52 ME. T. J. PAEKEK 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, PI. 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, PI. X., to be only 7^, while in fig. 4, PI. X., it amounts 

 to 9^. 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 (PI. XI. fig. 1), ventral (PI. X. fig. 4), right or left (PI. 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 (PI. 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 fii-st 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, PI. 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, PI. 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 



