352 COMPARATIVE ANATOMY 



ectoderm being limited to the outer parts of the branchial septa and 

 only being of secondary importance. The true external or integu- 

 mentary gills arise as projections of the surface, covered by ecto- 

 derm, at points between which the clefts are subsequently formed, 

 but it has recently been maintained that the endoderm takes part 

 in their formation also. 



Fishes possess gills throughout life. Amongst Amphibians this 

 is only the case in the Perennibranchiata : all the others simply 

 pass through a gilled stage, and later nearly always breathe by 

 means of lungs. 



Thus the study of this one Order furnishes us with an excellent 

 representation of the course of phylogenetic development through 

 which all the higher Vertebrates must have passed, and which 

 is still indicated in them by the appearance in the embryo of 

 gill-clefts and gill-arches with a corresponding arrangement of 

 the blood-vessels : these occur throughout the entire series of the 

 Amniota that is, in forms in which they no longer possess a 

 respiratory function. 1 



Amphioxus. The small mouth leads from the cavity of the oral 

 hood into that of the pharynx, and is provided with a muscular 

 fold, the velum. The numerous (80 100 or more) gill-clefts, 

 which are arranged in pairs and supported by elastic cuticular 

 rods, extend backwards nearly to the middle of the body. At first 

 they open freely to the exterior, but at a later period of develop- 

 ment they become enclosed by a paired fold of the integument 

 which gives rise to an atrial or peribranchial chamber, opening by 

 a single pore situated somewhat behind the middle of the body 

 (for details, cf. Fig. 258). 



The relative extent of the branchial apparatus is considerably 

 limited, even in the lowest Craniata, as compared with Amphioxus. 



Cyclostomes. In the Ammocoete-larva of the Lamprey the 

 oesophagus is continued directly backwards from the pharynx 

 (Fig. 259, A), and at the anterior end of the latter there is a 

 muscular velum, covered by the mucous membrane (Fig. 260). 

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

 brane which are present in the Ammoccete, persist in the adult ; 

 but, with the formation of a suctorial mouth, the portion of the 

 oesophagus into which they open becomes closed posteriorly, 

 the gullet apparently growing forwards above the latter, and 

 joining the mouth-cavity at the velum. Thus two canals pass 



1 Thus indications of five or six clefts are seen in the embryos of most 

 Reptiles, and of five in Birds and Mammals ; in many cases, however, the}' do not 

 become open to the exterior. Their order of disappearance is from behind for- 

 wards, and the most anterior (mandibulo-hyoid) cleft persists in a modified condi- 

 tion even in the adult, undergoing a change of function in connection with the 

 auditory organ (p. 294). Certain of the anterior arches persist in a modified 

 form (cf. under (Skull and Larynx). 



