350 BIOLOGY. [BOOK v. 



muscular envelopement and distinct muscles, traversing the; 

 cavity of the body and serving to open and to shut the valves. 

 The most of the muscular fibres constituting the motory ap- 

 paratus have still as in worms, the form of long filaments a 

 little flattened, which sometimes are subdivided into fibrils. A 

 certain number of those fibres are however slightly striated 

 transversally. 1 We perceive therein also some nuclei, which are 

 probably the last vestiges of the primitive embryoplastic cells, 

 at the expense of which the fibre has been developed. 



A new and decisive progress is accomplished in the arthro- 

 pods. No longer any contractile envelopement. The muscular 

 fibres are all grouped into distinct masses, having a definite form, 

 into muscles, which are inserted into such and such a part of the 

 body by means of fibrous tendons. Lastly those muscles are almost 

 solely formed of fibres bearing very numerous transversal striae. 2 

 Under the microscope they scarcely differ from those which 

 constitute the muscles of animal life in the vertebrates, that is 1 

 to say, muscles charged to excite the voluntary movements. 



The voluntary muscles of the vertebrates represent the most 

 perfect form of contractile substance. They are supported by 

 the diverse pieces of the osseous framework, into which they insert 

 themselves, and on which they impress the movements which the 

 will commands. The cutaneal muscular apparatus disappears, 

 unless we are disposed to see vestiges thereof in the small 

 muscles which put in movement the scales of serpents, the 

 pilose bulbs, and the hair of the mammifers, the feathers of 

 birds, in those which act on the skin of the human face, and so 

 on. Yet according to Gegenbaur, these dermic muscles are 

 completely lacking in fishes, and do not at all represent the 

 contractile tube of the inferior animals. 



From the point of view of elementary form the contractile 

 substance which may be called muscular presents itself under 

 many aspects. There is first of all the amorphous or sarcodical 



1 Gegenbaur, loc. tit., p. 465. 



2 Leydig, loc. cit., p. 152. 



