26 CLASS XIV. 



and Polypterus \ enlarges the absorbent surface and delays the pas- 

 sage of the food. In the sturgeons this spiral valve extends to the 

 anus. Usually it is turned obliquely, like a staircase, but in Zy- 

 gcena, and some other sharks, it is attached lengthwise to one side, 

 and is convoluted on itself, so that, when unrolled, it appears like 

 a broad lamina 2. 



The internal surface of the mucous membrane of the intestinal 

 canal usually presents longitudinal projecting lines or folds, which 

 often have a sinuous edge, or are incised transversely, or are united 

 like a net, as in Lojjhius jnscatorius. Only in very few fishes do 

 true villi occur, like those commonly met with in the small intestine 

 of the higher vertebrates. 



The position of the anus is very various. When fishes have 

 a ventral fin it lies behind this, more or less remote from it. In the 

 jugular fishes (p. 13), and such as have no ventral fins {apodes), the 

 vent is situated below the gullet, and the intestine towards its ter- 

 mination bends directly forwards, as for instance in the genus Ster- 

 narchus. In some Pleuronectes {Solea) a part of the intestinal canal 

 lies external to the abdominal cavity, or rather the abdominal 

 cavity, with its lining of peritoneum, is prolonged behind the first 

 interspinal bone of the pinna analis, which constitutes the osseous 

 boundary between the ventral cavity and the taiP. The perito- 

 neum has, in the Plagiostomes and some osseous fishes, two aper- 

 tures near the anus which conduct outwards {pori abdominales) , by 

 which in these last-mentioned the eggs and the sperma are dis- 

 charged ; the internal sexual organs lie with the intestinal canal in 

 the sac of the peritoneum. External to and behind, or rather above, 

 the peritoneum lie the kidneys and the swimming bladder upon 



^ In the so-named Ganoids. See a figure of the intestinal canal of Polypterus hichir 

 in J. Mueller's notice of this fish, Ahhandl. der Konigl. Ahad. der Wisscnsch. zu 

 Berlin, Physih. Klasse, 1844, Tab. vi. fig. i. In Lepidosfeus also Mueller observed 

 an indication of the valve in the intestinal canal at the lower part; ibid. p. 91. 



^ Meckel's System der vergl. Anat. iv. 1829, s. 35, in Zyrjcena; in Galeus Thalassi- 

 nus {ThalassorMnas vulpecula), and Glaucus, Duvernoy Ann. des Sc. nat. sec. S<^rie, 

 III. 1835, pp. 274 — 284, and Steenstra Toussaint Tijdschr. voor nat. Geschied. er 

 Physiologie, X. 1843, pp. 103 — 107, PI. 3. (According to Mueller and Henle this 

 arrangement occurs commonly in Zy<jcena, Carcharias, Galeocerdo and Thalassorhinus.) 



^ Numerous figures of the stomach and intestinal canal, in different fishes, are to 

 found, amongst others, in Home's Lectures on comp. Anat. II. Tab. 85 — 97, and espe- 

 cially in Eathke Beitrdge zur Gesch. der Thierwelt, 11. Tab. i — iv., and in his memoir 

 on the intestinal canal of fishes in Mueller's Arcldv, 1837, s. 335 — 356, Taf. 17 — 19. 



