SPIRAL VALVE 265 



seems to depend on several factors, the effect of which, in different 

 Elasmobranchs, is best studied in a series of valves of progressively 

 higher differentiation.^ 



In a hypothetical simple type of valve, easily derivable from 

 the more primitive type of Petromyzon, it may be conceived that, 

 while not exceeding in width the semi-diameter of the intestine, 

 the valve becomes disposed in several complete and more or less 

 closely approximated spiral turns, the free edge of the valve being 

 on the same level as its attached margin, and leaving an open 

 axial canal along the centre of the gut. The nearest approach 

 to this hypothetical type, which has been compared, not inaptly, 

 to un escalier tournant sans noyau, is perhaps to be found in the 

 Thresher-Shark (Alopecias vulpes). 



The structure of the more complicated spiral valves of other 

 Elasmobranchs are well illustrated within the limits of the single 

 genus Saia. 



In one specimen of Baia sp. (Fig. 159, A) the last four coils 

 of the valve are similar to those of the hypothetical type, but the 

 more anterior ones, owing to the greater width of the valve, 

 which here exceeds the semi- diameter of the intestine, have their 

 free margins deflected downwards, while that portion of the valve 

 which forms the first half turn is coiled inwards upon itself, so as 

 to form a hollow cone, open dorsally, and having its apex directed 

 forwards. In other examples a further modification is intro- 

 duced by the increasing width of the valve, which now, through- 

 out its whole length, equals the semi-diameter of the intestine ; 

 and by the formation of an axial columella by the thickened free 

 edge of the valve, which is traversed by a central band of 

 unstriped muscle, as well as by the intra-intestinal artery and 

 vein, and takes the place of the central canal of the preceding 

 types. The valve is, however, still regular, and its free margin 

 remains on the same level as the corresponding portion of the 

 attached edge. In other specimens, again, additional complica- 

 tions are introduced by a still further increase in the width of 

 the valve, which now exceeds, often considerably, the semi- 

 diameter of the intestine, and the consequent deflection of the 

 free edge of the valve either forwards or backwards (0 and D). 

 As shown in C the valve, in consequence of the backward deflec- 

 tion of its free margin, presents the appearance of a nest of 

 1 T. Jeffery Parker, Trans. Zool. Soc. xi. 1879, p. 49. 



