92 GENERAL PRINCIPLES OF ZOOLOGY. 



to a peculiar distribution of the muscle-substance, the smooth 

 musculature being chiefly distributed to the internal organs, 

 which are not under control of the will (involuntary muscles), 

 while the musculature of the body, subject to the will and hence 

 demanding more rapid action, is cross-striated (voluntary muscles). 

 We must not conclude that the difference between smooth and 

 cross-striated musculature coincides with the distinction between 

 visceral and body musculature; it should be noticed that the body 

 musculature of all molluscs is smooth, the visceral as well as the 



FIG. 48. Epithelial muscle-cells, o, of a medusa; b, of an actinian. 



body muscles of many insects and Crustacea, and the muscles of 

 the heart of vertebrates are cross-striated. 



It was pointed out above, in connexion with epithelia and 

 connective tissue, that these tissues differed fundamentally. This 

 contrast has its bearing in dealing with the muscles, for both 

 epithelial and mesenchymatous cells may form contractile sub- 

 stances and therefore there are two genetically different kinds of 

 muscles, the epithelial and the mesenchymatous (contractile fibre- 

 cell). Both kinds of muscle-cells can a priori form smooth as 

 well as cross-striated muscle-substance; but the collection of con- 

 nective (mesenchymatous) tissue around inner organs has caused 

 most contractile fibre-cells to be smooth, while most of the 

 epithelial muscle-cells are cross-striated. 



Epithelial muscle-cells are cells of which one end extends to the 

 surface of the body or the surface of an internal cavity (body 

 cavity, lumen of the gut, etc.), and may here have a cuticle, cilia, 

 or flagella, while at the opposite end it has secreted contractile 

 substance in the form of muscle-fibrils (fig. 48). They combine 

 the double function of epithelial and muscle cells. 



Contractile fibre-cells, on the other hand, are connective-tissue 

 cells, which usually have surrounded themselves with a layer of 

 contractile substance; corresponding to their origin, they have the 

 form of connective-tissue cells, and are spindle-formed or 

 branched; the branches arising from the ends of the cells (fig. 49). 

 The similarity of form renders the distinction between ordinary 

 connective-tissue cells and fibre-cells difficult; if the contractile 



