STRUCTURE OF FISHES. 7 



projecting from the front and hind walls of the gill-slits, and 

 supported by skeletal bars called branchial arches. An air-bladder 

 is frequently present and serves as a hydrostatic organ or float, 

 while in a few cases it may act as a lung and help the gills in 

 their work of respiration. The organs of smell are paired, and 

 only in rare cases (Lung-fishes) communicate with the mouth- 

 cavity by internal nostrils. Except in the Lung-fishes the heart 

 has but one auricle and receives only venous blood, which it 

 forces, first through the blood-vessels of the gills and thence as 

 arterial blood through the vessels of the body generally. The 

 skin is either soft and bare or is hardened by the development of 

 spines or denticles, or overlapping scales, or bony plates (scutes). 

 Peculiar cutaneous sense-organs are distributed along the sides of 

 the body (lateral line organs) and on the head, and appear to be 

 specially associated with an aquatic mode of life. Such organs 

 only occur elsewhere in Amphibians ; in the tailless Amphibians 

 (Frogs and Toads) they exist only in the larval or tadpole stages 

 (except in the Cape-toad, Xenopus). The principal organ of 

 locomotion is the powerful muscular tail ; this is assisted by the 

 pectoral and pelvic limbs, paired fins corresponding with the fore 

 and hind limbs of terrestrial Vertebrates. The skeleton of these 

 paired fins cannot readily be compared with the limb-skeleton of 

 other Vertebrates, there being no such bones as humerus, radius, 

 carpal and phalangeal bones, and the edge of the fin. is not 

 divided into the five toes, which are, with exceptions, so regularly 

 present in all other Vertebrates. Fishes also possess median tins 

 on the back (dorsal fins) and between the anus and tail (anal fin) ;. 

 these fins are supported by skeletal bars or rays, whereas in Newts 

 and other Vertebrates with median fins there are no skeletal 

 structures in those fins. 



The scheme of classification adopted in the arrangement of the 

 Fish Gallery is set out on pages 200-201. The systematic 

 series of Fishes is exhibited in the Wall-cases, commencing with 

 Wall-case 1 (Sharks and Dog-fishes) in the S.W. corner of the 

 Gallery, and ending with Wall-case 20 (Angler-fishes, File-fishes,. 

 Globe-fishes, and Sun-fishes) in the S.E. corner. Standing on 

 the floor of the Gallery or suspended from the roof are other 



