638 PISCES. 



head, where they take an extensive attachment to the largely-developed 

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

 vening 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 (Exocetiis), 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 thunder- 

 ing cataract to stop its course towards the locality where it is instructed 

 by Nature to deposit its eggs. 



(1736.) There are sundry tribes of Fishes which, being destined to 

 remain at the bottom of the sea, present certain peculiarities of struc- 

 ture, whereby they are not only distinguished from all others of the 

 class, but form most remarkable exceptions to the general law in accord- 

 ance with which the Yertebrata are organized. 



(1737.) The animals presenting this anomalous configuration are the 

 Pleuronectidce, or Flat-fishes, as they are generally termed, which when 

 at rest lie quietly upon the ground, where, from the colour of the upper 

 part of their bodies, they are scarcely distinguishable. To an ordinary 

 observer the Pleuronectida3 would seem to have their bodies flattened 

 and spread out horizontally, so that, while resting upon their broad and 

 expanded bellies, their eyes, situated upon the back of the head, are 

 thus disposed for the purpose of watching what passes in the water 

 above them ; and this, the vulgarly-received opinion, is considerably 

 strengthened by the fact that what is usually called the belly is white 

 and colourless, while the back is darkly coloured and sometimes even 

 richly variegated, so as to harmonize with the prevailing tints of the 

 sea-bottom. Hence the appearance of these fishes is deceptive, and 

 few imagine that, in applying the terms back and belly to the upper and 

 under surfaces of a Plaice or a Turbot, they are adopting a phraseology 

 quite inadmissible in an anatomical point of view. 



(1738.) On examining the skeleton of a Flat-fish, we at once see 

 that what we suppose to be the dorsal and ventral regions are in reality 

 the two sides, which are thus strangely different in colour, and that 

 the great peculiarity of their structure is the want of symmetry between 

 the lateral halves of the body, arising from the anomalous circumstance 

 that both the eyes are placed upon the same side of the head. Their 

 cranium, indeed, is composed of the same bones as that of an ordinary 

 fish ; but the two lateral halves are not equally developed, and the 

 * TrXevpd, the side ; vijKTrjs, a swimmer. 



