MORPHOLOGY OF ANIMALS 



{c) Muscular movement^ the principal type of move- 

 ment in higher animals, is caused by the contraction of 

 muscle fibres consisting of a kind of protoplasm especially 

 differentiated for this purpose (Fig. 9, C, D). During the 

 contraction or expansion of a muscle there is no change in 

 its volume, the shortening of a fibre in one axis being com- 

 pensated for by its expansion at right angles to that axis. 



All of these types of movement are found in certain 

 Protozoa and in many Metazoa. Amoeboid movements 

 are, however, usually restricted to free cells without mem- 

 branes or dense cortical layers of protoplasm, such as 

 certain egg cells, embryonic cells, endoderm cells, excre- 

 tory, pigment, and lymph cells of various Metazoa ; in no 

 case is this type effective in the movement of large bodies. 

 In the larvae of all phyla except the nemathelminths 

 and arthropods, locomotion is brought about, at least in 

 part, by cilia, and even among the adult forms of many 

 lower metazoans this is the principal type of locomotion 

 (ctenophores, turbellarians, nemertines, rotifers). Among 

 the nemathelminths and arthropods cilia are usually lack- 

 ing throughout the whole life-cycle. In large animals 

 locomotion is effected entirely by muscular contractility, 

 while cilia are limited to certain regions where by their 

 beating they produce currents. Muscle fibres are found 

 in all Metazoa; they are of two kinds, striped and non- 

 striped or smooth (Fig. 9, C, D) ; the latter are of very 

 wide distribution throughout the Metazoa, the former 

 are limited to a few phyla (mollusks, arthropods, chor- 

 dates). Smooth muscle is contractile to a much greater 

 extent than striped muscle, but is much slower in action. 



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