52 CLASS PISCES. 



dactyle plan. The ichthyopterygium, thougli clearly homolo- 

 gous with the cheiropterygium, is sharplj'' marked off from it, 

 and there are no intermediate forms connecting the two. 



Though the structure of the limb skeleton, and possibly the 

 jDOSsession throughout life of the paired posterior cardinal veins, 

 are the only absolutely characteristic features of fishes, there is a 

 number of features which, while not distinctive, are highly 

 characteristic. We may enumerate some of these : — 



1. Median unpaired folds of the integument, constituting 

 the unpaired fins and supported by dermal structures called 

 fin-rays or dermotrichia, would be absolutely characteristic 

 were it not for the fact that the fin-rays are absent from the 

 median fins of Marsipobranchii. 



2. The absence of an internal opening of the nasal sac would 

 be absolutely characteristic of the class were it not for the 

 presence of internal narial openings in Dipnoi and of the pharyngo- 

 nasal duct in Myxinoids. 



3. Respiration by means of lateral pharyngeal apertures and 

 gills is found in the adult in no other group excepting in a few 

 genera of the Amphibia. 



4. The absence of a cloacal bladder might perhaps be cited 

 as a distinctive character, for it is present at least in the embryos 

 in all the higher Vertebrata. 



5. Excepting in the Dipnoi the auricle is undivided, and the 

 ventral aorta is a tube of considerable extent. 



6. Excepting in the Dipnoi there is nothing corresponding 

 to the median inferior vena cava of the other classes. 



7. The absence of a tympanic cavity and membrane, and of 

 anything corresponding to the auditory ossicles of the higher 

 types may, we think, be cited as a distinctive character, for 

 these structures are very rarely, if ever, completely absent in 

 the other classes. 



8. The presence of the peculiar sense organ known as the 

 lateral line might almost be regarded as a piscine character, 

 were it not for the fact that it is not clearly present in all 

 Marsipobranchii, and that it is found in some Amphibia. 



9. The permanent division of the great lateral longitudinal 

 muscles of the body into segments (myotomes) (corresponding 

 in number to the vertebral segments is only found outside 

 Pisces in some Amphibia, and in the tail of some Repbelia. 



