SECT. IV THE MUSCULATURE 65 



function as jaws are also, like the closing muscles of 

 the mandibles, derived from the sinewy mass, and 

 run slantingly backwards, across the opening leading 

 into the under lip. 



The muscles of the second maxillae are very slightly 

 developed as thin slips running into the dorsal and 

 ventral parapodia ; the former, as already described, 

 arising from a point close to the opening of the shell 

 fold. 



The rings of muscles round the eyes will be de- 

 scribed in the section on the sensory organs, and we 

 shall see that they are developed from the longitu- 

 dinal musculature, and join the two bands which are 

 attached to the proximal end of the upper lip. Certain 

 bands which run from the sternal plate to the open- 

 ing of the shell fold, to join the dorsal longitudinal 

 bands, are probably to be referred to the dorso-ventral 

 longitudinal muscle septa. 



We find the expected histological difference between 

 the musculature of Apus and that of the Annelids, 

 that of the former being striped, that of the latter 

 unstriped. Perhaps the primitive character of the 

 striped muscles of Apus may be seen in that the 

 muscle cells form a thick irregular layer of nucleated 

 protoplasm round each bundle of fibres, without any 

 investing membrane or sarcolemma. 



This brief chapter by no means exhausts this 

 interesting subject ; further study will doubtless 

 reveal other, and perhaps more conclusive, homologies 

 between the muscles of Apus and those of a car- 

 nivorous Annelid. We have here selected only the 



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