232 LECTURE IX. 



We have seen that the jaws and mouth are subservient to the 

 respiratory as well as the digestive functions : but in the lowest of 

 fishes, viz. the Lancelet, this community of offices extends through 

 the whole cesophageal and seemingly gastric part of the alimentary 

 canal, which is dilated into a capacious sac, and is richly provided 

 with branchial vessels and vibratile cilia arranged in transverse linear 

 series, like those in the respiratory pharynx of Ascidians (the 

 arrow a extends from the pharynx into the intestine v^fig- 46.) : 

 the cesophageal portion of the alimentary canal is here seen to be 

 longer than the whole gastric aad intestinal portions. In the Myxi- 

 noids lateral diverticula are derived from the oesophagus and me- 

 tamorphosed into special respiratory sacs, communicating by narrow 

 canals both with the oesophagus and with the external surface 

 {Jig.66,f,m.) : in other fishes the respiratory apparatus is more con- 

 centrated and brought more forwards, so as to communicate with the 

 pharynx, and to leave the oesophagus free for the exclusive transmis- 

 sion of food to the stomach. 



The oesophagus {Jig. 61. d) is usually a short and wide funnel- 

 shaped canal with a thick muscular coat and a smooth epithelial 

 lining, more or less longitudinally folded to admit of increased 

 capacity for the deglutition of the often unmasticated or un- 

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

 the outer ones being usually circvdar, the inner ones longitudinal. 

 Some fasciculi from the abdominal vertebrte are attached to the 

 oesophagus in the Cottus scorpius (xcix.). The cardiac half of the 

 oesophagus is characterised by increasing width in most Cyprinidce, 

 and by a more vascular or otherwise modified texture in the Pharyn- 

 gognathi, Lopho-branchii, the Gobioids, Blennies, Flying-fish, Gar- 

 fish, and some others. The inner surface of the oesophagus sends off" 

 short processes, papilliform in Box and Ccesio, obtuse in Acipenser, 

 (prep. 463.), hard and almost tooth-like in Rhombus xanthtirus, 

 Stromatceus Jiatola, and Tetragonurus or the keel-tailed Mullet. 

 The inner sui'face 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 gi'ey 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 (prep. 664.). In the great Basking Shark {Selache) the 

 homologous processes, near the cardia, acquire unusual length, divid- 

 ing and subdividing as they extend inwards, so that the cardiac 

 opening is surrounded by ramified tufts directed towards the stomach. 

 This valvular mechanism (pi-ep. 464. a), seems intended to prevent 



