GILLS 275 



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

 pass through a gilled stage, and later 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 up to Man that is, in forms in which they are na 

 longer concerned in respiration. 



Thus rudiments of five clefts are seen in the embryos of most Reptiles 

 and Birds, and of four in Mammals ; in many cases, however, they do not 

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

 forwards, and the most anterior (hyoid) cleft persists in a modified condition 

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

 auditory organ (p. 224). Certain of the anterior arches persist in a modified 

 form (p. 69). 



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

 Amphioxus, which are arranged in pairs and are supported by elastic 

 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 in an atrial or peribranchial chamber ,. 

 which opens by a single pore situated somewhat behind the middle 

 of the body (for details compare Fig. 219). 



The relative extent of the branchial apparatus is considerably 

 limited even in the lowest Craniata. 



Cyclostomes. In the larval Ammoccete the oesophagus is 

 continued directly backwards from the pharynx (Fig. 220, A), and 

 at the anterior end of the latter there is a muscular fold, the velum,. 

 covered by the mucous membrane (Fig. 221). 



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

 brane which are present in the Ammocoete, persist in Petromyzon 

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

 oesophagus into which they open becomes closed posteriorly, and 

 the oesophagus apparently grows forwards above the latter, and 

 joins the mouth-cavity at the velum. Thus two canals pass back- 

 wards from the mouth, a ventral branchial or respiratory tube, and 

 dorsal oesophagus (Fig. 220, B). 



In Petromyzon and Bdellostoma l the individual branchial sacs, 

 which communicate directly with the pharynx, open freely to the 

 exterior : in Myxine this original condition becomes modified by the 

 outer parts of the gill-passages growing out into long tubes, which 



1 In Bdellostoma there are usually six or seven pairs of branchial sacs, and 

 behind these, on the left side, an cesophageo- cutaneous duct opens directly into the 

 pharynx, as is also the case in Myxine. Bdellostoma polytrema possesses thirteen 

 or fourteen pairs of gill-pouches. 



T 2 



