52 MR. T. J. PARKER ON THE INTESTINAL 
the last but one and the last is somewhat greater, and that between the first and second 
is, as a rule, although not invariably, about twice as great. The second branch of the 
mesenteric artery and vein (mentioned above) runs across the middle of this large space 
between the ventral portion of the first and second turns. 
A comparison of the figures will show how uniform in this respect the attached edge 
is: the almost isolated cases represented by figs. 1 & 4, Pl. X., are the only ones show- 
ing any marked deviation from the arrangement described. 
The proportion between the length of the attached edge of the valve and the dia- 
meter of the gut is best judged of by a consideration of the number of turns of the 
former. This is seen in fig. 8, Pl. X., to be only 73, while in fig. 4, Pl. X., it amounts 
to 94. In some cases, not figured, I have found it to be considerably lower, and in 
others higher. 
The proportional length of the attached edge is thus extremely inconstant ; and, in con- 
sequence, the position of the posterior end of the valve varies indefinitely. It may be 
dorsal (Pl. XI. fig. 1), ventral (Pl. X. fig. 4), right or ieft (Pl. X. fig. 6), or may occupy 
any intermediate position. 
§ 6. The free edge of the valve (represented in the diagrams by the thin line) starts at 
the same place as the attached edge. It passes at first directly inwards or to the right, 
nearly at right angles to the wall of the intestine, and then sweeps more or less sharply 
backwards. In consequence of this, as will be seen by an examination of the figure, 
and especially of the transverse section (Pl. X. fig. 3), the first half-turn of the valve, 
namely that part contained in the bursa entiana, is rolled upon itself, and forms a sort 
of hollow cone open along its dorsal side and having its apex directed forwards. This 
disposition of the first half-turn of the spiral valve is very constant, occurring, with but 
slight modification, in every specimen examined. 
Beyond this point the free edge begins to acquire more or less of a spiral twist. 
Its simplest course would be to form a spiral line parallel with that constituted by the 
attached edge, in which case the whole of the valve, with the exception of the first 
half-turn, would be a regular screw surface. The nearest approach to this is shown in 
fig. 4, Pl. X., in which it will be seen that from its second turn to the last but one the 
valve forms a regular spiral inclined plane, the width of which is very nearly equal to 
the semidiameter of the intestine. For the last turn the width gradually diminishes, so 
that at the posterior extremity of the valve it does not exceed half the semidiameter. 
The specimen from which fig. 4, Pl. X., is taken, as well as that containing the only 
other perfectly regular valve I have met with, was unfortunately dried, the effect of 
which treatment has been to make the valve shrink somewhat, and so appear narrower 
than it really is. If the specimen had been preserved in chromic acid, there can be no 
doubt that the thickened inner edge would have approached more nearly the axis of the 
intestine than it is seen to do in the figure; it would then have tended to coincide with 
