ALIMENT AEY CANAL OF FISHES. 413 



these, in the Lamprey, open into a pair of membranous pouches, 

 wliieli discharge the secretion each by a small orifice below the 

 side of the tono-uc' 



O 



Tlicre are neitlier tonsils nor velum palati in Fishes : the folds 

 of membrane behind the upper and lower jaws, of Avhich ' internal 

 lips' the Sword-fish and Dory afford good exami)les, seem intended 

 to prevent the reflux of the respiratory streams of water rather 

 than the escape of food from the mouth. In the Lepidosiren these 

 folds or inner lips are papillose and glandular. That of the 

 upper jaw in the Ray has a marginal fringe. 



In the aberrant Dermopteri and FLaijiostomi, at the two extremes 

 of the class, in which there are numerous branchial apertures on 

 each side, and the respiratory streams do not necessarily enter by 

 the mouth, the last pair of branchial arches are not metamorphosed 

 into pharyngeal jaws, and the entry to the gullet is simply con- 

 stricted by a sphincter ; in the Lepidosiren it is further defended 

 by a soft valvular fold like an epiglottis.^ 



The alimentary canal is usually short, simple, but capacious, in 

 Fishes ; in a few instances, e.g. Br ancldo stoma (fig. 169, ph, as), 

 Myxinoids,' Exocetus, Lepidosiren,^ it extends in almost a straight 

 line Irom the pharynx to the anus : l3ut it is generally disposed in 

 folds and sometimes in numerous convolutions. In Dermopteri 

 the stomach is hardly defined : in the rest of the class the alimen- 

 tary canal is primarily divided into a gastric and an intestinal 

 portion by the constriction called ' pylorus,' fig. 28 1, e. The gastric 

 portion is subdivided into ' oesophagus,' i)3. a., and stomach, ib. h, 

 the boundary line being more commonly indicated by a change of 

 structure of the lining membrane than by a cardiac constriction : 

 the intestinal portion is suljdivided into a ' small ' and a ' large 

 intestine ; ' the latter usually answering to the ' intestinum 

 rectum,' and the boundary, when well defined, being a constric- 

 tion and an internal valvular fold ; Ijut very rarely marked by 

 an external Cfccum. From the ocsopliagus the alimentary canal is 

 situated wholly or in part in the abdominal cavity, to the walls of 

 which it is usually suspended by mesogastric and mesenteric 

 duplicatures of the peritoneal lining memljrane of the abdomen. 

 When not wholly so situated, the part extends beyond the peri- 

 toneal region into the muscidar mass of the tail ; a portion of the 



branchials and the sterno-hyoid muscles in Cartilaginous Fishes, and which exists also 

 in Gadus, Salmo, and some other Osseous Fishes, has been compared to a sublingual 

 salivary gland: but it is a " vaso-ganglion " like the thyroid. 



' ccxxiT. '' XXXIII. p. 342, fig. j, d. 



' XXI. Neurologie, tab. iii. fig. 6. ^ xxxiii. pi. 2.5. 



