4-J-2 ANATOMY OF VERTEBRATES. 



Orthagoriscus it is both reticulate and villous, the villi being 

 longest at the beginning; of the canal. There is often a well- 

 marked difference in the character of the lining membrane of the 

 small and large intestines : thus, in the Salmon, the rugae become 

 fewer, larger, 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 valvulas conniventes in the human 

 jejunum. 1 The straight 'large intestine,' which is relatively 

 longer in the Amia, Polypterus, Paddle-fish, fig. 27 6, h, i, Sturgeon, 

 and Chimaerae, is characterised by the continuity of such transverse 

 folds as those in the Salmon, producing an uninterrupted spiral 

 valve of the mucous membrane. In the Lepidosiren the entire 

 tract of the straight and short intestine is traversed by this pecu- 

 liarly piscine extension of the inner coat. 2 The spiral valve 

 characterises the large intestine, fig. 278, k, 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 in the Rays, expanded and some- 

 times sacciform, fig. 284, g, in the Sharks, where it seems to form 

 the commencement 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 Alopias or Fox-shark. The economy of space in 

 the abdominal cavity is, however, effected at the expense of the 

 serous and muscular coats, not of the mucous membrane. The 

 required extent of secreting and absorbing 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 lengthwise, 

 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 in fig. 283. 

 The longitudinal form of the spiral valve may be seen in the 

 squaloid genera Carcharias, Scoliodon, Galeocerdo, Thalassorhinus, 



1 xx. vol. i. p. 199, prep. no. 635. 2 xxxiii. p. 343, pi. 25, fig. 2. 



