BEAXCHLE OF VEETEBEATA. 545 



arborescent processes of the branchial arches, which are placed 

 in special prolongations of the branchial cavity, where they support 

 a respiratory vascular plexus, also belong to this series (Hetero- 

 branchus, Clarias). 



Diverticula of its investing mucous membrane have the same 

 respiratory function as the cavity itself. Thus, in Saccobranchus, a 

 long tube extends from the branchial cavity, on either side, as far 

 as the lateral trunk muscles ; in Amphipnous there is a similar sac 

 behind the head, which opens just above the first branchial cleft. 

 Both these organs contain respiratory vascular plexuses. ■ 



§ 409. 



External gills in the form of integumentary structures were not 

 primitively possessed by the Vertebrata, for the so-called external 

 gills of the Selachian embryo are nothing more than filaments of the 

 internal gills which protrude through the branchial cleft. Gills, how- 

 ever, may come to the surface, and take the form of tegumentary pro- 

 cesses; such gills may be seen in the young stages of Polypterus; cer- 

 tain gills of Protopterus, and the gills of the Amphibia generally are 

 of this character. In the Amphibia the gills have the appearance 

 of two or three pairs of branched processes, which spring from as 

 many branchial arches. In the Perennibranchiata this apparatus is 

 permanently functional. In the rest of the Amphibia (Caducibran- 

 chiata) these external gills disappear ; in the Anourous forms, where 

 they are found for a short time only, they are replaced by shorter 

 internal gills. A membrane which grows from before backwards 

 covers the gills, so that there is only one efferent orifice. The 

 orifices on either side may continue to grow out, and get nearer to 

 one another, so as to unite into a single ventral orifice. 



When the larval stage ceases, the inner and outer gills of the 

 Derotremata and Salamander are atrophied ; in the latter, as in 

 the Anura, the branchial clefts are completely closed, but in the 

 Derotremata a cleft is left on either side. 



When the gills disappear, the branchial cavity, which constitutes 

 the respiratory antechamber, is converted into the primitive buccal 

 cavity, which is limited by essentially the same parts as it was 

 before. 



Branchial Clefts, and Palate of the Amniota. 



§ 410. 



In the Amniota, also, the arrangement which has been trans- 

 mitted from their branchiferous ancestors is retained during certain 

 stages of embryonic life, in the form of clefts in the wall of the 

 pharynx. These branchial or visceral clefts are never more 

 than four in number, and they appear in such a way from before 



2 N 



