92 C. F. MARSHALL. 



Salamander, in both of these the fibrils in the muscle-cells are 

 very evident, but the intranuclear networks do not show at 

 all distinctly, which is a most unusual result in this mode of 

 preparation. However, in preparations from the mesentery 

 of the newt, made by Klein's method, the intranuclear net- 

 works come out very distinctly in many fibres ; and in one case 

 I could trace the connection of the intranuclear network with 

 the fibrils of the cell. The longitudinal fibrils do not show 

 so well in these preparations as in those made by the gold 

 method (figs. 12, 13). It thus appears that the vertebrate un- 

 striped muscle differs from all the invertebrate uustriped 

 muscle that I have investigated, in that the cells contain an 

 intracellular network in the form of longitudinal fibrils. 

 This may perhaps represent a form of network intermediate 

 between the typical irregular network of other cells and the 

 highly modified network of the striped muscle-cell. 



From these investigations it appears that the peculiar intra- 

 cellular network of striped muscle is developed in all muscles 

 which have to perform rapid or regular movements. 



A brief review of the chief animals mentioned in the 

 preceding pages will make this clear. Commencing with the 

 Actinia and the Medusa, these are both highly organised 

 Coelenterates, but the Actinia is a sluggish animal which 

 exhibits slow and irregular movements, while the Medusa 

 propels itself through the water by rapid and regular contrac- 

 tions of its disc. Now, in the Actinia we find no striped 

 muscle, but in the Medusa the network is present. In the 

 worms such as the Leech and Earthworm striped muscle is 

 absent; these animals only performing comparatively sluggish 

 movements. In the Polyzoa the retractor muscles of the 

 stomach, and in the Rotifers the retractors of the trochal disc, 

 perform rapid movements, and have been described as striated 

 transversely ; this is probably due to the network, although, 

 as stated above, I have so far been unable to determine this 

 myself. In the Mollusca the movements are as a rule 

 sluggish, and unstriped muscle is the prevailing type in this 

 group. But in the odontophore muscles of the Snails the 



