DIGESTIVE SYSTEM OF FISHES. 239 



and less oblique as they approach the rectum ; the commencement of 

 this intestine is marked by a large transverse fold or circular valve, 

 which is succeeded by several others less produced, and resembling 

 the valvular conniventes in the human jejunum. The straight 'large in- 

 testine,' which is relatively longer in the Amia, Polypterus, Paddle-fish 

 (Jiff. 61.), Sturgeon, and Chimajra), is characterised by the continuity 

 of such transverse folds as those in the Salmon, producing an unin- 

 terrupted spiral valve of the mucous membrane. In the Lepidosiren 

 the entire tract of the straight and short intestine is traversed by 

 this peculiarly piscine extension of the inner coat.* The spiral 

 valve characterises the large intestine in all the Plagiostomes, and 

 establishes the essential difference between the short and apparently 

 simple intestinal canal of these cartilaginous fishes, and that of the 

 low-organised Myxinoid species. 



The true homologue of the small intestine is extremely short in 

 the Plagiostomes ; it is narrow iu the Rays, expanded and sometimes 

 sacciform (Jig. 65. g) in the Sharks, where it seems to form the com- 

 mencement of the suddenly expanded large intestine : this is straight, 

 and though constituting the chief extent of the intestinal canal, it is 

 very short in proportion to the body ; not exceeding, for example, 

 one eighth of the entire length of the body in the Alopecias or Fox- 

 shark. The economy of space in the abdominal cavity f is, however, 

 effected at the expense of the serous and muscular coats, not of the 

 mucous membrane. The requii'ed extent of secreting and absox'bing 

 superficies is gained by raising or drawing inwards, from the intestinal 

 parietes, the mucous membrane in a broad fold at the beginning of 

 the large intestine, and continuing it in spiral volutions to near the 

 anus. The coils may be either longitudinal and wound vertically 

 about the axis of the intestinal cylinder, or they may be transverse 

 to that axis. In the first case, when the gut is slit open length Avise, 

 the whole extent of the fold may be uncoiled and spread out as a 

 broad sheet ; and, if the gut be divided transversely, the cut edges of 

 the valve present a spiral disposition, as '\\\ Jig. 64. The Ilunterian 



preparation (No. 645.) shows the longitudinal form igr 1| 64 



of the spiral valve ; as it may be seen, also, in the 

 squaloid genera Carcharias, Scoliodon, Galeocerdo, 

 ThalassorJmius, and Zi/gcena. \ In the second and 

 more common modification, the fold of mucous mem- ^'"''i^Jiverm)^-''^"^^"'' ' 

 brane is disposed in close transverse coils, as shown in 

 the longitudinal section of the Selache's gut (Jig.6o.h.); and a trans- 

 verse section exposes only the flat surface of one of the coils. Prep. 



* XXXIII. p. MS. pi. 25. iig. 2. f Roget, c. ii. p. 205. 



t XLVi. t. iv. p. 314. ; t. xcvi. p. 277. pi. 2 and 3. 



