THE STRUCTURE OF FISHES 



59 



fresh-water dogfishes, a modified heterocercal type is present. In this 

 type the ventral lobe only is developed in adults, leaving a rounded 

 tail. The homocercal type is found in most fresh-water fishes. It is the 

 type of tail fin in which the lobes are equal, although in many species 

 it may not be forked. The backbone does not extend into either lobe. 



The fins are supported by bony structures consisting of soft rays 

 and spines. The soft rays are usually flexible, segmented structures, 



c 



Diagrams. (A) Heterocercal tail oi Scaphirhynchiis platorynchu^. (B) Modified 

 heterocercal tail of Lepisosteus osseus. (C) Homocercal tail. 



which often branch or diverge at their outer ends. The true spines are 

 often hard, pointed structures which are unsegmented and restricted 

 to median fins. Although true spines are found only in the median fins, 

 the stiffened rays of paired fins are usually referred to as spines. Many 

 fishes have fins containing both spines and rays. In some fishes, such 

 as the trout and the catfishes, a small fleshy lobe is present between the 

 dorsal fin and the caudal fin; this is the adipose fin and is entirely with- 

 out rays or spines. Usually the number of spines is expressed in Roman 

 numerals and the ravs in Arabic numerals. 



