A OOMPARATIVE STUDY OF STRIATED MUSCLE. 45 
in the case of Syllis, and the zones of simple non-striated 
fibres in Polynoé, becomes very obvious, and it becomes clear 
that not one but a bundle of the fine non-striated fibres is 
the equivalent of the compound fibre of Syllis. 
This, however, seems but a slight advance on the way from 
the gizzard of Hesione to that of Syllis corruscans. But 
there is another important point to be noticed about the 
radiating fibres in some species of Polynoé. It is that when 
examined with the polariscope the fibres are found not to be 
optically homogeneous, but to be divided with perfect regu- 
larity into two nearly equal halves (fig. 25), an outer singly 
refracting portion and an inner doubly refracting ; moreover 
the inner doubly refracting half is much more darkly stained 
by hematoxylin than the outer. Near the middle of the 
external singly refracting zone are situated the nuclei of the 
fibres. These are approximately on a level in all the fibres, so 
as to form a transverse line running through the singly 
refracting zone. They are extremely narrow, spindle-like 
bodies, which lie in the axis of the fibres, with a small amount 
of surrounding protoplasm (fig. 26). 
In Syllis nigropunctata the gizzard (figs. 21 to 23) has 
essentially the same structure asin S. corruscans. There 
are the same hollow columns of muscle with the ganglion-cells 
and the core of granular protoplasm with its network of 
delicate threads and nuclei. But there is only a single 
transverse network in each fibre. This runs through a 
singly refracting zone crossing the middle of the fibre, with, 
on either side of it, a doubly refracting segment, succeeded 
again at the extreme outer ends of the fibres by singly refract- 
ing zones. In Syllis kinbergiana (fig. 24) instead of one 
transverse network in each fibre there are three, with cor- 
responding zones in the muscle substance. In a fourth 
species, S. schmardiana (fig. 4), the fibres have all the cha- 
racters presented by those of the other species, but the 
transverse networks are from five to nine in each fibre. 
The inference to be drawn from these facts seems very 
obvious. In the same organ of related forms we have, taking 
