ORIGIN OF THE GILL-OPENINGS. 329 



reproduce, in accordance with the fundamental principle of 

 Biogeny, the same process which was originally of the 

 greatest importance to the development of the whole verte- 

 brate tribe. This process was the differentiation of the 

 f.ntestinal canal into two sections : an anterior, respiratory 

 part, the gill-intestine, which serves only for breathing, 

 and a posterior, digestive part, the stomach-intestine, which 

 serves only for digestion. As we meet with this very 

 characteristic differentiation of the intestinal tube into tw^o, 

 physiologically, very distinct main sections, not only in 

 the Amphioxus, but also in the Ascidian and the Appen- 

 dicularia, we can safely conclude that it also existed in 

 our common ancestors, the Chorda Animals (Ghordonia), 

 especially as even the Acorn Worm (Balanoglossus) has 

 it (Fig. 186, p. 86). All other Invertebrate Animals are 

 entirely without this peculiar arrangement. 



The number of the gill-openings is still very large in the 

 Amphioxus, as in Ascidians and in the Acorn Worm. In 

 the Skulled Animals it is, on the contrary, very much 

 lessened. Fishes mostly have from four to six pairs of gill- 

 openings. In the embryos of Man and the higher Verte- 

 brates also, only three or four pairs are developed, and these 

 appear at a very early period. The gill-openings are perma- 

 nent in Fishes, and afford a passage to the water which has 

 been breathed in through the mouth (Figs. 191, 192, p. 113; 

 Plate V. Fig. 13, ks). On the other hand, the Amphibians 

 lose them partially, and all the higher Vertebrates entirely. 

 In the latter, only a single vestige of the gill-openings remains, 

 the remnant of the first gill-opening. This changes into a 

 part of the organ of hearing ; from it originates the outer 

 ear-canal, the tympanic cavity, and the Eustachian tube. 



