372 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



and the ceratodus of 

 Australia, besides their 

 gill breathing by organs 

 of the pattern of the tele- 

 osts, we have nostrils 

 opening into the throat 

 and very good lungs, al- 

 most as good as some 

 amphibians, and there- 

 fore both lung breath- 

 ing and gill breathing 

 in about equal propor- 

 tions. These are there- 

 fore called lung fishes. 



Classification of 

 Fishes by Respira- 

 tory Organs. — The 

 form of the respiratory 

 organs is so characteris- 

 tic of the various orders 

 of fishes that Huxley 

 has made it the basis of 

 classification. Accord- 

 ing to Huxley, fishes 

 may be divided into six 

 orders — viz., (i) Dipnoi, 

 or nostril breathers, rep- 

 resented by the protopterus, lepidosiren, etc. These 

 breathe equally by lungs and gills. (2) Ganoids, so called 

 spc nc 



cut. op 



FlO. 254. — Anterior portion of a lamprey 

 with parts cut away so as to show 

 the structure of the gill pouches : int 

 op, interior openiDg ; ext op, exte- 

 rior opening ; r s, respiratory sac. 



Fig. 255. — Amphioxus : spc, spinal cord ; nc, notochord ; dv, dorsal vessel ■ 

 st, stomach ; aa, aortic arches or gill arches. 



