CHAPTER III 

 PHYSIOLOGICAL 



ANIMAL LOCOMOTION.— It is very characteristic of animals 

 that they can mostly move about in search of food, foothold, and 

 mates, or away from enemies and influences that are hurtful; and 

 this locomotion is effected in a great variety of ways. Among multi- 

 cellular animals there are four chief methods, which, following F. W. 

 Gamble, may be illustrated by picturing a man in a boat by the side 

 of a stream. 



(I) The man may reach forward with a boathook, fasten it to 

 some prominence like the root of a riverside tree, and pull the boat 

 forwards; then loosen his attachment and quickly refix it farther 

 ahead. This is the pulling method, and it is well illustrated by 

 leeches and starfishes. WTien it is not swimming, but looping along 

 over a stone, the leech exhibits the following sequence. Fixing its 

 mouth and loosening the posterior sucker, it pulls its body forward, 

 contracting its longitudinal muscles. At the end of this movement 

 the posterior sucker has been brought forward almost to touch the 

 margin of the mouth, and the body is arched steeply 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 the 

 circular and diagonal muscles, which, so to speak, squeeze the body 

 forward. Then the mouth is re-attached, the posterior sucker is 

 loosened, and the sequence recommences. The locomotion is often 

 surprisingly rapid, and a land-leech can fasten itself to a man 

 walking at leisure. 



A starfish makes a number of its tube-feet tense with water by 

 contracting the muscular ampullar inside the arm; the suctorial 

 tij^s of the tube-feet are pressed against the rock like so many firm 

 fingers; some of the water from the tube-feet is then allowed to 

 flow back into the ampullae, so that a partial vacuum is formed 

 between the tip of each tube-foot and the surface of the rock. Thus 

 the tulx?-feet adhere firmly and a forcible pulling away of the starfish 

 may break the tube-feet rather than loosen their attachment. The 

 next step is a contraction of the longitudinal smooth muscles on the 

 walls of the tube-feet, and this contraction pulls the starfish nearer 

 to the points of attachment. Then by a strong contraction of the 

 internal, somewhat sjTinge-like ampulla?, water is forced into the 

 tube-feet at a pressure sufficient to do away with the partial vacuum 

 and thus lilx^rate the suckers. If the starfish is climbing up the face 

 of shore-pool rock, it would slip back at tlus juncture were it not that 



