110 GENERAL PRINCIPLES OF ZOOLOGY. 



looks very similar to protoplasm without granules, the 

 boundary-line between it and the muscle-corpuscle is more 

 difficult to recognize than in the case of the cross-striated 

 muscle-substance, which by its finer structure gives a quite 

 different appearance from the protoplasm. In cross- 

 striated muscles the contractile portion consists of two sub- 

 stances regularly alternating with one another in the direc- 

 tion of the contraction of the muscle, of which the one is 

 doubly, the other singly, refractive (Figs. 23, 45, 48). 



Smooth and Cross-striated Muscle-fibres. The 

 smooth muscle-substance betokens a lower stage of de- 

 velopment than the cross-striated, since it chiefly occurs in 

 the less highly organized and more inactive animal forms. 

 Interesting in this respect is the fact that in the two stages 

 of development of one and the same animal, the simply 

 formed and inert polyp has smooth muscles, while the 

 more highly organized and actively motile medusa has 

 cross-striated muscles. The difference in their action has 

 led in the vertebrates to the peculiar distribution of the 

 muscle-substance, the smooth musculature being chiefly 

 distributed to the internal organs, which are not under 

 the influence of the will, while the musculature of the 

 body, subject to the will and hence demanding more rapid 

 action, is cross-striated. We must, however, guard against 

 concluding that the difference between smooth and cross- 

 striated musculature coincides with the distinction be- 

 tween intestinal and body musculature. To prevent at 

 the outset this erroneous view, it should here be noticed 

 that the body musculature of all molluscs is smooth, the 

 intestinal musculature of many insects and crabs, as well as 

 the body musculature, is cross-striated. 



Epithelial and Connective-substance Muscle-cells. 

 The epithelium and the connective substance, the first 

 and second of the divisions of tissues, are two fundament- 

 ally .different forms of tissue. This distinction is of im- 

 portance in speaking of the musculature ; for it is shown 

 that epithelial cells as well as connective-tissue cells have 

 the power of forming contractile substance, and that geneti- 



