264 CHARACTERS OF VERTEBRATE ANIMALS 



on the under side of the head, and undoubtedly have to do with 

 sensation. Little is known of the use of these organs, but they 

 no doubt respond to certain vibrations in the surrounding water. 

 It is important to avoid the common error, made when dealing 

 with the sense-organs and sensations of lower animals, of trying 

 to explain them by reference to ourselves. 



Development. — The Dog-Fish is developed from an &gg, which 

 looks almost like the " yolk " of a fair-sized bird's &gg were it not 

 for its greenish tint. This &gg is enclosed in a horny " purse ", 

 the corners of which are drawn out into tendril-like threads which 

 curl round sea-weeds and other firm objects. 



We may now proceed to the consideration of the chief groups 

 of Fishes. 



Sub-class I.— LUNG-FISHES (Dipnoi) 



Of all known fishes these come nearest to the Amphibia, and 

 some zoologists place them in a class of their own, distinct from 

 that containing more ordinary fishes. They are a very ancient 

 group, now represented by only three genera, all of which are 

 found in fresh water. They are the Barramunda or Burnett 

 Salmon {Ceratodus) from the Burnett and Mary rivers in 

 Queensland; the African Mud- Fish (^Protopierus), native to 

 some of the rivers of tropical Africa; and the South American 

 Mud- Fish iyLepidosireti), found in the Amazon and upper part 

 of the Paraguay rivers and their tributaries (fig. i6i). 



There are certain characters common to all three forms. 

 Taking first the external characters, the head is somewhat 

 amphibian-like, and has been compared to that of a salamander, 

 while the tail tapers considerably and is symmetrically margined 

 by a fin which is not expanded into lobes (protocercal or diphy- 

 cercal caudal fin). There is no spiracle, but gill-clefts are present, 

 protected by a flap or gill-cover. The paired fins are much 

 elongated, and the body is covered by thin overlapping scales. 



The most characteristic feature of the internal organs is the 

 presence of one or two bag-like lungs opening on the under 

 side of the pharynx, so that these animals possess, like some 

 adult Amphibia, both gills and lungs at the same time, and it 

 is from this circumstance that they derive the name of Dipnoi 

 (Gk. dis, twice; pnoe, breath). 



