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

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200 EDWJN S. GOODRICH. of sphincter (figs. 33 and 36). A shallow branchial chamber lined with epidermis is thus formed, leading from the external to the internal opening. It is in this chamber that the nephridium opens, at a place corresponding apparently to the point of junction of the ectoderm with the endoderm (fig. 41). The position of the nephridiopore can be seen in figs. 36 and 39. When the atrium becomes formed by the closing off of the space between the metapleural folds, with which the branchial cavities become merged, the pores open into the atrium. A ventral view of a stage where the atrium has just begun to be formed posteriorly shows one or two nephridia behind the last open gill-slit (fig. 35). Probably these nephridia belong to the posterior gill-slits, which have closed up (Willey, 15); they open now directly on the surface (fig. 30). The young nephridium is a flattened sac, without internal opening (figs. 36 and 39). From its inner end spring a large number oE solenocytes; their tubes pierce its wall, and their flagella pass into the lumen of the sac. The majority of the solenocytes spread over the blood-vessel which runs along the future dorsal edge of the slits. The solenocytes of the first few slits scarcely extend beyond this limit; but, passing backwards to more posterior nephridia, we find that the solenocytes spread farther and farther up towards the dorsal aorta, the tubes lengthening out as the cells lie farther from the nephridial sac. At about the fifth or sixth nephridium some of the solenocytes actually reach the aorta (fig. 40). The tubes in this case may attain a really astonishing length, stretching right across the field of a T^-th oil-immersion objective with oc. 8. Fig. 34 represents the posterior gill region of a living larva, in which the remarkable development of the solenocytes is well shown. Here a group of the longest solenocytes, some twelve to eighteen in number, spread out over the aorta in a most beautifully regular fan-like arrangement in each segment. A section of this region is shown in fig. 31; the fan-like disposition is found in each segment to the hindmost
STRUOTUBE OP THE EXCRETORY ORGANS 01? AJII'HIOXTJS. 201 limit of the series of nephridia. Presumably the dorsal solenocytes degenerate later, since they are not known to exist in the adult. The observations on the larval nephridia recorded in this part may be summarised as follows:—To every giil-slit corresponds a nephridium, consisting of a sac closed internally, but opening to the exterior apparently at the point where the ectoderm joins the endoderm iu the shallow branchial chamber. From the internal blind end of the nepliridial sac spring numerous solenocytes, some of which reach and spread over the aorta at every segment in a fan-like arrangement. This structure is only fully developed from about the eighth segment backwards to the last nephridium. July 3rd, 1909. LIST OF REFERENCES. 1. Boveri, Th.—"Die Nicrenkaniilchen des Amphioxus," 'Zool. Jahrb. Anat. Abt.,' Bd. 5, 1892. la. " Bemerk. iiber cl. Bau. d. Nierenkaniilclien des Ampliioxus," ' Anat. Ann;.,' Bd. 25, 1904 2. Felix, W.—'• Entw. des Hamapparates," ' Hertwig's Handbuch d. Entw. d. Wirbeltiere,' Bd. 3, 1904. 3. " Entw. des Exkretions-systeni," ' Anat. Hefte. Ergeb. Anat. u. Entw.,' Bd. 13, 1904 4. G-oldschmidt, B,.—"Amphioxides," ' Wiss. Ergeb. d. deutsch. Ticfseo- Expedition " Valdivia," ' Bd. 12, 1905. 5. Goodrich, E. S.— " On the Ccelom, etc ," ' Quart. Journ. Micr. Soi.,' vol. 37, 1894 6. " On the Nephridia of tlie Polychaita," Parts I, II, and III, ibid., vols. 40, 1897, 41, 1898, and 43, 1900. 7. " On the Structure of the Excretory Oi-gans of Amphioxus," Part I, ibid., vol. 45, 1902. 8. " On the Body-cavities and Nephridia of the Actinotrocha larva," ibid., vol. 47, 1903. 9. Hatschek, B.—" Mittb. iiber Amphioxns," ' Zool. Anz., ! Bd. 7, 18S4.
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