THE SYMMETRY OF ANIMAL FORMS. 85 



we speak of the attached end as the proximal, or near 

 endj and of the free end as the distal, or far end. 

 The limbs of vertebrates are provided, like their bodies, 

 with supporting structures called bones. In the lowest 

 vertebrates, the fishes, the limbs divide at the ends 

 into a number of bony rays ; these are called fin-rays. 

 Many fishes have also a median,^ or middle, fin, along 

 some part of the middle line of the body. 



In the higher vertebrates the fin-rays are usually 

 represented by digits, that is to say, fingers and toes, 

 larger in size than the fin-rays of the fish, and usually 

 protected by a hard dermal tip at the end forming a 

 nail or claw. But they are subject to great modifica- 

 tions. 



The Tail. — From the median fin-ridge of fishes, 

 which gives rise to the median fin just referred to, are 

 also formed the fins of the tail — the most important 

 fins of all, with which the fish not only propels but 

 steers itself. The lobster's tail, of four loose flaps, 

 should be noticed by the student, and compared and 

 contrasted with that of the fish. 



The tail is not always developed; and when it is, 

 it is often merely as an ornament. But to the birds 

 it is nearly as useful as to the fishes ; and in monkeys, 

 and other animals of arboreal habits, it is often 

 prehensile, and used in climbing. Among English 

 animals, the little harvest mouse is the only one that 

 can use its tail in this way. The tail does not ap- 



' This is usually spoken of as the Azygous, or unpaired fin. 



