14 



ELECTRO-PHYSIOLOGY 



CHAP. 



distributed. The several primitive fibrils are rarely detached 

 from the formative plasma of the muscle-cell as a single layer, 

 but are usually clustered together, and arranged in strata, which 

 gives the appearance of radial striation above alluded to, in 

 transverse sections of the contractile layer. 



While the elaborate structure of single muscle -cells in 

 Annulata undergoes no appreciable change as development 

 progresses, there is on the other hand a marked 

 variety in regard to arrangement of the muscle 

 elements into filaments and bundles. The principle 

 of surface growth by involution is still paramount, 

 and just as in single muscle-cells the flat, fibrillated 

 lamina curves round to make room for greater 

 mass -development of the contractile substance, 

 the same process is repeated in the grouping- 

 together of a number of muscle-cells in the longi- 

 tudinal muscular layer of many Annulata. 



In other Lumbricidse the arrangement of 

 muscle-cells inside a " case " is still more regular, 

 since they surround the central hollow in a single 

 layer, which gives a feathered appearance to the 

 transverse section. The axis, which corresponds 

 to the shaft of the feather, is bordered on either 

 side by the oblique sections of the myoblasts, which 

 cover the converging sides of each pair of cases 

 (Fig. 6). In contrast with those, the longitudinal 

 muscle-fibres of Lumbricus olidus and many 

 Oligochetse lie in a mass of irregular layers, or 

 little groups divided by septa of connective 

 FIG. 6. Transverse tissue, as also occurs in Hirudina3. The original 



section of body- .,,,.,, ~ n , ., -, , 



muscles of Lum- epithelial character of the longitudinal muscles 





5 ' is thus no lon g er distinct in the arrangement of 

 the individual elements in such cases ; but the 

 structure of the single cells is otherwise perfectly conformable. 

 A contractile, fibrillated, cortical layer can always be distinguished 

 from a medullary substance (sarcoplasm), which it wholly or 

 partially encloses. The usually solitary nucleus either lies to 

 one side on the margin or surface of the separate fibres, or (e.g. 

 in- Hirudinse) within the central protoplasmic hollow. In many 

 Annulata, as in Cnidaria, the involution of the muscular lamella 



