Class 2. Pisces. 



361 



posterior joint of the vertebral column ; below this apparently last 

 joint, there is usually a portion supported by rays, and almost 

 congruent with its upper portion, so that the tail appears to be 

 diphyceroal ; this is termed a homocercal tail. As a matter of 

 fact, however, it is just like the heterocercal, since the end of the 

 vertebral column is bent upwards, and the dorsal portion of the fin 

 is smaller than the ventral (see Fig. 299 D). In many, actually 

 heterocercal, forms, in which a great length of the vertebral column 

 is bent up, there is an approach to the same structure ; since the 

 end of the tail, as regards the exterior, is divided into two almost 

 equal portions : a dorsal, into which the vertebral column is pro- 

 longed, and a ventral, consisting exclusively of rays (Fig. 299 A, B). 



Fig-. 299. End of the tail of various Fish : A Sturgeon, B Pike, Salmon, 

 P C o d . h vertebral column, h' bent up termination of the same, ci upper arch, t neural 

 ;spine, n lower arch, n' last lower arch, united with h'. In the bent portion of the 

 vertebral column is still fairly well developed (it is enclosed between the two halves of the 

 caudal fin rays of which the left are removed in the figure), in D, which represents the usual 

 'Teleostean condition, it is on the contrary very small. — Partly original, partly a copy. 



In the embryo, and in many cases also, in the newly-hatched animal 

 (Teleostei), the notochord is for some time a straight rod; later the 

 posterior end bends up, and is relatively much larger than in the 

 adult. 



Well developed, bony, or partly ossified ribs are attached to the 

 transverse processes of the trunk vertebrae in most Ganoids, Teleo- 

 steans, and Dipnoans ; in the Selachians the ribs are wanting or 

 very short; they are absent also from the Cyclostomes. There is 



