182 SPECIAL PHYSIOLOGY. 



commonly far greater than the vertical ; secondly, of the intercostal muscles 

 acting upon the ribs, which represent long levers attached to the sides of the 

 vertebrae ; and thirdly, of certain transverse cuticular plates, situated on the 

 abdominal surface, attached at either end to the extremities of the correspond- 

 ing pairs of ribs, covered with strong epidermis, and named the abdominal 

 scutcc, ; these scutse are imbricated, each overlapping the one behind it, so that 

 their free edges are directed backwards ; and, when elevated by proper muscles, 

 they take a powerful hold upon the ground, or upon other surfaces, such as 

 projecting rocks, and the trunks and boughs of trees. Certain serpents coil 

 themselves up into concentric rings, and then, resting the tail firmly upon the 

 ground, holding by their scutae, rapidly unfold their spires, and dart them- 

 selves for a certain distance through the air. The extraordinary locomotive 

 powers of serpents, which can glide, or creep, or climb, or swim, or even spring, 

 through the air, are very remarkable, especially when their apparent locomo- 

 tive inferiority, in the total absence of limbs is considered. In the Chelonian 

 Keptiles, or tortoises, the gait is slow and labored, owing to their wide-spread, 

 and laterally attached, limbs, the shortness of the levers of which these are 

 composed, the comparative feebleness of their muscles, and the great propor- 

 tionate weight they have to carry ; the tortoise is proverbially slow. The 

 turtles walk still more awkwardly, their extremities being adapted rather as 

 paddles for swimming purposes. 



Amongst the Amphibia, the motion is quadruped in the frogs, toads, and 

 newts ; but in the more defective species, as the proteus, the movement is 

 rather of a creeping kind, the body being supported on its under surface. The 

 energetic leap of the frog is due to the great comparative length and strength 

 of its hinder extremities ; whilst the position of the animal, inclined at about 

 an angle of 45 from the horizon, is that which is best adapted for obtaining 

 the longest trajectory over the surface of the ground, with a given expenditure 

 of power. The toads, which hop much more feebly, and often walk or run, 

 have the body placed more horizontally and the hinder limbs shorter than in 

 the frog. 



Amongst Fishes, the eel, in its migrations, sometimes crawls over soft mud- 

 banks or grass, moving by lateral undulations, after the manner of a serpent, 

 but much less easily, as it has no abdominal scutse. There is a species offish, 

 the Anabas, of Tranquebar, which occasionally leaves the water for a time, 

 and even ascends the trunks and branches of neighboring low trees, accom- 

 plishing this by means of its pectoral and abdominal fins,"w r hich are, in fact, 

 its anterior and posterior limbs. This animal is provided with a number of 

 large cells on the side of its head, in which it can receive and carry a supply 

 of water for its gills during its temporary aerial journey from its proper 

 element. 



Passing now from the Vertebrate to the Molluscous subdivision of the ani- 

 mal kingdom, we find comparatively few of this group which move over solid 

 surfaces. The air-breathing terrestrial Pulmogasteropods (snails, &c.) creep 

 over a solid surface by means of their muscular foot, which adheres closely, 

 without the intervention of air, to the object to which it is attached, moving 

 over it by means of longitudinal undulations, so minute and rapid, in some 

 cases, as to require a magnifying glass for their detection ; they may, how- 

 ever, be easily seen by watching, through a common lens, the under surface 

 of a small slug or snail, creeping up a piece of glass. Certain Lamellibranchiata 

 (the pectens) can project themselves from the bottom of the ocean, a short dis- 

 tance through the water, by means of a strong curved foot, which they thrust 

 from their shell in a bent direction, and then suddenly straighten. Others have 

 the power of turning themselves over, or dragging themselves along, step by 

 step, by fixing and contracting a long muscular appendage. 



The Molluscoida present no examples of motion on solids. 



In the Annulose animals locomotion over solids is a characteristic mode of 

 progression, as in Insects, Spiders, Crustaceans, Mvriapods, and Worms. In 

 the higher forms this is accomplished by means of limbs, many in number, 

 provided with numerous joints, and acted upon, after the manner of levers, by 

 powerful muscles. These muscles, however, as seen as in the familiar example 

 of the crab, are contained within the moving levers, instead of the levers 



