VI. Biology. 3. Different Kinds of Locomotion, etc. 65 



sometimes suck the blood of larger ones. Other animals, as certain Fly-larvae, 

 may be both carrion feeders and parasites. Again, there are fonns which are a 

 transition to the parasites, in so far as they live in other animals, but do not 

 feed at their expense ; at most they take a moderate share at meal - times 

 (commensalism) . 



3. Diflferent Kinds of Locomotion. — Their Effects 

 upon the Structure of Animals,— Sessile Forms. 



The movements of animals are known to be very diverse. Many, 

 especially lower animals (Worms, etc.), crawl bymeans of contractions 

 of the muscles of the body -wall, or by movements of the cilia upon the 

 surface. Others s wim : this is often accomplished by movements of 

 the whole body, or by the greater part of it ; or it may be effected by 

 means of limbs. Walking and running, on the other hand, are 

 absolutely dependent on the presence of limbs. Springing move- 

 ments are of various kinds : amongst aquatic animals, some part of 

 the body strikes upon the water (decapodous Crustacea) ; amongst 

 terrestrial forms, a jump generally results from violently jerking the 

 limbs off the ground. Flying is always effected by means of 

 specially developed limbs, just as burrowing and climbing, 

 well-known forms of locomotion. 



Bach of the kinds of movement mentioned, which may be com- 

 bined, to a certain extent, in the same individual, may influence the 

 structure of the animal more or less deeply. This is clearly manifest 

 when allied groups, typically distinguished from one another by 

 different modes of locomotion, are compared. A great many of the 

 specially marked differences between Pish and the higher Vertebrates, 

 are to be ascribed to diversities of movement. In Pisces, limbs are but 

 sHghtly developed, the body muscles, and those of the powerful tail, 

 much more so, in accordance with the needs of an aquatic life. In the 

 most decided walking types of Vertebrata, the Mammalia, the tail is 

 reduced, the dorsal and caudal muscles also, but the limbs are well 

 developed. That all this is a result of a difference in locomotion is 

 borne out by the fact that if Mammalia are adapted, as an exception, 

 to a swimming life, they again come to resemble Pish. This is the 

 case in Whales, which are derived from terrestrial Mammalia : their 

 limbs have atrophied under the influence of an aquatic life, whilst 

 the tail is developed as a powerful locomotor organ. In like 

 manner the difference between a Shrimp and a Crab depends princi- 

 pally upon the former being a springing and swimming animal, 

 whilst the latter is distinctly an ambulatory form. The different 

 burrowing types (Mole, Mole- cricket), may also be compared with 

 their nearest relations. The kind of movement does not, however, 

 always leave so clear a mark upon the structure as in the cases 

 mentioned. The adaptation is not always so exact, the modification 

 not always so complete as in these : other swimming Mammals may 



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