Ix DIGESTIVE GLANDS 271 
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, ete. In 
at least two genera (Gastrosteus and Cobitis), 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. Scyl/vwm), 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 Lieberkiihn 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 
1 Archiv f. mikr. Anat. xiii. 1876. 
2 Krukenberg, quoted by Miss Alcock, Journ. Anat. and Phys. xiii. (N.S.), 
1899, p. 613. 
