A COMPARATIVE STUDY OF STRIATED MUSCLE. 47 
visceral nerves in causing the contraction of the anisotropous 
half. 
With the necessity for rapid and simultaneous contrac- 
tion must be connected also the transition from a line of 
internal nuclei running across a bundle of fibres to a trans- 
verse network connecting the fibres together to form a com- 
pound fibre; and the multiplication of the transverse networks 
and alternating zones of muscle substance is only a further 
development in the same direction. 
It would be superfluous to indicate hypothetical inter- 
mediate stages by which the transition from the simple non- 
striated to the compound striated fibres may have been effected. 
But there is an observation of Rohde’s on the ordinary non- 
striated fibres of the body-wall and other parts in Polycheta 
that is of some little significance in connection with this 
question. He finds that in many cases numbers of the simple 
fibres are in contact with, and are apparently developed from 
and nourished by, a single common mass of granular proto- 
plasm—an arrangement which, if it presented itself in a 
regular manner among the radiating fibres of the annular 
bands in the gizzard of such a form as Polynoé, would 
obviously bridge over still more the gap between the two 
equivalent forms of tissue. 
SUMMARY OF GENERAL RESULTS. 
The general conclusions to which these observations appear 
to lead may be summarised as follows: . 
There are two principal types of striated muscle in the 
animal kingdom—the simple and the compound—which are 
not in any way genetically related to one another. 
Compound striated muscular fibres are found in their most 
primitive, as well as in a more highly developed, form in 
certain Polycheta, where they occur as the equivalents of 
bundles of simple non-striated fibres found in a corresponding 
situation in related forms. Each compound striated fibre is 
derived from a bundle of simple non-striated fibres. In its 
VOL. XXX, PART 2,—NEW SER, - D 
