PISCES 



FISHES. 



501 



Similar fasciculi spread out or approximate the rays of the tail, 

 increasing or contracting at will the extent of surface presented 

 by that organ. The muscles of the pectoral and ventral limbs are 

 small in proportion to the feebleness of these extremities ; the 

 muscles of the trunk alone constitute the great bulk of the body, 

 and form the efficient agents in progression. 



Fig. 224. 



These great lateral masses commence at the back of the head, 

 where they take an extensive attachment to the largely developed 

 cranium : from this point backwards, they fill up the entire space 

 intervening between the skin and the vertebral column, with both 

 of which they are intimately connected, reaching even to the origin 

 of the tail fin. The whole force of these powerful muscles is 

 evidently exerted in bending the spine from side to side, and in 

 effecting those vigorous lateral movements of the tail whereby the 

 fish is propelled through its liquid element. We need, therefore, 

 feel little surprise at the strength with which this part of the body 

 of fishes is not unfrequently endowed, or at the velocity of their 

 movement ; at seeing how easily their speed outstrips our fleetest 

 ships ; how the Flying-fish (Exocetus), urged on by fear, darts like 

 an arrow to a distance through the air; or how the Salmon, in 

 obedience to an imperious instinct, defies even the thundering 

 cataract to stop its course towards the locality where it is in- 

 structed by Nature to deposit its eggs. 



(539.) There are sundry tribes of fishes, which, being destined 

 to remain at the bottom of the sea, present certain peculiarities of 

 structure, whereby they are not only distinguished from all others 



