240 



LECTURE IX. 



Spiral valve, Selache ; Clift. 



652. A, shows a typical example of this disposition of the mucous 



membrane in the Fox-shark {Alo- 

 pias Vulpes) ; the valve describes 

 thirty-four circumgyrations within 

 seven inches extent of the intes- 

 tine ; the mucous membrane is mi- 

 nutely honey-combed : a few scat- 

 tered fibres of elastic or involuntary 

 muscular tissue may be traced in 

 the vasculo-cellular layer included 

 within the mucous fold, and they 

 form a slender band within the free 

 border of the valve, retaining much 

 elasticity in the dead intestine, and 

 drawing that border into festoons. 

 Besides Selache and Alopias, the 

 spiral valve is transverse in Galeus, Lamna, and all the Dog-fishes 

 ( Scylliidce and Spinacidce). The trunk of the veins of the longitudi- 

 nally convoluted valve runs along its free thickened border, and 

 quits its commencement to join the vena portte*: that of the 

 transversely spiral valve is external to the gut. 



One may connect the peculiarity of the spiral valve with the 

 necessity for reducing the mass and weight of the abdominal contents 

 in the active high-swimming Sharks, which have no swim-bladder ; the 

 essential part of an intestine being its secerning and absorbing surface, 

 we see in them the requisite extent of the vasculo-mucous membrane 

 packed in the smallest compass and associated with the least possible 

 quantity of accessary muscular and serous tunics by the modifications 

 above described. Analogous ones exist, however, in other Plagi- 

 ostomes, and in the Lamprey, to which the above physiological ex- 

 planation will not apply ; and the spiral valve is associated with the 

 air-bladder in some of the highly organised Ganoids, and in the 

 Lepidosiren. Nevertheless, it is to be remarked that the intestinal 

 canal is shortest, and the spiral valve most complex and extensive, in 

 the Sharks. In both these, and the Rays, the valve subsides at a 

 short distance from the anus ; and into the back part of this terminal 

 portion of the rectum an elongated C£ecal process with a glandular inner 

 surface opens {fig. 75. i). The anus itself communicates with the fore- 

 part of a large cloacal cavity in the Plagiostomes. In other Fishes, 

 where it opens distinctly upon or near the external surface, it is 

 anterior in position to the orifices of oviducts, or sperm-ducts, and 



Duvernoy, xcvi. p. 274. pi. 10. 



