ELEMENTARY VITAL PHENOMENA 



241 



musde-fibriUcc ; the fibrillae lie embedded in the protoplasm 

 of the fibre, which can be termed with Rollett, in brief, sarco- 

 plasm, arranged in various ways but all in the same direction. 

 They represent specially differentiated organoids of the cell- 

 protoplasm. 



In accordance with the varying structure of the contractile 

 muscle-fibrillse, two groups of muscle-fibres or muscle-cells are 

 distinguished, the smooth and the cross-striated. In smooth 

 muscle-fibres the fibrillae, which lie embedded ia the sarcoplasm 

 parallel to one another, are completely homogeneous threads in 



A B C 



Pig. 100. — Stentor ci^i-ab t'x, a ciliate infusoriau containing numerous parallel muscle-fibrilla? 

 (myoid- fibres) in the exoplasm. A, Extended ; B, half -contracted (free-swimming) ; €, 

 wholly contracted. 



which every cross-section is like every other one. Cross-striated 

 muscle-fibres, on the other hand, contain fibrillae that from one 

 end to the other are divided iato many segments, all of which 

 possess a corresponding but complicated structure. 



The simplest forms of smooth miisde-celh are found among 

 Infasoria. In many ciliate Infusoina, such as Stentor, the cell-body 

 represents such a muscle-cell of the simplest kind ; it contains, 

 embedded in the external layer of its protoplasm, smooth 

 muscle-fibrillse, the so-called myoids, arranged approximately 

 parallel to one another (Fig. 100). Other Infusorm, especially 



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