LOCOMOTION OF ANIMALS IN FLUIDS. 185 



anterior limbs are altogether paddle-like, the integuments extending quite up 

 to the last phalanges ; the hinder-limbs are placed almost in a line with the 

 body, and are used somewhat like the tail of a fish. 



In the Cetacea, which, though breathing by lungs, are adapted entirely to 

 living in the sea, or the mouths of rivers, the swimming action is, in a few 

 cases, aided by short, paddle-like, anterior limbs (dugong), but it is, in all, 

 mainly effected by powerful alternate movements of the trunk and tail, which 

 strike the water upwards and downwards like the body of the flat fishes, and 

 not laterally like an ordinary fish. The caudal fin-like expansion of the ceta- 

 cean has accordingly its surfaces directed upwards and downwards, instead of 

 laterally, like the fish. 



The specific gravity of birds, with their feathers perfect, is, as a rule, less 

 than that of water. This is owing to the large proportionate size of their 

 respiratory organs, the lightness of their bones, the cavities of which are very 

 large, though filled with fat instead of air in most swimming birds, and lastly, 

 to the quantity of air which is entangled in their down and feathers, and held 

 there, because the oily secretion with which they cover themselves, prevents 

 its displacement by the water. Hence birds of this kind are so buoyant on the 

 water that a much smaller proportion of their body is submerged than in the 

 case of quadrupeds or of man. The form of the body, too, is suitable for float- 

 ing, being boat-like, and so curved at the fore part, and gradually narrowed 

 behind, as to present but slight resistance to the water, and the smallest 

 amount of friction in the displacement of that fluid behind. The legs of swim- 

 ming birds are placed further back, and wider apart, than in land birds, and 

 are articulated in such a direction, as to spread away from the body, arrange- 

 ments calculated to give lateral play to the actions of the feet, and to increase 

 the efficacy of their propelling power over the body ; moreover, the tarsi are 

 flattened sideways, so as to present the smallest possible resistance in being 

 moved forward through the water ; whilst the phalanges of the toes, also long, 

 compressed, and, when flexed, folded very flatly together, spread out widely 

 when extended, and are more or less completely webbed, so as enormously to 

 increase the power of the stroke in the water in swimming. This stroke is 

 backwards and a little outwards, so that both the limbs act on the water, along 

 two lines diverging outwards and backwards from the middle of the pelvis of 

 the bird, the water reacts in the opposite direction, and the converging forces 

 thus obtained are combined, according to the rule of the composition of forces, 

 into a resultant force, which impels the body directly forwards. 



Swimming birds are ungainly in their walking gait, the backward position 

 of the legs necessitating a more horizontal position of the trunk ; the great 

 width between the legs, the looseness of the joints, and the softness of the 

 feet, give them a waddling and feeble motion in walking. Diving birds have 

 generally the centre of gravity of the body situated further back than other 

 birds, the head, neck, and anterior part of the trunk presenting a narrow or 

 pointed form ; besides the impetus with which they throw themselves into the 

 water, these birds aid themselves by movements, not only of their feet, but 

 also of their short and almost paddle-like wings. 



A large number of the Saurian reptiles swim perfectly well in water, mainly 

 by lateral strokes of the hinder part of the body and tail, the latter being 

 usually more or less flattened sideways for that purpose, and acting in the 

 same manner as a fish's tail. In some cases, as in the crocodiles, the nostrils 

 are placed at the tip of their long muzzle, so that they can completely sub- 

 merge themselves, with the exception of the nostrils. The Ophidian reptiles 

 can probably all of them swim, the water-snakes enjoying that faculty, how- 

 ever, in an eminent degree ; in this movement, the head of the snake is raised 

 above the surface of the water for respiratory purposes, whilst rapid lateral 

 undulations of the body are effected, by means of which the water is struck 

 obliquely backwards by a series of flexures, at intervals, on one side, and by 

 another series of flexures on the other side ; and the resultant action is repre- 

 sented by two oblique lines diverging, outwards and backwards, from the 

 middle part of the elongated body ; but the forward lines of reaction of the 

 water on each side, converge, and meet in the body, so that the resultant force 

 of all the lines of reaction on the two sides, impels the animal in an inter- 



