On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

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192 JiDWJJf S. GOODBICH. due to theoretical bias. It is true that I hold that the renal organ of Amphioxus is a nephridium homologous with the nephridia of Annelids and Platyhelminths, and not homologous with the kidney tubules of the Oraniata (5, 7); but it is now well known that the true nephridia of Annelids may open into the coeloin. There is no a priori reason why they should not do so in Amphioxus. However, no nephridium has yet been found possessing both solenocytes and an internal opening, though such intermediate stages must presumably have existed. The Relation of the Nephridium to the Bloodsupply.—The general blood-supply has been well described and figured by Boveri (1). But according to my observations the vessels occur not so much as narrow capillaries, as in the form of a large expanded vessel spreading over the area occupied by the excretory organ. This is shown in sections (figs. 7, 23), and also in the reconstructions given on Plate 11. It will, moreover, be noticed that, although the greater part of the bloodvessel lies on the inner or atrial surface of the nepln-idium, yet several loops pass round to the outer or coelomic surface. Thus a considerable part o£ the nephridial canal is entirely surrounded by the bloodvessels. The soleuocytes radiate out from the canal, and always lie on the wall of a bloodvessel, being attached to it by a protoplasmic process (figs. 4, 15). The way in which these cells are distributed is shown in figs. 14, 19, and diagrams 2 and 3, and the text-figure. It will there be seen that the longer tubes, which are of course those belonging to cells furthest away from the canal, pass over the shorter tubes to reach their destination. Never do the solenocytes project freely into the ccelom; when they appear to do so in sections this is, I believe, due to the cell having become detached accidentally, either during the process of preservation or of cutting. The tubes are therefore fixed at both ends. In the text-figure may also be seen the peculiar disposition of the solenocytes at the top of the secondary gill-bar. Here
STItUCTCJBK 01' THK EXCKRTOIiY 01WANS 01' AJIFHIOXUS. 193 the canal of tlie nephridium gives off two or three short diverticuln, which are turned away from the coelom towards the middle line. The numerous solenocytes projecting from these diverticula lie in a sort of pocket or chamber (figs. 1, 2, 3, 23), which only communicates with the coelom by means of a dorsal opening, over which pass a large number of solenocyte tubes. In one region the inner wall of this chamber is formed by the skeletal rod of the gill-bar (figs. 3; 28). Somewhat similar pockets are found occasionally in connection with other parts of the nephridinm, as, for instance, the anterior limb of the canal. The cavity in the chambers is, I believe, rather of the nature of a lymph space than of a true ccelomic cavity. The Relation of the Nephridium to the Coclomic Epithelium.—It is important to determine exactly what is the disposition of the ccelomic epithelium in the neighbourhood of the nephridium. Boveri (1) and Weiss (13) have already shown that the canal is covered by the ccelomic epithelium ; but this epithelium only clothes the outer or coelomic surface (text-figure). It passes on to the nephridium from the atrial wall, covering the canal and its blind branches to their extremity. Here it is not reflected so as to pass over to the inner or atrial surface of the organ, but ends abruptly nenr the base of the solenocyte tubes (figs. 2, G, 8, 9). Thus the nephridium and the bloodvessels which accompany it maybe said to lie "morphologically" entirely outside the coelom j between the coelomic epithelium and the atrial epithelium. The nephridium is, in fact, retroperitoueal. This is true, I believe, of the solenocytes themselves, though less easy to prove. For the coelomic epithelium stops short where the solenocytes begin (figs. 6, 8, 2), passing neither on the inner side over the bloodvessel, nor outside them over their coelomic surface. For a long time I was under the impression that a very delicate membranous extension of the epithelium covered over the coelomic surface of the solenocytes; but I am now satisfied that this is not the case,
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On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

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