56 MR. T. J. PARKER ON THE INTESTINAL 
kingly shown in the annexed diagram, in which the areas given above are expressed as 
squares. The square ai cd represents the internal area of the intestine itself, or, in 
other words, the absorption-surface of a valveless gut; ad’ c'd’ is the entire absorption- 
surface of A, ae’ d@ of B, alc'd® of C, and ab‘c'd* of D. By taking away the 
square abcd we have the gnomons dc'b, dc’b, dc’b, dc‘b representing the entire 
4 
¢ * 
d* 
F 
SS 
a b ob? Uda in 
areas of the spiral valves of A, B, C, and D respectively, and, of course, the trapezia 
dcc'd', deed, deed’, dcc d representing the areas of one side of those valves. 
§ 11. But these numbers by no means adequately express the full advantage accruing 
to the possessor of a perfect form of spiral valve ; for, in the complicated forms, not only 
is the absorption-surface greater than in the less complicated, but the resistance offered 
to the passage of food is immensely increased, and, consequently, the time to which it 
is exposed to the action of the digestive fluids. For instance, in fig. 1, Pl. X., there is 
a clear central passage from one end of the intestine to the other nearly as wide, pos- 
teriorly, as the corresponding space in a rabbit’s cecum; in fig. 4 the whole of the 
food is compelled to take a spiral course, the columelliform arrangement of the free 
edge of the valve quite preventing any direct passage; in fig. 1, Pl. XI., although there 
is a central passage, yet it is so constricted at one part of its course, namely at the 
apex of the conical cavity formed by the third turn, that a very small aperture is left, 
so small, indeed, that a dried specimen of it often becomes entirely closed; moreover a 
good deal of food, taking a spiral course, will become wedged in between the closely 
adhering turns of the valve, and very considerably delayed ; finally, in fig. 3, Pl. XL, a 
