34 A HISTORY OF FISHES 



leaves no doubt that gill-breathing represents the more primi- 

 tive method of respiration, and must have gradually given 

 place to lung-breathing as the ancestors of modern terrestrial 

 vertebrates left the water for the dry land. All the higher 

 vertebrates, whether amphibians, reptiles, birds or mammals, 

 provide us with indubitable proof of their fish ancestry by the 

 possession of gill-like structures of one kind or another at some 

 stage of their lives. In a human embryo, say of about three 

 weeks, the sides of the throat are provided with four pairs of 

 clefts, which not only correspond in position to the gill-slits 

 of a fish, but their supporting skeleton and associated 

 blood-vessels provide further resemblances. This fish-like 

 apparatus is, of course, never used for breathing, and as 

 development proceeds it becomes modified out of all recog- 

 nition. 



A proper understanding of the functions of the gills is im- 

 possible without some idea of their anatomy, and at the risk 

 of introducing what may appear to be needless technicalities, 

 it will be necessary to give a brief account of the salient features 

 of their structure. The principles of respiration are essentially 

 the same in all fishes, but a marked difference in the type of 

 gills is found in the three main classes, and it will be con- 

 venient to describe first of all the conditions found in the 

 Selachians, and then to compare them with those existing in 

 the Cyclos tomes and Bony Fishes. All breathing organs, no 

 matter what their form, are closely associated with the upper 

 part of the food channel or alimentary canal {cf. p. i68). Such 

 a connection between the nutritive and respiratory apparatus 

 is highly characteristic of backboned animals, and persists even 

 in man himself, but in fishes the two remain more or less 

 intimately connected throughout life, whereas in the higher 

 animals the association is generally a temporary embryonic 

 phase. 



In a typical shark the side walls of the pharynx, that is to 

 say, of that portion of the alimentary canal at the back of the 

 mouth immediately in front of the commencement of the 

 narrow gullet, are perforated by a series of narrow openings 

 (Fig. 15A, ph.). Each of these pharyngeal openings leads into a 

 kind of flattened pouch, which in turn communicates with the 

 exterior by a comparatively narrow slit, the external gill-cleft, 

 lying on the side of the head between the eye and the pectoral 

 fin (Fig. 14A, g.c.l.). As a rule these clefts are not very long, 

 but in the huge Basking Shark {Cetorhinus) they extend from the 



