Walter Holbrook Gaskell

“ It does not seem to me at all necessary that the immediate invertebrate ancestor of the vertebrate should have possessed excretory organs which opened out separately to the exterior on each segment; a fusion may already have taken place in the invertebrate stage, and so a single duct have been acquired for a number of organs. Such a suggestion has been made by Rückert, because of the fact discovered by Cunningham and E. Meyer, that the segmental organs of Lanice conchilega are on each side connected together by a single strong longitudinal canal. ”

“ Starting with the working hypothesis that the central nervous system of the vertebrate has arisen from the central nervous system of the arthropod, but has involved and enclosed the alimentary canal of the latter in the process, so that there has been no reversal of surfaces in the derivation of the one form from the other, we have been enabled to compare closely all the organs of the head-region in the two groups of animals, and in no single case have we been compelled to make any startling or improbable assumptions. ”

“ These observations show that the Crustacea possess not only true excretory organs in the shape of coxal glands, i.e. antennary glands, shell-glands, etc., in the cephalic region, but also a series of segmental glands situated at the base of the appendages, especially of the respiratory appendages: a system, that is to say, of coxal glands which have lost their excretory function, through having lost their external opening, but have not in consequence disappeared, but still remain in situ, and still retain an important excretory function, having become lymphatic glands containing leucocytes. ”

“ These observations of Claus and Spangenberg suggest not only that the primitive arthropod of the trilobite type possessed segmental organs in every segment directly derived from those of a polychæte ancestor, but also that such organs were partly somatic and partly appendicular in position. Such a suggestion is in strict accord with the observations of Sedgwick on the excretory organs of the most primitive arthropod known, viz. Peripatus, where also the excretory organs, which are true segmental organs, are partly somatic and partly appendicular. ”

“ Such a phylogenetic history is most probable, because it explains most naturally and simply the facts of the development of the vertebrate body-cavity; for the mesoblast always originates in the neighbourhood of the notochord and central nervous system, and the lumen of the body-cavity always appears first in that region, and then extends laterally and ventrally on each side until it reaches the most ventral surface of the embryo, thus forming a ventral mesentery, which ultimately disappears, and the body-cavity surrounds the gut, except for the dorsal mesentery. ”

“ It is to me most interesting to find that the very group of annelids, the Polychæta, which possess solenocytes so remarkably resembling those of the excretory organs of Amphioxus, are the highest and most developed of all the Annelida. I have argued throughout that the law of evolution consists in the origination of successive forms from the dominant group then alive, dominance signifying the highest type of brain-power achieved up to that time. ”

“ Yet another marked peculiarity of this region is the segmental arrangement of the excretory organs. Just as our body-musculature has arisen from the uniformly segmented simple longitudinal musculature of the lowest fish, so, as we pass down the vertebrate phylum, we find more and more of a uniform segmental arrangement in the excretory organs. ”

“ In the vertebrate, after the somatic dorsal part or myocœle has separated off, a ventral part is left, which forms a nephrocœle in the trunk-region, and gives origin to the splanchnic striated muscles in the cranial region, i.e. to the muscles which, according to my theory, were once appendicular muscles. This ventral nephrocœlic part is divisible in the trunk into a segmented part, which forms the excretory organs proper, and an unsegmented part, the metacœle or true body-cavity of the vertebrate. ”

“ Many observers have been led to the conclusion that the pronephric duct is epiblastic in origin, although from its position in the adult, it appears far removed from all epiblastic formations. However, at no time in the developmental history is there any clear evidence of actual fusion of any part of the pronephric organ with the epidermis, and the latest observer, Brauer, is strongly of opinion that there is never sufficiently close contact with the epidermis to warrant the statement that the epiblastic cells take part in the formation of the duct. ”

“ Even if it be possible to find some special reason why the branchial and cloacal parts of the gut are freed from yolk and lined with serviceable epithelium a long time before the midgut, why should a bit of the midgut, which Shipley calls the œsophagus, which is connected with the region of the pronephros and not of the branchiæ, differ so markedly from the rest of the midgut? ”

“ The importance ascribed by Beard to the thymus in the formation of leucocytes in the lowest vertebrates would be considerably reduced in value if the branchial region of Ammocœtes possessed neither thymus glands nor anything equivalent to them. Such, however, is not the case. Schaffer has shown that in the young Ammocœtes masses of lymphatic glandular tissue are found segmentally arranged in the neighbourhood of each gill-slit—tissue which soon becomes converted into a swarming mass of leucocytes, and shows by its staining, etc., how different it is from a blood-space. ”

“ The mesonephric tubules belong to segments posterior to those of the pronephros, are strictly segmental with the myotomes, and open into the pronephric duct.
3. All observers are agreed that the two sets of excretory organs resemble each other in very many respects, as though they arose from the same series of primitive organs, and, according to Sedgwick and Brauer, no distinction of any importance does exist between the two sets of organs. ”

“ In my opinion, nests of sympathetic ganglion-cells necessarily mean the supply of efferent fibres to some organ, for all such ganglia are efferent, and also, if they are found in the organ, would have been brought into it by way of the blood-vessels supplying the organ, so that Aichel's statement of the origin of the suprarenals in the Elasmobranchs seems to me much more probable than a derivation from nerve-cells. ”

“ In the lower vertebrates, the only glandular masses apart from the cell-lining of the body-cavity itself, which give rise to leucocyte-forming tissue, are these segmental branchial glands, or possibly also the modified post-branchial segmental glands, known as the head-kidney in Teleostea, etc. ”

“ Now, these lateral roots were originally the nerves supplying the prosomatic and mesosomatic appendages with motor as well as sensory fibres. The absence, therefore, of lateral roots in the spinal region implies that in the vertebrate none of the musculature belonging to the metasomatic appendages has remained. Consequently, as far as muscles are concerned, the clue to the origin of the spinal segments must be sought for in the segmentation of the body-muscles. ”

“ Such an attempt is possible, it seems to me, because there exists throughout the animal kingdom striking evidence that excretory organs which no longer excrete to the exterior do not disappear, but still perform excretory functions of a different character. Their cells still take up effete or injurious substances, and instead of excreting to the exterior, excrete into the blood, forming either ductless glands of special character, or glands of the nature of lymphatic glands. ”

“ According to this explanation, the whole ventral portion of the somatic musculature of the adult Amphioxus belongs to the extension of the pleural folds, the original body-musculature being confined to the dorsal region. This is expressed roughly on the external surface of Amphioxus by the direction of the connective tissue septa between the myotomes (cf. Fig. 162, B) . These septa, as is well known, bend at an angle, the apex of which points towards the head. The part dorsal to the bend represents the part of the muscle belonging to the original body ”

“ Then, from some cause, the appendages were rendered useless, and dwindled away, leaving only the pronephric organs to indicate their former presence. At the end of this stage the animal possessed vertebrate characteristics.
4. For the purpose of increasing mobility, of forming an efficient swimming instead of a crawling animal, the body-segments increased in number, always, as is invariably the case, by the formation of new ones between those already formed and the cloacal region, and so of necessity caused an elongation of the pronephric duct. ”