34 WILLIAM A. HASWELL. 
protoplasm present in the fully-developed fibre and its relation 
to the contractile substance ; and (4) on the presence or absence 
of a sarcolemma. 
The number of fibrils in a fibre varies in extreme cases to a 
very great extent, there being instances of fibres consisting of 
only about half a dozen fibrils, and others in which the fibrils 
constituting a single fibre are extremely numerous. 
In the closeness and fineness of the transverse networks there 
are also considerable differences, and the two qualities run 
more or less exactly parallel: the less the distance between the 
transverse network the finer they are. The finest and most 
numerous striations are to be found in the muscular fibres of 
the higher Vertebrata, the coarsest in those of the Chzetopoda, 
the fibres of the Arthropoda occupying an intermediate 
position in these respects. 
The amount of protoplasmic matter which in the adult 
fibre is not differentiated either into contractile substance or 
muscle-networks is in the muscular fibre of the Vertebrata 
usually very small in amount, consisting for the most part of 
a patch on which the motorial end-plate rests, with, in some 
instances, other patches lying between the fibrils. The same 
holds good of the muscular fibres of many Arthropoda. But 
many of the latter (many Spiders for instance, and some 
Insects) have running through every fibre a nucleated core 
of protoplasmic material which is the remnant of the formative 
substance of the muscle-cell. In the Chzetopoda this core is 
of considerable extent as compared with the thickness of the 
muscle-substance, and it contains the nerve-endings as well 
as a reserve store of formative material. 
The most primitive form of compound striated fibres is that 
found by myself in the gizzard of various species of Syllis, 
and described generally in a paper in the ‘ Quarterly Journal 
of Microscopical Science’ for 1886.1. I am _ inclined to 
think that the two species (Syllis corruscans and Syllis 
Schmardiana) in which I mainly worked out the structure 
1 “Qn the Structure of the so-called Glandular Ventricle (Drisenmagen) 
of Syllis.” 
