246 COMPARATIVE ANATOMY. 



there may be every means for its contained oxygen to become 

 absorbed, leaf- like or thread-like vascular processes, or gills, 

 become developed in the region of each gill-cleft. These are 

 internal or external according to their position. 1 



While Fishes possess gills throughout life, this is only the 

 case in a small section of the Amphibia, viz. in the Perenni- 

 branchiata; all the others simply pass through a gilled stage, 

 and later come to breathe by means of lungs, so that the study of 

 this one group of animals furnishes us with an excellent representa- 

 tion of the course of phylogenetic development through which all 

 the higher Vertebrates must have passed. 



The best proof of this, as well as of the important meaning of 

 the branchial apparatus of animals in general, lies in the appear- 

 ance of gill-clefts and gill-arches throughout the entire series 

 of the Amniota up to Man, that is, in forms in which they no 

 longer possess a respiratory function. They are thus repeated 

 ontogenetically, but have undergone a change of function, 

 coming into relation with the auditory organ and tongue, as 

 already described in connection with the skull and auditory organ 

 (see pp. 78, 80, 84, and 198). 



Amphioxus. The numerous (80, 100, or more) gill-clefts of 

 Amphioxus, which are supported by elastic rods, extend back- 

 wards nearly to the middle of the body. At first they open freely 

 to the exterior, but in a later period of development they become 

 covered by two lateral folds of the skin. 



The water passing through the gill-slits is conducted backwards 

 by means of the peribranchial chamber thus formed, and passes 

 out through an aperture, the atrial pore, which lies somewhat 

 behind the middle of the body (Fig. 199, c). 



This extension of the branchial apparatus over such a large 

 section of the body, which points back to a very primitive condition, 

 becomes considerably limited even in the Cyclostomi. 



Cyclostomi. In Ammoccetes the oesophagus is continued 

 directly backwards from the branchial cavity (Fig. 200, A), and at 

 the entrance of the latter there is a muscular fold covered by the 

 mucous membrane, the velum (Fig. 201, F). 



The seven 2 gill-sacs provided with leaf-like folds of mucous 

 membrane which are present in Ammocoetes, persist in Petro- 

 my zon ; but, with the formation of a suctorial mouth, the portion 

 of the oesophagus into which they open (respiratory tube) be- 

 comes closed posteriorly, and the ossophagus apparently grows 



1 External gills persist after hatching as functional respiratory organs only in 

 Protopterus and the Amphibia, and even in the latter they are often soon replaced 

 by internal branchiae (comp. pp. 250 and 251). 



2 In Ammocoetes there are primitively eight gill-clefts ; but the first pair, which 

 give rise in most Elasmobranchs and many Ganoids to the spiracle, and in Amniota 

 to the tympano-Eustachian passage,. does not perforate the skin. 



