SPIRAL VALVE IN THE GENUS KAIA.- 39 



possess what may be called a scroll valve — that is, a fold of the intestinal wall, the fixed 

 edge of which is usually stated to " run straight and parallel with the axis of the intes- 

 tine," while the fold is " rolled up upon itself into a cylindrical spiral." 



This description is not strictly correct. On opening the gut along its ventral wall 

 by a longitudinal incision (PI. XI. fig. 8), the valve appears quite like a second intes- 

 tine within and nearly filling the first. It may be unrolled from right to left ; and 

 it is thus seen that the attached edge is not in a straight line, but is slightly (though 

 distinctly) curved, beginning just posterior to the pylorus on the right side, curving 

 gently outwards until it actually passes to the left of the median ventral line of the 

 intestine, and then back again to the right, to end on the dorsal side at the commence- 

 ment of the rectum. The free edge is very strongly curved — the width of the valve, in 

 the middle being equal to two thirds of its length, while at either end it gradually 

 diminishes until the free and attached edges meet. Thus the form of the unrolled 

 valve may be compared to that of a vertical section of a biconvex lens one surface of 

 which has a very slight, the other a very strong curvature. 



The valve thus constituted is rolled upon itself from left to right, the successive 

 turns being comparable to a series of cylinders placed one within the other and be- 

 coming gradually larger, in length as well as in diameter, from within outwards. This 

 is well shown in fig. 8, in which the ventral portion of each turn is cut away. 



I give this description and the accompanying figures of the scroll valve of Zygcena 

 for the sake of comparison with the spiral valve ; my account adds nothing to Duver- 

 noy's excellent description of the similar valve in Thalassorhinus vuljjecula^. 



§ 19. In Lej)idosiren I have found the spiral valve to be a well-marked, that ot 

 Ceratodus a less perfect, example of type D. To the latter Dr. Giinther's description ^ 

 applied perfectly well. 



§ 20. Lastly, in the Lamprey there is, as is well known, a ridge of mucous membrane 

 projecting into the intestine, round the inner surface of which it takes a spiral course, 

 the spiral being a very open one, the whole width of the valve not more than half the 

 diameter of the gut. The valve is therefore an extremely simple example of type A. 



A valve of this sort is, of course, to all intents and purposes, a typhlosole, only dif- 

 fering from the structure of that name in worms from the fact that its course is spiral 

 instead of straight — just as the papillose ridges of the hind gut are spiral in Astacus, 

 while they are straight in Homarus. Such a valve also bears a close resemblance to 

 the embryonic condition of the spiral valve in the Elasmobranch^. 



§ 21. Thus the spiral valve, reduced to its simplest expression, becomes a typhlosole ; 

 and the scroll valve, indefinitely reduced in width, becomes the same thing. Even in 

 the fully developed structures we get a sort of hint of a connexion between the two ; 



' Ann. des Sci. Nat. 2' serie, 1835, t. iii. 



^ PhU. Trans. 1871, part ii. p. 511. 



' Balfoiir, ' Elasmobranch Pishes,' pi. xvii. fig. 2. 



