18 ELECTEO-PHYSIOLOGY CHAP. 



above and below the axis. Hhode (Lc.) noticed the same appear- 

 ance in the bi-obliquely striated muscle -cells of many worms 

 (Arenicola, Nephthys). The bisected adductor muscle of Molluscs 

 often shows, even to the naked eye, a division into two parts, 

 distinct in colour and general appearance, the one white and 

 tendon-like, the other glassy and transparent, of a grayish-yellow. 

 The spindle-shaped muscle-cells of the former generally exhibit a 

 well-marked longitudinal striation as the expression of fibrillated 

 structure, while the more extended and flattened fibres of the 

 gray -part are often striped bi-obliquely (Ostrea, Anodonta, etc.), 

 and in many cages they also show a definite transverse striation 

 (Lima, Pecteri). As we shall see later, these varieties of structure 

 are closely allied to differences of function, and it may be con- 

 cluded, at least for Pecten and Lima, that the quick, flapping 

 movements made by these animals are served by the striated 

 part of the adductor muscle, while the smooth fibres effect the 

 sustained persistent closure. The relation between cross-striation 

 of muscle - fibrils and rapidity of the movement engendered is 

 already apparent in the epithelial muscles of Cnidaria, where, as 

 we have stated, the comparatively swift, swimming movements 

 of the Medusa are produced by striated fibrils. This also accounts 

 for the almost universal cross-striation of muscle -cells in the 

 heart and masticatory apparatus of Mollusca, which otherwise, in 

 regard to the finer structure of single elements, follow closely the 

 forms of muscle-cells described above. There are still the spindle- 

 shaped, freely branched, plexiform fibre -cells, which, as seen 

 especially in transverse section, are generally rich in axial sarco- 

 plasm, wholly or partially surrounded by the small fibrillar cortical 

 layer. The latter once more exhibits frequently a distinct radial 

 striation in transverse section, representing a regular alternation 

 of layers of fibrillated substance and sarcoplasm. Sometimes the 

 fibrils (bundles of ?) surround the plasma bodies of the cell in a 

 single row of dots (Fig. 10, ) ; in other cases the cortical layer 

 seems to correspond with a continuous stratum of contractile 

 substance. 



Even a superficial comparison of cross-sections through the 

 cardiac or masticatory muscles on the one hand, and the adductor, 

 or pedal, muscle respectively on the other, in Lamellibranchs and 

 Gasteropods, shows that the relative distribution of sarcoplasm, 

 and of contractile fibrils separated out from it, varies very consider- 



