ALIMENTARY CANAL OF FISHES. 415 



lining, more or less longitudinally folded to admit of increased 

 capacity for the deglutition of the often unmasticated or undivided 

 food. The muscular fibres are arranged in different fasciculi, the 

 outer ones being usually circular, the inner ones longitudinal. 

 Some fasciculi from the abdominal vertebrae are attached to the 

 oesophagus in the Coitus scorpius. 1 The cardiac half of the 

 oesophagus is characterised by increasing width in most Cyprin- 

 idce, and by a more vascular or otherwise modified texture in 

 the Pharyngognathiy Lophobranchii, the Gobioids, Blennies, 

 Flying-fish, Garfish, and some others. The inner surface of the 

 oesophagus sends off short processes, papilliform in Box and Ccesio, 

 obtuse in Acipenser? hard and almost tooth-like in Rhombus xan- 

 thurus, StromatcBus Jiatola, and Tetragonurus. The inner surface 

 of the gullet presents longitudinal papillose ridges in Planirostra. 

 But the most striking peculiarities of the oesophagus are met with 

 in the Plagiostomes. A layer of grey parenchymatous substance 

 is interposed between the muscular and inner coats at the cardiac 

 half of the oesophagus in the Torpedo. Numerous pyramidal 

 retroverted processes, jagged or fringed at their extremity, project 

 from the inner surface of the oesophagus in the Dog-fish (Spinax 

 acanthias), 3 and some other Sharks, fig. 278, a. In the great Basking 

 Shark (Selache) the homologous processes near the cardia acquire 

 unusual length, dividing and subdividing as they extend inwards, 

 so that the cardiac opening is surrounded by ramified tufts directed 

 towards the stomach. 4 This valvular mechanism, fig. 278, b, would 

 prevent the return of such fishes or mollusks as may have been 

 swallowed alive and uninjured by the small obtuse teeth of this 

 great Shark. In many Osseous Fishes we may, finally, notice 

 the communication of the ' ductus pneumaticus ' with the oeso- 

 phagus, usually by a small simple foramen ; but provided with 

 special muscles in the Lepidosteus, where it opens upon the 

 dorsal aspect of the oesophagus, and with a sphincter and cartilage 

 in the Polypterus, and Lepidosiren, where it communicates like a 

 true glottis with the ventral surface of the beginning of the 

 oesophagus. In the Globe-fishes (Diodon, Tetrodoii) the great 

 air-sac seems to be a more direct developement, as a cul de sac, 

 of the oesophagus. 5 These singular fishes blow themselves up 

 by swallowing the air, which escapes through a large anterior 

 oblique orifice into the sac : and this again communicates with 



1 xcix. 2 xx. vol. i. p. 126, prep. no. 463. 



3 Ib. prep. no. 664. 4 Ib. prep. no. 464. A. 



5 Ib. vol. iii. p. 271, pi. 47, preps, nos. 20932095. 



