40 A HISTORY OF FISHES 



gressively shorter, until the condition described in the Salmon 

 is finally attained (Fig. 17). 



The type of gill structure described here is that found in 

 almost all Bony Fishes, but in the Sea Horses and Pipe-fishes 

 {Syngnathidae), sometimes spoken of as Lophobranchs (tuft 

 gills), the filaments are reduced to small rosette-Hke tufts 

 attached to quite rudimentary arches. 



When a fish breathes, the initial movement consists of an 

 expansion of the hoop-like gill-arches, with a consequent en- 

 largement of the cavity of the pharynx; at the same time the 

 mouth is opened a little, and a stream of water is drawn into 

 the pharynx, the external gill-opening or openings being kept 

 tightly closed. This is known as the inspiratory phase. It will 

 be noticed that as a general rule the fish does not use the 

 nostrils for breathing purposes, and, although water is inhaled 

 through these apertures in a few forms hke the Chimaeras and 

 Lung-fishes, they usually serve only as organs of smell {cf. p. 182). 

 The expiratory phase, which follows immediately, consists in 

 closing the mouth tight, and at the same time contracting the 

 pharynx, thus driving the water outwards over the gills and 

 through the external openings. The actual exchange of gases 

 takes place as the v/ater passes over the gills. The walls of the 

 fine blood-vessels with which the deHcate filaments are suppHed 

 are exceedingly thin, so that the blood circulating through 

 them is separated from the water by nothing but an infinitesi- 

 mally fragile and permeable membrane. Fish blood, just hke 

 our own, consists of a fluid substance, the plasma, in which 

 float multitudes of red and white corpuscles. The red cor- 

 puscles, forming flat, circular discs, contain that remarkable 

 substance known as haemoglobin, which has the power of taking 

 up oxygen under certain conditions and, later, of giving up 

 this oxygen to the body cells, receiving in exchange their waste 

 products in the form of carbon dioxide. 



Most fishes breathe in the manner described, but there are 

 sonie which, owing to their pecuHar mode of life, have been 

 obliged to modify this process in accordance with their change 

 in habits.^ The Lamprey {Petromyzon) , for example, has adopted 

 a parasitic habit, and spends a good deal of its time attached 

 to other fishes by means of its sucker-like mouth. It is quite 

 obvious that while in this position it would be impossible to 

 inhale water through the mouth without losing its hold, and, 

 instead, water is often taken into and expelled from the branchial 

 sacs by their external openings, through the alternate expansion 



