238 CONTRIBUTIONS TO THE NATURAL HISTORY 



and, as the inner and outer layers correspond to each other, they form a sort of partition 

 thr oiighthe gelatinous mass, though they do not meet in the centre. The form of the 

 bundles themselves is very peculiar, inasmuch as the isolated fibres can be traced to more 

 or less bottle-shaped cells. 



The circular system of muscles (Plate V. Fig. 11,1, and 2, i, i), lining the main 

 cavity of the body, covers the inner vertical system, and forms a layer of fibres placed 

 at right angles with it. But it does not merely pass over the vertical muscles, it also 

 covers the vertical chymiferous tubes which diverge from the centre of the disk. These 

 muscular bundles are parallel to each other, and cover regularly the whole inner surface 

 of the disk, there being only a thin layer of loose epithelium over them. Fully to ap- 

 preciate their position and mode of action, it is necessary to have a correct idea of the 

 relation of parts on this inner surface of the body. From what has been said above, 

 it is plain that the inner surface is divided into eight zones ; the four inner vertical 

 muscular columns and the four chymiferous tubes, which alternate with each other, 

 form the lines of demarcation ; and between two such lines, that is, alternately between 

 one bundle of muscles and one chymiferous tube, and so on, there are muscular fibres, 

 extending in gentle curves all over the inner surface of the body, beginning at a short 

 distance from the central digestive cavity, which this muscular system does not surround, 

 down to the lower part of the disk. 



That this system of concentric muscles consists of eight vertical rows of concentric 

 bundles, and not of circular fibres extending all round the inner surface, is shown by the 

 different forms which these parts assume during strong contraction and after death, where 

 evidently each system of these fibres, that is to say, each vertical row between a chymif- 

 erous tube and a vertical muscular bundle, contracts independently, and in its more or less 

 powerful contractions forms more or less arched curves ; and as the whole cavity, in its 

 contraction, has more or less curved walls when seen from above or from below, these 

 eight rows of concentric fibres appear to intersect each other in opposite curves, and to 

 form two distinct systems in the upper and lower half of the body. But it can easily 

 be understood from Fig. 1, 2, and 11, Plate V., that the fibres which appear arched 

 inwards above belong to the same system as the fibres which appear arched outwards in 

 the lower parts. It is not difficult to ascertain that this system of fibres covers the ver- 

 tical chymiferous tubes, at its junction with them, and that there are some fibres running 

 up and down along these tubes, especially near the eye-specks upon the posterior or inner 

 surface of their bulb, where various fibres (Plate V. Fig. 2, o, o, o, o) rise from the margin 

 accompanying the chymiferous tube behind the bulb. 



Finally, there is a fourth muscular system in the lower partition (Plate V. Figs. 2, 4, 



