72 ORGANIC FUNCTIONING 



There are various mechanisms : (i ) the segments of the earthworm 

 are armed with spines placed Uke paired appendages : these catch 

 on the ground or sand and rhythmic contractions and relaxations 

 of the body drag the animal along — even in a burrow in soil ; 

 (2) the body (as in the limpet, or common snail) has a broad, 

 fleshy base, or '' foot." This adheres to, say, the surface of a 

 stone, or the ground, or some plant stem, leaves, etc. Rhythmic 

 contractions and relaxations of the muscles of the foot drag the 

 animal along ; (3) Echinoderms (starfishes, sea urchins, etc.) have 

 the lower, or all the bodily surface covered by numbers of " tube- 

 feet." These are hollow tubes provided with a kind of piston- 

 sucker mechanism and they are full of liquid (mainly sea water). 

 This is forced into or withdrawn from the tube-feet, actuating 

 the sucker and extending or contracting the tube-feet. The latter 

 adhere, in groups, to the surface on which the animal crawls, 

 and the whole body is thus pulled along. Very complex and 

 movement-groups of the tube-feet occur and complex and 

 powerful movements of the body result — for instance, the starfish 

 is able to pull open the valves of an oyster, right itself if turned 

 upside-down, and so on. 



24^. Rocket-Propulsion. — The Cephalopods, which may be 

 very large and most predatory animals (some giant cuttle-fishes 

 are competent to fight a sperm-whale), move about in two ways : 

 (i) the " arms," or tentacles, have rows of suckers which can 

 adhere with ereat force to some surface. These tentacles are 

 strongly muscular and so the animal can grip, transport and crush 

 other animals to which its tentacles are applied ; (2) there is a 

 *' mantle-cavity " in the body and this can be made to close. 

 The cavity is filled with water and the muscles of the body 

 violently contract so as to expel the contained water through a 

 spout, or *' siphon." The reaction produced by this expulsion 

 makes the animal bound backwards. Somewhat analogous to 

 this are the swimming movements of the medusoid Jelly-fishes. 



24^. Ciliary Movements. This is the main mode of locomo- 

 tion, and of general action, of most of the smaller, microscopic 

 animals. (Many Protozoa, such as Amoeba, crawl on rigid surfaces 

 by " pseudopodial " actions) but most micro-organisms swim. 

 The surface of the body is covered with minute, hair-like organs 

 called cilia. A cilium '' lashes," being flexed quickly and thus 

 gripping on the water and pulling the animal along in the direction 



