CHAPTER XXIII 



THE TAIL 



An extension of the body behind the anus, containing all the 

 chief tissues of the body, is a structure characteristic of chordate 

 animals. Its original function was to assist the animal in swim- 

 ming, for it contains myotomes and a portion of the notochord, 

 and so is able to take part in the undulatory movements from 

 side to side which propel the animal forwards through the 

 water. The area of the tail is commonly increased by the 

 formation of a fin in the middle line, in the lower chor dates. 

 In Amphioxus, the fin is not very large, but it extends sym- 

 metrically from the middorsal and midventral lines of the tail, 

 and tapers to a point behind. This primitive type of tail 

 is called diphycercal. It is present also in the Cyclostomes, 

 where it is supported by cartilaginous radials, and in early 

 stages of development of other forms. 



In Selachians the tail is asymmetrical, for the vertebral 

 column is bent slightly dorsally, and the dorsal (epichordal) 

 lobe of the caudal fin is reduced while the ventral (hypochordal) 

 lobe is increased. The ventral lobe is supported by the 

 elongated haemal arches of the vertebral column, known as 

 the hypurals, and not by separate radials. This type of fin 

 is called heterocercal. In addition to the Selachii, it is present 

 in the sturgeon, the Osteolepidoti and the fossil Dipnoi. 

 Since the axis of the tail in such forms is bent up, in swimming, 

 the head of the fish tends to be turned down, as when the 

 fish noses along the bottom in search of its prey. 



In the higher bony fish (Teleosts) the dorsal lobe of the 

 caudal fin is further reduced and the ventral lobe enlarged, 

 with the result that the tail presents an externally symmetrical 



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