IIO THE EVOLUTION OF MAX. 



Fishes have in common with all other Double-nostrils up 

 to Man, and it is on this parallelism that we found our 

 claim of relationship to Fishes. (Cf. Table X., vol. i. 

 p. 466.) The following characters of the Double-nostrils 

 must be especially indicated as the systematic anatomical 

 features of the highest importance : (1) the double structure 

 of the nose; (2) the internal gill-arch apparatus, together 

 with the jaw-arches ; (3) the swimming-bladder, or lungs ; 

 and (4) the two pairs of limbs. 



As to the nasal structure, on which is based the distinc- 

 tion of the Single-nostrils (Monorhina) from the Double- 

 nostrils (Amphirhina), it is certainly significant that even in 

 Fishes the earliest rudiment of the nose consists of two en- 

 tirely distinct lateral grooves or pits in the outer surface of 

 the head, just as is the case in the embryo of Man and of all 

 higher Vertebrates. On the other hand, in Single-nostrils 

 and Skull-less Vertebrates the first rudiment of the nose is. 

 from the first, a single pit in the centre of the forehead 

 region. No less important is the higher development of the 

 skeleton of the gill-arch and of the jaw apparatus connected 

 with it, as it occurs in all Double-nostrils from Fishes to 

 Man. It is true that the primitive modification of the 

 anterior intestine into the gill-intestine, which occurs even 

 in Ascidians, is developed in all Vertebrates from one simple 

 rudiment; and in this respect the gill-openings, which in 

 all Vertebrates and also in Ascidians pierce the wall of the 

 gill-intestine, are quite characteristic. But the external 

 framework of the gills, which in all Skull-less and Single- 

 nostrilled Animals (Acraniota and MonorJdna) supports 

 the gill-body, is displaced in all Double-nostrils (Amphi- 

 rhina) by an internal gill-skeleton which replaces the former. 



