Walter Holbrook Gaskell

“ In any organ the nerves which are specially of value in determining its innervation, are of necessity the efferent or motor nerves, for the limits of their distribution in the organ are much more easily determined than those of the afferent or sensory nerves. The question therefore of primary importance in endeavouring to determine the nature of the origin of the alimentary canal from its innervation is the determination of the efferent supply to the musculature of its walls. ”

“ At first sight the second essential of a gut-tube—the power of digestion—appears to present an insuperable difficulty to this method of forming an accessory gut-tube, for it necessitates the formation of a secretion capable of digesting proteid material by the external cells of the body, whereas until recently it was supposed that such a function was confined to cells belonging to the so-called hypoblastic layer. ”

“ It is by no means certain that these two sets of nerves possess equal value from a morphological point of view. The meaning of an inhibitory nerve is at present difficult to understand, and in this instance, is rendered still more doubtful owing to the presence of Auerbach's plexus along the whole length of the intestine—an elaborate system of nerve-cells and nerve-fibres situated between the layers of longitudinal and circular muscles surrounding the gut-walls, which has been shown by the recent experiments of Magnus, to constitute a special enteric nervous system. ”

“ It is possible to look at the question the other way round, and suppose that the organ whose development is finished first is older than the one still in process of making. In this case it would be more right to say a ventral extension of the tissue, which gives rise to the notochord, takes place and forms the alimentary canal. It is, to my mind, perfectly possible, and indeed probable, that the formation of the vertebrate alimentary canal was a repetition of the same process which had already led to the formation of the notochordal tube. ”

“ The evidence of comparative anatomy bears out most conclusively the suggestion that in the original vertebrate the gut was mainly a respiratory chamber. In man and all mammals the oral chamber opens into a small pharynx, followed by the œsophagus, stomach and small intestine. ”

“ The position, then, of the notochord and its method of formation suggests that the mid-ventral surface of the arthropod ancestor of the vertebrate formed a deep groove between the bases of all the prosomatic, mesosomatic, and metasomatic appendages, which was subsequently converted into a tube extending along the whole of the body between mouth and anus, and finally, by the proliferation of its lining cells and their conversion into notochordal tissue, became the notochordal rod of the vertebrate. ”

“ Just posterior to the branchial part a diverticulum of the gut was formed at an early stage, as seen in Amphioxus, and provided the commencement of the liver. This simple liver-diverticulum became the tubular liver of Ammocœtes, and formed, curiously enough, not a glandular organ of the same character as the liver of the higher vertebrates, but a hepato-pancreas, like the so-called liver of the arthropods, which also is a special diverticulum of the gut, or rather the main true gut of the animal. ”

“ Similarly, on the invertebrate side, all those forms which resembled Limulus must have possessed a very short region between the branchial and cloacal parts of the body. The original cloacal part of the vertebrate gut may well have been the original cloaca of the arthropod, into which its intestine emptied itself, especially when we see the tendency of the scorpion group of animals to form an accessory cloacal pouch known as the stercoral pouch or pocket. ”

“ As already frequently stated, Apus and Branchipus are the two living arthropods which most nearly resemble the extinct trilobites. The beautiful specimens of Triarthrus (Fig. 165) found by Beecher give an idea of the under surface of the trilobite such as has never been obtained before, and demonstrate how closely the condition of things found in Apus (Fig. 164) was similar to that occurring in the trilobites. ”

“ In many of these, new individuals are formed from the parent by a process of budding, and it has been proved that frequently the gut of the new individual thus budded off arises not from the gut or hypoblastic layer of the parent, but from the surface or epiblastic layer. ”

“ Such a tube was able to act as a digestive tube, owing, as already pointed out, to the digestive powers of the skin-epithelium, and I imagine at first the respiratory chamber, seeing that it composed very nearly the whole of the gut, was at the same time the main digestive chamber; even in Ammocœtes its digestive power is superior to that of the intestine itself. ”

“ Experiments were made now years ago of turning a Hydra inside out so that its internal layer should become external, and vice versâ, and they were said to have been successful. Such an animal could go on living and absorbing and digesting food, although its epiblastic surface was now its digestive internal surface. ”