CHAPTER VI 



STRUCTURE AND CLASSIFICATION OF FISHES AND 

 PRIMITIVE VERTEBRATES 



The 7300 odd species of existing fishes known to science 

 are divided into the following five sub-classes: 



I. Lung-Fishes (Dipnoi). 



II. Bony Fishes (Teleostomi). 



III. Sharks and Rays (Elasmobranchii). 



IV. Chimaeras (Holocephali). 



V. Round-Mouths (Cyclostomata). 



Probably the best idea of the general structure of the group 

 will be obtained by briefly describing such a common example 

 of the Shark kind (Elasmobranchs) as the Spotted Dog-Fish 

 (Scyllium canicula), abundant on our shores, especially during 

 the herring season. It may to all intents and purposes be 

 regarded as a small shark. 



External Characters (fig. 166). The spindle-shaped body is 

 well suited for progression through the water, and its outline is con- 

 tinuous, there being no sharp boundary between head and trunk, 

 or trunk and tail. We have seen that in tadpoles and some adult 

 Amphibia (see p. 246) there is a membranous fringe bordering 

 the tail above and below and running forwards on the upper side 

 of the trunk. Such a longitudinal fringe running in the middle 

 line is known as an unpaired fin, and is especially characteristic 

 of Fishes, where, however, it is not a mere soft membrane as in 

 Amphibia, but is supported by firm rod-like structures, the Jin- 

 rays. We find that in the Dog-Fish, as in most fishes, this 

 membrane is not continuous, but is represented by a number of 

 separate pieces, each of which is named with reference to its 

 position. Here, for example, there is a caudal fin bordering the 

 tail, two dorsal fins in front of this above, and an anal or ventral 

 fin in front of it on the under side. Special attention may be called 



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