AQUATIC RESPIRATION OF FISHES 25 



the bottom. Some of these fishes do not make active respiratory 

 movements when swimming, but rely upon the current entering 

 the mouth as a result of their forward movement. This mechan- 

 ism is found in some sharks but the best-known example is the 

 mackerel which is unable to maintain full oxygenation of its 

 blood when prevented from active swimming in an aquarium. 

 In these instances, then, the passage of water across the gills is 

 mainly due to a greater pressure in the mouth and is analogous 

 to the pressure-pump mechanism. Indeed, pelagic fish which do 

 not rely on the current resulting from their swimming move- 

 ments depend on the buccal pressure pump far more than the 

 suction pumps for ventilation of the gills. At the other extreme 

 fish which spend most or the whole of their lives on the sea 

 bottom depend to a far greater extent on suction-pump mechan- 

 isms (fig. 8). Such teleosts as the dragonet, plaice, sole, and other 

 flatfishes are notable teleost examples and rays and skates 

 among the cartilaginous fishes also show greater dependence on 

 their parabranchial suction pumps. Further adaptations of these 

 bottom-living forms are found in the position and nature of the 

 opercular openings. In the gurnard, for example, the exit of 

 water from the opercular cavities is restricted to a small dorsal 

 portion of the total opercular opening (fig. 8). The dragonet 

 shows a similar dorsally-directed respiratory current and in this 

 case the opercular cavity only communicates with the exterior 

 through a small hole. In flatfishes, both bony and cartilaginous, 

 it appears that the opercular and gill-slit openings may be actively 

 closed and so prevent the entry of any sand grains into the gill 

 chambers. Fish which lie on one side, such as plaice, use the gills 

 on both sides which are equally developed as are the opercula. 

 Most cartilaginous fishes are heavier than water and when at 

 rest they sink to the bottom. The presence of the dorsaUy-directed 

 spiracle may be associated with this feature for it enables clean 

 water to be respired when the fish is on the bottom. There is a 

 rough correlation between size of the spiracle and habit of the 

 fish. It is absent in the pelagic Holocephali (which also have an 

 operculum) and is relatively small in pelagic sharks but is ex- 

 tremely well developed in the bottom-living skates and rays. In 



