256 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 Lutodeira, 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, 1t 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- 
branchs (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” 
(Gymnotidae), the ventral surface of the throat (Fig. 351.) 
