INTRODUCTION ii 



would more nearly resemble one another than do the fish and 

 the dog. But the slightest analysis of their structure will show 

 that they do not. 



The fish has a distinct head provided with jaws, and in the 

 jaws are teeth. On its snout are the openings of the organs of 

 smell, usually called olfactory organs, and they have the form 

 of a pair of pits with external apertures. Behind them is a 

 pair of eyes, placed in sockets, moveable by muscles fixed on 

 a definite hollow eyeball. Behind the eyes we find, embedded 

 in the skull, a pair of organs of hearing — the auditory organs. 

 All these structures are present in the dog also, and though 

 the details differ largely, the general arrangement is the same. 

 The muscles which move the eyeballs are almost identical. 

 In both the dog and the fish the head is largely formed of a 

 bony and gristly case or box, which contains a mass of nervous 

 structure — the brain. The fish has no distinct neck, the head 

 passing without any distinct intervening region into the trunk 

 or body. The dog has a neck. Both the fish and the 

 dog have a distinct backbone, or, as anatomists call it, a 

 vertebral column, composed of numerous short bony joints 

 or pieces, arranged in a row down the middle of the back, 

 ending behind in a tail and jointed in front to the brain-case 

 or skull. And we find that in both dog and fish there is a 

 little arch placed on the top of each joint of the backbone, 

 and in the canal formed by all these arches there lies a long 

 nervous cord, the spinal marrow or spinal cord, which is 

 directly continuous in front with the brain. The dog has two 

 pairs of limbs used for walking and running, the fish has two 

 pairs of fins used as aids in swimming. The fins are not at 

 all like the dog's legs, but they occupy the same relative posi- 

 tions, and are comparable to some extent. Other noticeable 

 features show considerable differences between dog and fish. 

 The dog has a coat of hair covering its body — the fish a coat 

 of scales. The dog breathes air by lungs, the fish breathes 

 water by gills, which are really a series of apertures passing 

 from the throat to the exterior, the walls which separate the 

 apertures being covered with folds of tissue richly supplied 

 with blood-vessels. A legion of other characters, some of 

 similarity, some of difference, between dog and fish might be 

 enumerated, but enough has been said to show that, different 

 as dog and fish are, they do resemble one another in funda- 



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