20 ELECTRO-PHYSIOLOGY CHAP. 



catory muscles of many molluscs are yellow, pink, and even 

 deep red. 



Knoll discovered a remarkable instance of " plasmic " muscle- 

 cells among Invertebrates, in the thin muscle-bands of the mantle 

 of Salpa (S. maxima, africana, pelesii). The cross-striped, cylin- 

 drical fibres are very long, with conical ends. They are easily 

 split up longitudinally into finer bundles and fibrils, owing to the 



excessive abundance of sarcoplasm, 

 which intersects the plexus of fibrils, 

 and as we see in transverse section, 

 not only collects in the axis of each 

 fibre, but also runs out in wide, 

 radial tracts towards the periphery, 

 thus dividing the contractile and 



FIG. ll.-Transverse section of two muscle- distinctly fibrillated COrtical Stratum 

 cells from Salpa pelesil. (Knoll.) 



into separate laminae (Fig. 11). 



The same plan of structure is here to some extent repeated, 

 on a larger scale, that prevails in the far more delicate radial 

 striation of the cortical zone of muscle-cells in many worms and 

 molluscs. But there is one important difference ; the contractile 

 substance is no longer (as in all previous cases) exclusively at the 

 periphery of the formative cell, but appears in more or less con- 

 spicuous bundles (muscle-columns) within the central sarcoplasm 

 also. Hence, as Knoll has pointed out, the Salpa 

 muscles in a measure represent the transition to 

 certain arthropod and vertebrate muscles, in which 

 the same structural arrangement is present. A 

 transverse section through the cardiac muscles of 

 Crustacea often exhibits an unmistakable similarity FlG 12 _ Trans . 

 to Salpa in disposition of sarcoplasm and con- verse section of 



, ., , 1 . cardiac muscle- 



tractile substance, except that the sarcoplasm is, ceii of Lobster. 



where possible, even more richly developed, and ( Kno11 -) 



all the "muscle-columns" lie within the formative cell (Fig. 12). 



The unusual quantity of protoplasm is explained in both 

 cases by the sustained and strenuous work which is served by 

 these muscles. 



From these observations we may conclude that there is no 

 fundamental difference in structure between the different muscle- 

 cells of Invertebrates (excepting only the muscular fibres of 

 Arthropoda) ; whereas among Vertebrates we shall find striking 



