THE HIGHER ORGANISMS 137 



In the fishes the heart is tubular and consists of two 

 chambers, a posterior auricle and an anterior ventricle. 

 As in the Crustacea, the blood is forced by the anteriorly 

 situated ventricle into the aorta, which gives off large 

 branchial arteries in pairs, itself dividing to form the 

 anterior pair, passes through the branchial arteries to 

 the gills where it is aerated, and is then collected, beyond 

 the gills, into two dorsal arteries, by which it is distributed 

 throughout the body of the fish. After passing through 

 the capillaries, it is collected by two large cardinal veins, 

 from which it is brought through two vessels ducti 

 cuvieri into the sinus venosus, passed into the auricle, 

 and then into the ventricle to renew the circuit. 



It is interesting to find three genera of fishes, survivors 

 of forms common in past geological periods, which 

 occupy a position intermediate between fishes and 

 batrachia in so far as their circulatory apparatus are 

 concerned. Of these Ceratodus, the Australian "lung 

 fish," is more like other fishes, while Protopterus and 

 Lepidosiren, the African "mud fish," are more like the 

 batrachians. All are peculiar in possessing lungs as 

 well as gills, the former a single lung, the latter a pair of 

 lungs, and in having their circulatory apparatus modi- 

 fied in consequence. 



In Ceratodus there is one lung which is small and of 

 far less value as an aerating organ than the gills. Indeed 

 the quantity of blood that is carried to it is very small, 

 and has already passed through the gills, so as not to re- 

 quire this supplementary aerating action except when 

 the fish is prevented from using its gills, during periods 

 of drought when the ponds dry up or the water they con- 

 tain becomes thick and muddy, through evaporation, 

 and charged with offensive substances and fermentative" 

 gases. It is then that the lung subserves a useful pur- 

 pose by tiding the fish over what might be called a period 

 of air famine, and permitting the blood to come into 

 contact with just enough air to enable life to be main- 

 tained. This extremely primitive pulmonary develop- 



