3o6 



FISHES 



CHAP. 



^r--i-^A 



in. 



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- 

 come Physoclistous or remain 

 Physostomous depends entirely 

 on the abortion or retention 

 of the primitive communica- 

 -Trausverse section tiirough the tion between the air-bladder 

 '''^^"f^t ff ™ ?{ ^"^j'^'^l'^y? ^""■'^«; and the alimentary canal. 



a.o^ Air-Dladder ; d.o and t.o, the dorsal '^ 



and ventral branches of the air-bladder ; When present in TeleOStS, the 



I, liver; m, mesentery: s, stomach; vx, j„„j-,,„ ^„„,,,-„„4-:„ „,:^i, 



vertebral column. The doited and broken ^^^^tus pneumatlCUS, With a 



lines surrounding the bladder and its feW exceptions (e.g. Noto- 



branches represent the peritoneal invest- , \ i ■ • ■ ^ i_^ ^ j. 



meut of these parts. (From Giinther.) pierus), 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 



Fio. 183.- 



