ALIMENTARY CANAL OF FISHES. 



423 



283 



SpnilvalM, Zi/t/fnia. 



284 



and Z^r/cena.^ In the second and more common modification, the 

 fold of mucous membrane is disposed in close 

 transverse coils, as shown in the longitudinal 

 section of the Selachc's gut, fig. 284, h ; and a 

 transverse section exposes only tlie fiat surfirce of 

 one of the coils. In the Fox-shark (Alopias 

 I ulpes) ; the valve describes thirty-four circum- 

 gyrations within seven inches' extent of the 

 intestine ; the mucous membrane is minutely honeycombed : a 

 few scattered 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 Seladie, fio;. 

 284, li, and Alopias, the spiral valve 

 is transverse in Galeus, Lamna, and 

 all the Dog-fishes {^Scylliidce and 

 SpinacidfE). The trunk of the ' arteria 

 meseraica intestinalis,' and that of 

 the corresponding veins of the longi- 

 tudinally convoluted valve, run along 

 its free thickened border, and the 

 vein quits its commencement to join 

 the vena porta; : ^ the arterial and venous trunks of the trans- 

 versely spiral valve are 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 secern- 

 ing and absorbing surface, we see in them the requisite extent of 

 the vasculo-mucous membrane packed in the smallest comp)ass, and 

 associated with the least possible (tuantity of accessory muscular 

 and serous tunics, by the modifications above described. Analogous 

 ones exist, however, in other Plagiostomes, and in the Lamprey, 

 to which the above physiological exijlanation will not ajiply ; and 

 the spiral valve is associated with the air-bladder in some of the 

 highly organised Ganoids, and in the Lepidosiren. Nevertheless, 



' XLVI. t. iv. p. 314 ; t. xcvi. p. 277, pi. 2 and 3. See also, xx. prep. no. 645 ; 

 probably from ScoUodon. 



- Duvernoy, xcvui. p. 274, pi. 10. 



