RESPIRATION 43 



As a general rule, gill-rakers are wanting in the Sharks, most 

 of which feed on other fishes, but the huge Basking Shark 

 {Cetorhinus) and Whale Shark (Rhineodon) are both provided 

 with many close-set, flattened and tapering gill-rakers, each 

 perhaps four or five inches long (Fig. 18). In appearance they 

 recall the baleen plates of the Whalebone Whales, which have 

 exactly the same function, namely to act as a filter to strain 

 off the most minute forms of animal life. When feeding, the 

 Basking Shark merely opens its mouth and takes in a mass of 

 water containing myriads of the minute crustaceans forming its 

 usual food. The water rushes out over the gills, and the animals 

 are left sticking to the inner walls of the throat and to the 

 filtering mechanism, where they can be conveniently swallowed. 



The rate of breathing seems to vary greatly in different fishes, 

 ranging from about twelve to fifteen respirations per minute in 

 the Wrasse (Labrus) and the Rockling [Motella) to as many as 

 150 in the Minnow {Phoxinus) and Stickleback (Gasterosteus) . 

 Professor Bashford Dean noticed that in the Australian Lung- 

 fish [Epiceratodus) the normal rate was about twelve per minute 

 on a cool day, but rose to as many as thirty-one as the water 

 warmed up. If the water is at all deficient in oxygen the rate 

 of breathing is naturally accelerated and the fish appears to 

 "pant" or respire hurriedly, while other factors which lead to 

 an increased rate are excessive activity or the stress of some 

 emotion such as greed or fear. 



All the breathing organs so far described have been what are 

 known as internal gills, but in certain fishes external organs 

 are developed similar to those found in larval newts and frogs. 

 In the young Selachians they take the form of long, delicate 

 filaments, protruding for some distance through the external 

 gill-clefts. These seem to aid in some way the breathing of the 

 embryo while enclosed in its egg-case, for they completely 

 disappear when it is hatched {cf. p. 332). Similar structures are 

 developed in the young of certain Bony Fishes, but are soon 

 discarded in favour of the more adequately protected internal 

 gills. In the young Bichir [Polypterus) there is a leaf-like external 

 gill projecting backwards from each side of the head above the 

 ordinary gill-opening. Here they are retained for a somewhat 

 longer period, but gradually become reduced in size as growth 

 proceeds (Fig. 19). 



The vast majority of fishes are quite incapable of living for 

 any length of time out of the water, and, as a general rule, 

 those with wide external gill-openings expire more rapidly than 



