Locomotion of Animals

LOCOMOTION OF ANIMALS. It is characteristic of animals that they can move about in search of food, foothold and mates, or away from enemies and hurtful influences; this locomo tion is effected in a great variety of ways. Among multicellular animals there are four chief methods, which, following F. W. Gamble, may be illustrated by picturing a man in a boat.

Pulling.

The man may reach forward with a boathook, fasten it to some prominence like a tree-root, and pull the boat forwards. This is the pulling method, and is well illustrated by leeches and starfishes. When looping along, the leech exhibits (see fig.) the following order: Fixing its mouth and loosening the posterior sucker, it pulls its body forward, contracting its longi tudinal muscles. At the end of this process the posterior sucker has been brought forward almost to touch the margin of the mouth, and the body is arched upwards like a croquet hoop. Then the posterior sucker being fixed, the mouth is freed, and the body is protruded forwards to a new position of oral attachment. This protrusion is effected by a contraction of circular and diag onal muscles, which squeeze the body forward. The mouth is re-attached, the posterior sucker is loosened, and the sequence re-begins.

Punting.

The man may stand up in the boat and use a pole as a lever, pressing it against the floor of the stream. This "punt ing" is one of the commonest modes of animal locomotion, being exhibited by all the diverse types that have firm appendages useable as levers against a base. A beetle hurrying across the roadway, a crab walking over a rock, a frog jumping among the grass, an ostrich sprinting at full speed, a man walking—all are using levers which propel the body forwards by pressing against a hard substratum. Sometimes there are complications, which do not essentially affect the principle of the method employed. Thus the freshwater mussel may make its flabby "foot" tense with blood, close a sphincter muscle which prevents backflow, and then pull the ploughshare-like organ backwards against the sand, thus push ing its body forwards. The foot has to be protracted before the next step is taken. This method approaches pulling.

The movements of snakes are somewhat intricate. A rapid dart forward may be effected by a sudden straightening of one or more of the bays of the sinuous body, but let us take the ordinary smoothly continuous progression. Except in burrowing snakes

the ventral surface is covered by a single series of large scales, which can be raised and lowered. The posterior margins of these scales are sharp, strong and imbricating. When raised, which is effected by special muscles, they catch on roughnesses on the ground. Into the sides of these large ventral scales the lower ends of the ribs are attached by minute ligaments, and the upper ends are connected to the vertebrae by articulations which allow them ready movement forwards and backwards. Several ribs are drawn forwards by muscles and move the associated scales a minute distance headwards, a whole series of ribs and scales working simultaneously in the same direction. Then these ribs are drawn backwards, and the pressure of the raised scales against the hard ground pushes the body forwards. While one series of ribs is being drawn backwards, another series is being drawn forwards, and thus a continuous flowing movement is brought about. This case is perhaps intermediate between "punting" and "rowing." Sculling.—The man in the boat may stand in the stern and "scull," using a single oar to displace masses of water alternately to right and left. This is a common method among swimming animals, such as fishes and whales. In most fishes the swimming organ is the post-anal body, which consists almost entirely of strong W-shaped blocks of muscle, dovetailed into one another, and centred in the flexible backbone. By alternately bending and straightening the posterior body, masses of water are displaced, and thus the fish is propelled forwards. In cetaceans locomotion is similar, but 4 complication is introduced by the adaptive shape of the tail-flukes. In true seals (Phocidae) the hind-limbs are permanently turned backwards and bound up with the short tail, forming a unified functional propeller. The principle is the same in cases like sea-snakes and swimming leeches, where the gripping of the water and the using of it as a resistant mass, against which to contract, extend over the whole length of the body.