186 SPECIAL PHYSIOLOGY. 



mediate direction, that is, directly forwards. The Chelonian swimming rep- 

 tiles (turtles) move in the water by means of both pairs of limbs, which have 

 a paddle-like shape, and a lateral, widespread action, rotating on their axes, 

 so as to be feathered, or to present their edge, in being drawn forwards, while 

 they offer their flat surface to the water in the backward stroke. The com- 

 pressed shape of the body of the swimming chelonia, offering so thin a trans- 

 verse section in passing through the water, contrasts remarkably with the 

 dome-shaped shell of the terrestrial chelonia or tortoises. 



The tailed Amphibia (newts) swim, after the manner of the saurians, or 

 ophidia, by simple, lateral, undulatory, strokes of the body or tail. The tail- 

 less Amphibia (frogs) swim by the force of their powerful hind-legs, provided 

 with long and webbed toes. The stroke of the frog in swimming is very 

 similar to that of man, and it has often been noted that, in proportion to the 

 size of the body, the frog has a larger muscle of the calf (gastrocnemius), for 

 the extension of its foot upon the leg, than any other animal. 



In the previous examples of swimming animals, we have had under obser- 

 vation creatures which, breathing by lungs, and requiring therefore, from time 

 to time, and often at frequent and short intervals, to respire air, cannot be 

 said to inhabit the water, but rather enter it for temporary purposes, for cap- 

 turing food, or for other ends. They cannot endure continued submergence, 

 except in conditions of hibernation. But we now pass to the contemplation 

 of animals organized for permanent subsistence in, and complete submergence 

 beneath, the water, breathing by gills. Fishes, considered in reference to 

 their manner of swimming, exhibit three principal modes. First, the ordi- 

 nary mode, by lateral strokes of the body, tail, and caudal fin, in opposite 

 directions, as in common fishes ; secondly, by the vertical flapping of the 

 body, lateral fins, and tail, or by vertical undulations of large lateral fins, or 

 of a thin marginal fin, as in the flat fish, torpedo, turbot, sole, plaice, and 

 flounder ; and thirdly, by lateral undulatory movements of the body and tail, 

 as in lampreys and eels. Besides this, there are exceptional modes of pro- 

 gression which we cannot notice here. 



The first form of progression in Fishes, is the typical one. In the most 

 perfect cases, the body of the fish is elongated, and its centre of gravity, and 

 greatest transverse sectional area of displacement, are situated well forward, 

 the form being suddenly narrowed to the nose, whilst it is gradually narrowed 

 backwards to the tail, a shape which, as has been demonstrated by laborious 

 calculations made in reference to shipbuilding, offers the least possible resist- 

 ance to progression through water. The body of the fish is deeper vertically 

 than it is thick from side to side, a form which gives it stability in the water, 

 by preventing rolling, and affords more ample space for the attachment of the 

 lateral planes of muscle, destined to deliver the powerful side strokes of the 

 body and tail. There are no vertebrae which can be called cervical, lumbar, 

 or sacral, but all are either dorsal or caudal. There is therefore no neck in 

 the fish, so that the head is fixed directly and stiffly on the trunk, without the 

 intervention of any weak portion or neck. The fins are of two kinds, viz., 

 single and median, and double and laterally symmetrical, fins. The single 

 fins are dorsal, caudal, and post-abdominal or anal ; the first and last increase 

 the lateral area of the fish, and add to its balancing power, whilst the caudal 

 fin acts as an extension backwards of the tail, and so increases the power of 

 its stroke. The lateral, double, and symmetrical fins are the right and left 

 pectoral, and the right and left abdominal or ventral : these are the true anterior 

 and posterior limbs of the fish ; the pectoral pair are always situated at the 

 under and back part of the head, just behind the gill-openings ; the abdominal 

 vary in their point of attachment in different, species, from a place near or 

 even anterior to the pectorals, backwards to the hinder part of the abdomen. 

 These lateral fins are more used in the slighter balancing, ascending, descend- 

 ing, or turning movements of the fish, than as instruments of progression, 

 that is, in ordinary fishes ; for in the ray-tribe they are enormously developed, 

 and form the chief organs of locomotion, whilst by the flying fishes they are 

 employed in flight. 



In the ordinary swimming movement, the tail, being first curved slightly 

 forwards to one side, is then suddenly and powerfully extended backwards into 



