The Early Vertebrates and the Fishes 



motions in the water. It is probably by means of these ' lateral 

 line ' sense organs, for instance, that fishes are so easily able to 

 avoid obstacles when swimming in the dark. 



The heart has one auricle and one ventricle, except in a single 

 group which we shall afterwards mention. The heart is situated 

 immediately behind the gills, to which the blood is pumped 

 directly by the ventricle. From the gills, the blood is collected 

 and distributed throughout the body, is re-collected and returned 

 to the auricle. The circulatory system is provided with a set of 

 blood-glands, essentially similar to those in man himself. There 

 is a spleen, a thymus and a thyroid gland, and a pair of supra- 

 renal bodies. The several functions of these glands form an 

 extremely difficult chapter of physiology, but, broadly speaking, 

 they are concerned in the formation of the white blood corpuscles, 

 the removal of worn-out red corpuscles, and in certain obscure but 

 important chemical changes in the composition of the blood. 

 The blood itself consists of a fluid plasma in which float white 

 and red blood corpuscles, the latter being flat and oval, and con- 

 taining the same oxygen-carrying substance, haemoglobin, as is 

 found in mammals. 



The alimentary canal is simple. The mouth cavity is suc- 

 ceeded by the pharynx, the walls of which are perforated by the 

 gill clefts. Next follow the gullet, the stomach, and the intestine, 

 the division into the three portions being apparent often only 

 after close examination. There are generally gastric glands, of 

 simple form, a large liver, and almost always a pancreas. The 

 kidneys and the reproductive organs open to the exterior by a 

 common duct. A further characteristic feature of the fishes is their 

 external covering of scales. True teeth, comparable to those of the 

 higher vertebrates, appear first in this group. Some of the main 

 features that we have mentioned are illustrated in Fig. 70. 



Careful study of the fishes makes it evident that they have 

 very much in common with the higher groups of vertebrates. 

 It is not too much to say, with Haeckel, that there is far more 

 difference between Amphioxus and the fishes than between the 

 fishes and man. 



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