144 THE BOOK OF NATURE STUDY 



The gill-filaments composing a gill look very red because of 

 their rich blood supply. The use of the numerous filaments 

 is obviously that the blood is thus spread over a large surface, 

 and is able more effectively to gain oxygen from the water and 

 to part with its carbon dioxide. A gill may be compared to a 

 much cut-up leaf, or to a country with a much cut-up coast line. 

 To the inside of the gill-arch there is a series of gill-rakers which 

 form a sieve or filter preventing food-particles from coming 

 through the gill-clefts. Some gills should be neatly cut out 

 and examined in a glass saucer. A little patience will be needed 

 before clearness is reached in regard to the gill-clefts, the gill- 

 arches, the gill-filaments or lamella, the gill-rakers, the gill-cover, 

 the gill-chamber, and the gills themselves. 



(h) The Lateral Line. Most fishes show a very distinct line 

 running along each side of the body. It usually divides into 

 several branches on the head. It is a canal protected by the 

 scales, with groups of sensitive cells at intervals and with pores 

 opening to the exterior. There is no doubt that it is sensory, 

 and there is considerable evidence to show that it is an exceed- 

 ingly delicate organ of touch. Somewhat similar are the " jelly- 

 tubes" of the skate, etc., and the sensitive pits or spots very 

 common on bony fishes. It is interesting to notice that the 

 lateral -line system is found only in fishes and Amphibians. 

 Indeed in many Amphibians it is confined to the larval stages. 

 In higher animals its place is taken by the touch-spots and other 

 nerve-endings in the skin. 



(f) The Skin. As an exercise senior pupils might be asked 

 to compare the skin of a fish with that of a higher animal, such 

 as a rabbit. 



Under the term "skin" are included two very different layers, 

 the outer epidermis and the inner dermis. The epidermis is 

 always being worn away and is continually replacing itself. The 

 dermis is a more permanent layer and it contains blood-vessels. 

 It is easy to show that a scratch does not draw blood unless it 

 gets through the epidermis. 



In all animals higher than fishes the dermis contains con- 

 nective tissue, blood-vessels, nerve-fibres, and muscle-fibres. In 

 fishes there is very little development of muscle-fibres in the dermis. 



