GASTRIC DIGESTION IN THE LOWER VERTEBRATES

True gastric digestion, involving the action of pepsin, appears to be found in animals only in the Chordata, and in this group, at the present day, only from the Pisces upwards. It is suggested that its appearance may be correlated with the change from microphagy to macrophagy which occurred in the early history of the phylum.

Information relating to gastric digestion in the lower vertebrates is still too limited for any extensive generalizations to be possible, but many differences between conditions in these forms and in the mammals are evident. Certain facts illustrating the more primitive organization of the gastric mucosa in fish (and to a lesser extent in Amphibia) are briefly summarized.

There seems to be a general resemblance between the pepsin of mammals and that of the lower vertebrates, with perhaps some minor adaptation to medium. There is some positive evidence for the existence of enzymes additional to pepsin in the stomach of fish, but the source of these has not been made clear.

An acid fluid has been obtained from the stomach of fish and Amphibia by siphon or fistula. The secretion of acid may be continuous throughout life in selachians, but increases after feeding; in rays an alternation of acidity and alkalinity has been reported. The stomach of teleosts seems to be less acid than that of selachians and Amphibia, but the pH at the surface of ingested prey may be more suitable for the action of pepsin than is that of the general contents. Digestion proceeds much more slowly in poikilotherms than in mammals, and the food commonly remains in the stomach for one or more days.

The few observations at present available suggest that gastric secretion in selachians, unlike that of mammals, may be controlled entirely by humoral means, but the evidence is incomplete and not altogether consistent. There is no evidence for a ‘psychic phase’ in selachians or Amphibia, but there are indications that in the latter group the sympathetic nervous system may be an important factor in the control of secretion. Histamine, is reported to stimulate the secretion of acid and pepsinogen in frogs.

Extensive studies of the nervous control of gastric movements in fish and Amphibia have led to results which are often conflicting and difficult to reconcile with those established for mammals. Some possible explanations for this are discussed, and in particular it is emphasized that a clear-cut reciprocity between sympathetic and parasympathetic systems may be lacking in the lower vertebrates.

There is evidence for the absórption of fat in the stomach of fish, and it is pointed out that certain characteristics of gastric digestion in poikilotherms would appear to make that organ more adapted for absorption than it is in mammals.

It is concluded that the absence of a stomach from the Holocephali, the Dipnoi and certain Teleostei is likely to be a secondary rather than a primitive condition.

I am indebted to Profs. B. P. Babkin and A. B. Dawson for kindly reading the typescript of this article, which was prepared during the tenure of a Rockefeller Foundation Fellowship at McGill and Harvard Universities, and at the Marine Biological Laboratory, Woods Hole.