their name Dipnoi ; not only do these 

 animals breathe by means of gills like ordinary fishes, 

 but they have a highly developed apparatus for the respi- 

 ration of air a single lung in the case of Ceratodus, a 

 pair of lungs (united in front) in the other two genera. 

 They have bony scales and dermal fin rays ; but the paired 

 fins are constructed on a totally different type from those 

 of any other living fish. The fin, pectoral or pelvic as the 

 case may be, is leaf-like, or very long and narrow, and the 

 skeleton (Fig. 229) consists of a central axis in the form of a 

 slender, tapering, jointed rod of cartilage, with a row of smaller 



