FISHES AND SEA-SQUIRTS. 299 



the tail fin is unequally lobed, the upper lobe being larger 

 than the lower. The result of this arrangement is that at 

 each stroke the body is inclined downwards, for the larger 

 lobe naturally gives greater impetus than the smaller. The 

 reason for this is that those fish which have unequally lobed 

 tails have their mouths on the under surface, and are usually 

 ground feeders, so that each stroke drives them nearer their 

 food, which they reach from above. Fishes with equally 

 lobed tails, on the other hand, have terminal mouths, are 

 swifter and more highly specialised. It should also be 

 noted that in fish the tail is the main organ of propulsion, a 

 fact which has resulted in various modifications of the body. 

 One of the most interesting of these is that the internal 

 organs have been shifted forwards, so as to leave the tail a 

 mere mass of solid muscle. It is a familiar fact that the 

 posterior opening of the food canal in a fish is far forward, and 

 that the body organs, heart, alimentary organs, reproductive 

 organs, etc., are, roughly speaking, crowded into the small 

 space in front of this opening. On the other hand, in most 

 vertebrates, such as frog, bird, mammal, the viscera extend 

 to the posterior end of the body, and the limbs are the great 

 means of propulsion. The student will find it of much 

 interest to compare the conditions in prawn or lobster with 

 those obtaining in a typical fish. In both cases the tail is 

 used as an organ of propulsion, and in both cases is in conse- 

 quence converted into an almost solid mass of muscle, which 

 renders both sought after by man as food. There are, how- 

 ever, many interesting differences in detail in the mechanism 

 in the two cases, and some even more interesting resem- 

 blances. Thus, in both cases the kidneys are shifted far 

 forward into the head region. It must not, of course, be 

 supposed that there is any relation between lobster and fish, 

 even if the former is legally a " fish," but the two have both 

 solved a mechanical problem after a similar fashion. 



While swimming is effected in a fish by means of the 

 tail, the necessary steering is accomplished by means of 

 the fins. Of these there are two kinds the paired fins 

 corresponding to the limbs of other vertebrates; and the 

 unpaired fins, which are to be found in the middle axis of 

 the body, and vary much in different fish. It is especially 

 interesting to note the position of the paired fins. As they 



