46 WILLIAM A. HASWELL. 
the same place, in one case bundles of fine non-striated fibres, 
distinguishable into internal and external zones of different 
optical properties ; in another compound hollow fibres, consist- 
ing of bundles of fibrils similar to the fibres in the first case, 
but more closely united and bound together by a single trans- 
verse network crossing a median singly refracting zone of the 
muscle substance; in a third similar compound fibres with 
three transverse networks ; in a fourth the same with seven to 
nine; and in a fifth similar fibres, but with fifteen to twenty 
transverse networks, and undistinguishable from the fibres of 
triated muscle found in the Arthropoda and Vertebrata. It 
seems at all events highly probable, in view of these facts, that 
the striated muscular fibre, as we find it in Syllis corruscans, 
was derived from bundles of fine non-striated fibres, and that 
the fibrils of the latter represent the fibres of the former. The 
mode of formation of the transverse networks is less obvious; 
but I think the facts described above justify us in regarding it 
as probable that the primitive simple transverse network was 
the equivalent of the transverse line of intrinsic nuclei which 
occupy a corresponding position in the bundles of non-striated 
fibres. What the meaning of the division of the non-striated 
fibres in Polynoé into singly refracting and doubly refracting 
portions may be, it is hard to say precisely; but it seems 
highly probable that, like the corresponding division in the 
compound striated fibre, it is connected with a necessity for 
rapid contraction. 
If Engelmann’s view! be correct, that the anisotropous sub- 
stance alone is contractile, and the isotropous substance in 
striated muscle simply receives and transmits nervous impulses 
“to the anisotropous segments, this regular division of the fibres 
in Polynoé must have come about owing to the desirability 
of rapidity and simultaneity in the contractions of the fibres ; 
the outer isotropous half being that in which the nerve-branch 
ends, and being in fact itself virtually a nerve-ending, acts 
regularly and powerfully under stimuli received through the 
“ Contractilitat u. Doppellbrechung,” ‘ Pfliiger’s Archiv,’ xi. 
