2 S 6 FISHES CHAP. 



somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a 

 contracted sigmoid curve between the somewhat dilated stomach 

 and the spacious intestine is the only departure from the straight 

 course of the preceding groups. 



In the remaining Fishes the degree of convolution varies 

 within rather wide limits. The oesophagus is usually straight 

 and wide, but in Lutocleira, among Teleosts, it is long and even 

 convoluted, and in the Plectognath Teleosts it gives off a large 

 sac-like outgrowth (" air-sac "), which extends anteriorly as far as 

 the head, and posteriorly to the beginning of the tail, and 

 communicates with the oesophagus by two apertures. The 

 stomach may be U-shaped with the concavity directed forwards, 

 and consisting of a right limb passing backwards from the 

 oesophagus, and a left limb curving forwards to its junction 

 with the intestine (Fig. 153). In such instances as these the 

 stomach and the adjacent section of the intestine describe a 

 characteristic siphonal curve. In certain other Fishes (Fig. 160), 

 the oesophageal portion of the stomach terminates behind in a 

 tubular or sac-like dilatation at some distance posterior to the 

 laterally situated pylorus, which indicates the origin of the 

 intestine. The intestine is straight, or nearly so, in Elasmo- 

 branchs, Crossopterygii, and Dipnoi, and also in a few Teleosts ; 

 but sometimes, and very generally in Teleosts, it is more 

 or less convoluted, notably in some of the Mugilidae, and in 

 the Loricariidae, where, as in Plecostomus, it is disposed in 

 numerous spiral coils like a watch - spring. The terminal 

 portion of the intestine or rectum either opens into a cloaca, 

 which also receives the urinary and genital ducts, as in Elasmo- 

 braachs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally 

 by an anus, situated in front of the separate or united urino- 

 genital ducts, as is the case with all the remaining groups 

 of Fishes (Fig. 154). The cloacal aperture is invariably 

 situated near the junction of the caudal and trunk regions, and 

 as a rule is median in position, rarely, as in the Dipnoi, displaced 

 to the right or left of the middle line; but the anus differs 

 greatly in position, sometimes retaining its primitive position at 

 the hinder end of the trunk, as in the Holocephali, Chondrostei, 

 Crossopterygii, Holostei, and many Teleosts, or occupying almost 

 any position between that point and, as in the " Electric Eels " 

 (G-ymnotidae), the ventral surface of the throat (Fig. 851.) 



