DOUBLE-NOSTRILS AND SINGLE-NOSTRILS. Ill 



This internal gill-support consists of a consecutive series of 

 cartilaginous arches, which are situated between the gill- 

 openings within the wall of the throat (pharynx), and 

 extend round the throat. The foremost of these pairs of 

 gill-arches changes into the jaw-arch (maxillary arch), 

 which gives rise to the upper and lower jaws. 



A third essential character by which all Double-nostrils 

 are well distinguished from all those lower Vertebrates 

 which we have already considered, is the formation of a 

 blind sac which protrudes from the anterior portion of the 

 intestinal canal, and which in the Fishes becomes the air- 

 filled swimming-bladder (Plate V. Fig. 13, lit). As this 

 organ, in proportion as it contains a greater or less quantity 

 of air, or in proportion as this air is more or less compressed, 

 imparts a higher or lower specific gravity to the Fish, it 

 acts as a hydrostatic apparatus. By this means the Fish 

 can rise or sink in the water. This swimming-bladder is 

 the organ from which the lung of higher Vertebrates has 

 developed. The fourth and last main character of Double- 

 nostrils is the presence of two pairs of extremities or 

 members in the primitive arrangement of the embryo ; a 

 pair of fore limbs, which in Fishes are called pectoral fins 

 (Fig. 191, v), and a pair of hind limbs, which in Fishes are 

 called ventral fins (Fig. 191, A). The Comparative Anatomy 

 of these fins is of supreme interest, because they contain 

 the rudiments of all those parts of the skeleton which, in 

 all the higher Vertebrates up to Man, form the skeleton or 

 support of the extremities of the fore and hind limbs. In 

 Skull-less and Singie-nostrilled Animals there is, on the 

 contrary, no trace of these extremities. In addition to 



these four most important main characters of the Amphi- 



41 



