VERTEBRATA PISCES. 63 



water into its mouth by a process allied to swallowing. 

 This water is driven from the pharynx, through the slits, 

 into the gill chambers, and having passed over the gills, 

 escapes by apertures at each side of the neck, called the 

 " gill slits." These gill slits are closed in front by the 

 operculum (Lat. operculum, a lid) or gill cover. The 

 blood contained in the mumerous vessels with which 

 the gills are supplied, is aerated by the oxygen of the air 

 contained in the water which thus passes over them. 



Most fish are furnished with a sac filled with gas, called 

 the " swim bladder." It lies between the alimentary 

 canal and the kidneys. Its probable use is to enable the 

 fish to maintain the proper specific gravity. In the mud- 

 fish it opens into the gullet by a tube, and seems to have 

 some analogy to the lungs of the higher vertebrates. 



65. Nervous System and Senses. The brain of fishes is 

 very small compared with the spinal cord. It consists of 

 a series of lobes or ganglia connected by nerve fibres. 

 There are one or two nostrils, to which water is ad- 

 mitted, but the nasal sacs are closed behind, and, except 

 in the lepidosiren or mud-fish, do not communicate with 

 the pharynx as in the higher Yertebrata. The organs of 

 hearing consist of two cavities placed on the side of the 

 head filled with fluid, and containing each two otolithes 

 (Gr. OILS, otosj an ear; lithos, a stone). There is no 

 external ear. 



66. Covering. Fishes are covered with scales, which 

 usually overlap one another, like the slates on the roof of 

 a house. These scales are of four different kinds : 



1. Cycloid (Gr. Jeuklos, a circle; eidos, form). These 

 are thin, smooth, and horny, and are either circular or 

 oval in shape. Most of our common fishes have cycloid 

 scales. 



2. Ctenoid (Gr. kteis, ktenos, a comb ; eidos, form). 

 These are also thin and horny, but their hinder edges are 

 fringed with comb-like spines. The scales of the perch 

 are ctenoid. 



3. Ganoid (Gr. ganos, splendour; eidos, form). 



