1)2 OENERAL PRTNCIPLES OF ZOOLOGY. 



ti) :i peculiur distribution of tlie muscle-substance, the smooth 

 nnisculaturc 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). 

 Wc 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. — EpitheUal muscle-cells, a, of a medusa; h, 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 

 ej)ithelial muscle-cells are cross-striated. 



E'pitlieliul Miiscie-ccUs are cells of which one end extends to the 

 surface of the body or the surface of an internal cavitv (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. 



Con/riir/ile fibre-cells, on the other hand, are connective-tissue 

 cells, which usually have surrounded themselves witli a layer of 

 contractile substance; corresponding to their origin, they have the 

 form of connective-tissue cells, and are si)iudle-formed or 

 branched; the brandies arising from tlie ends of the cells (fio-. 40). 

 Tlie similarity of form renders the distinction between ordinarv 

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



