242 FISHES 



Amphibia and all terrestrial vertebrates. In bony fishes the 

 posterior nostrils have closed up, and the external nostrils are 

 again divided into two small apertures. The skin of the hyoid 

 arch grows out into a flat plate, the gill-cover or operculum, 

 which covers all the gill-clefts, leaving only one external aper- 

 ture ; the operculum is supported by several flat, broad, dermal 

 bones (Fig. 20, op.). The broad gill-septa are reduced to narrow 

 bars from each of which project two series of gill-filaments 

 corresponding with the gill-folds of the Elasmobranch ; the 

 hyoidean gill-series becomes rudimentary, and is called a 

 pseudobranch. There are thus said to be four gills in the 

 bony fish, a gill being the gill-bar with its double series of fila- 

 ments, and these gills can be seen as four red fringes beneath 

 the gill-covers in any common fish. 



The heart of the fish consists of one auricle and one ven- 

 tricle, and it is situated in a special cavity called the pericardium, 

 just behind the gills and ventral to the pharynx. The ventricle 

 is triangular in shape, and from its apex, which is anterior, 

 passes forwards the main artery which carries the blood towards 

 the gills and which is called the ventral aorta. In the dog-fish 

 the ventral aorta is elongated and the part nearest the ventricles 

 is muscular and contains several transverse rows of valves ; this 

 part is called the truncus arteriosus. In the bony fishes the 

 aorta is shortened and the truncus arteriosus is reduced, until in 

 the Teleosteans there is no truncus arteriosus but only a single 

 row of valves and beyond them a small dilatation called the 

 bulbus arteriosus or aortic bulb. From the ventral aorta or 

 aortic bulb are given off on either side the " afferent arteries " 

 which convey the blood to the fine blood-vessels or capillaries 

 of the gills ; "efferent arteries" convey the blood from the gills 

 to the dorsal side of the pharynx where they unite to form the 

 carotid arteries anteriorly and the dorsal aorta posteriorly ; the 

 former convey the blood to the head and brain, the latter gives 

 off branches to the various parts of the body including the 

 digestive organs. In its passage through the gills the blood 

 absorbs the oxygen dissolved in the water and gives up the 

 carbonic acid or carbon dioxide which it has absorbed from 

 the tissues. The respiratory movements of the mouth and 

 pharynx take in water by the mouth and expel it through the 

 gill-clefts, so that the water in contact with the gills is con- 



