MORPHOLOGY OF ANIMALS 



The muscular system may consist of isolated fibres such 

 as are found in many cnidarians, platodes, and rotifers, or 

 these fibres may be united into bundles or sheets as is the 

 case in most higher animals; these groups of muscles show 

 many differences and can be compared only in a general 

 way. 



In general the arrangement of the body muscles depends 

 upon the presence or absence of a skeleton. Animals such 

 as annelids, which have no skeleton, usually have the 

 body musculature arranged in the form of two coats, an 

 outer layer of circular fibres and an inner of longitudinal 

 ones; while the intestinal musculature is also arranged in 

 two coats, the outer (next the coelom) longitudinal and 

 the inner circular (Fig. 5, C, D). If an exoskeleton is 

 present, as in arthropods, these muscular layers of the body 

 wall are broken up into bundles which become attached 

 to the skeleton ; if an endoskeleton is present, as in verte- 

 brates, the muscles become attached to the bones, many of 

 which serve as levers. 



The locomotor apparatus of echinoderms is unique, 

 consisting of a great number of tube-feet, which are hol- 

 low muscular tubes, closed at the end by a sucking disk. 

 The cavity of each tube is connected with the water-vas- 

 cular {ambulacral) system within the body, from which 

 water can be forced into the tube-feet. In this way they 

 are protruded until the sucking disk touches and becomes 

 attached to some object; then by contraction of the mus- 

 cles of the tube-foot the water is forced back into the 

 water system, and by simultaneous action of many of these 

 feet the body is slowly warped along (Fig. 10). 



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