ORGANISATION AND STRUCTURE OF MUSCLE 



ir- 



is excessively developed, and produces very complicated figures in 

 transverse section. Certain Polychoetse, in particular, exhibit an 

 extraordinarily complex arrangement of the flat band-like muscle- 

 cells, which are individually of very insignificant proportions. 

 The cross-section of the longitudinal muscle-layer not infrequently 

 acquires a characteristic appearance. 



The muscle-fibres of Cephalopoda should be mentioned as 

 affording in many respects a remarkable instance of muscle-cells 

 in Invertebrates. Their peculiar structure has recently been 

 investigated by Ballo- 

 witz (10). 



With both the 

 high and low power 

 a system of parallel 

 lines appears, running 

 obliquely in opposite 

 directions, and seem- 

 ing with the medium 

 power to cross directly 

 over one another 

 (Fig. 8). The ex- 

 amination of partially 

 destroyed muscle-cells 

 shows this to be the 

 optical expression of 



fibres which " run in 

 a continuous spiral in 



the COrteX rOUnd the FlG - V. Transverse section of body-muscles of Protula protensa. 



medullary substance." 



The steepness of the spirals varies considerably with the state of 

 contractipn. In very flat muscle-cells both systems of striation 

 appear to lie almost in the same plane, giving an appearance of 

 "double oblique striation," first described by Schwalbe (11) for 

 several of the Invertebrates. Schwalbe explained these figures 

 on the assumption that the fibres were composed of rhombic 

 " sarcous elements," while Engelmann (12) at a later period 

 pointed out their fibrillated structure, and maintained "that 

 every fibre with double oblique striation consists of two systems 

 of fibrils in concentric layers parallel to the surface of the fibre, 

 which describe a spiral in opposite directions round its axis." 



