306 FISHES CHAP. 
septa give to the cavities of the lateral compartments the appear- 
ance of being occupied by a coarse spongy network. 
(e) In its relations to the oesophagus and to the air-bladder 
the ductus pneumaticus exhibits striking modifications in 
different Teleosts. With very rare exceptions, an open ductus 
is wanting in the Heteromi, Catosteomi, Acanthopterygii, Opis- 
thomi, Pediculati, Jugulares, and the Plectognathi, for which 
reason the term “ Physoclisti” has often been used as a collec- 
tive name for these Fishes. On the other hand, a permanently 
open ductus is generally pre- 
sent in the Malacopterygii, 
Ostariophysi, Apodes, and the 
Haplomi, which, in conse- 
quence, have been designated 
“Physostomi.” It must be 
emphasised, however, that all 
Teleosts are Physostomous in 
the embryonic condition, and 
whether they eventually be- 
ele come Physoclistous or remain 
RUNCS ian & 4 Physostomous depends entirely 
= yi on the abortion or retention 
7 of the primitive communica- 
Fra. 183.—Transverse section through the tion between the air-bladder 
sidomine| magn of Caickinv Tweide: aud the alimentary canal 
and ventral branches of the air-bladder ; When present in Teleosts, the 
tHe my mest gctomach ies ductus pneumaticus, with a 
lines surrounding the bladder and its few exceptions (e.g. Noto- 
branches represent the peitovesl MS pzorys), where it is both short 
and relatively wide, is almost 
invariably much longer and narrower than in the other orders 
of Teleostomi and in the Dipnoi, sometimes passing directly from 
the air-bladder to the oesophagus, but not infrequently describing 
a sigmoid curve, as in some Cyprinidae, or an even more tortuous 
course. The opening into the alimentary canal is, with perhaps 
a single exception, dorsal, but may vary from the commence- 
ment of the oesophagus to the hinder end of the stomach. In 
Erythrinus the oesophageal aperture is lateral. In two instances 
the air-bladder has acquired secondary openings to the exterior, 
and of these one occurs among the Physostomi and the other 
