THE ANIMAL ACTION-SYSTEMS 71 



as rudders and balancing organs. They are water-planes in the 

 mechanical sense. Swimmerets, parapodia^ etc., are bodily seg- 

 mental appendages in Crustacea, worms, etc., which belong to the 

 same general series that we are regarding as *' pedal." Their 

 actions are those of paddles. 



23«. Parts and Actions Associated with Pedal Organs. 

 In all animals that move pedally the limbs or appendages are 

 modified for other actions. The hands of man, the paws of the 

 cats, or the hooves of horses and cattle, are examples of limbs 

 used for " manufacture," aggression and defence. Among the 

 arthropods the appendages are nearly always modified in groups : 

 thus the antennae and eyes of the lobster (sensory appendages) ; 

 the mouth appendages which grasp and masticate food ; the great 

 claws (aggression) — the walking legs and swimmerets (abdominal 

 appendages) are locomotory, but some of the latter may be 

 copulatory. 



Similar modifications of the appendages occur in the insects 

 and worms. 



24. ON OTHER MODES OF LOCOMOTION 



A great part of the animal kingdom thus moves on limbs and 

 other appendages, articulated on the body and performing 

 rhythmic dragging, driving, or gliding motions on rigid surfaces, 

 in air or in water. There are many other kinds of locomotion. 



24^. Saltatory Mechanisms. Appendages may be used for 

 locomotion by jumps — those of the flea, for instance. The whole 

 abdomen of the shrimps, prawns and lobsters can violently flex 

 (or bend) and the action, by reason of the grip on the water 

 exerted by the telson (or tail) causes these animals to make 

 backward bounds. The saltatory movements of many crustaceans 

 (sand hoppers) appear to be made by violent flexions of the whole 

 body. Some marine molluscs (the scallop, Pecten) bound in 

 quite another way : the shell cavity is filled with water while 

 the valves are widely open. Then folds of the mantle partially 

 close the shell margins except in one place and the adductor 

 muscle quickly closes the valves. The outrush of water from 

 the shell cavity then causes the animal to bound backwards. 



246. Crawling Movements. Very many animals crawl 

 slowly on rigid surfaces — the ground, rocks, the sea bottom, etc. 



