IX DIGESTIVE GLANDS 27 1 



probable that in these Fishes the secretion of the digestive fluids 

 is effected by the ordinary lining epithelium of the stomach or 

 intestine, or both. In the remaining groups gastric glands are 

 generally present in the form of simple caecal structures em- 

 bedded in the submucosa and opening on the surface of the 

 mucous membrane into the cavity of the stomach. The glands 

 differ in different Fishes in the character of their lining epi- 

 thelium and in the extent to which their component cells are 

 differentiated from the epithelium of the stomach. There does 

 not appear, however, to be any distinction into " central " (pepsin- 

 forming) and " parietal " (acid-secreting) cells, as is the case in 

 the higher Vertebrata. Towards the pyloric end of the stomach 

 the true gastric glands are often replaced by mucous glands. 

 There are, nevertheless, not a few Teleosts in which special 

 gastric glands are absent, as, for example, Syngnathus acus, and 

 several species of Cyprinidae, Labridae, and Blenniidae, etc. In 

 at least two genera (Gastrosteus and Colitis), belonging to 

 widely different families, gastric glands are present in certain 

 species but absent in others. As suggested by Edinger,^ the 

 absence of these glands may possibly be due to degeneration. 



It may be remarked that the formation of such digestive 

 ferments as pepsin and trypsin, which are associated with the 

 stomach and pancreas respectively, in the higher Vertebrates, is 

 not nearly so strictly localised in Cyclostomes and Fishes. So 

 far from peptic digestion being limited to the stomach, it may 

 take place in the pharynx, stomach, and intestine of Ammocoetes, 

 and in some Elasmobranchs (e.g. Scyllium), and in such Teleosts 

 as the Pike, Eel, and Carp, the peptic region extends from the 

 stomach for some distance along the intestine, while trypsin has 

 been obtained from the mucous membrane of the stomach, intes- 

 tine and pyloric caeca, as well as from the pancreas.^ 



Intestinal glands analogous to the glands of Lieberktihn in 

 the higher Vertebrates seem to be entirely wanting in Fishes, 

 unless represented by the sac-like or tubular crypts which are so 

 generally present in the Teleostomi. 



The Liver. — Phylogenetically the oldest gland in connexion 

 with the Vertebrate alimentary canal, and in size by far the 



^ Archivf. mikr. Anat. xiii. 1876. 



" Krakenberg, quoted by Miss Alcock, Journ. Anat. and Pliys. xiii. (KS.), 

 1899, p. 613. 



