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

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19G EDWIN S. C400DR1CH. of Annelid larvee (trochosphores), but I can form no opinion as to the reality of any such resemblance." The next author to mention the organ is MacBride, who briefly describes its development, believing that it arises from the communication between the gut and the second myotome (II). 1 Van Wijhe (14) describes the canal of Hatschek's nephridium in the adult, applying to it the name Schlundforsatz: " eine enge Rohre, welche dem linkeu Seitenrande der linkeu Aorta angeschmigt ist. Das enge lumen wird von einem einschichtigen Cylinderepithel begrenzt und bildet streckenweise seitliche Ausbuchtungen. Wo eine solche angeschnitten wird, konnen zwei Lumina im Schnittbilde auftreten. Unmittelbar hinter dem Velum miindet die Rohre mit einer feinen Offnung in den Schlund aus." He denies, however, the presence of the ccelomic cavity described by Hatschek, and does not accept the latter's theory as to the organ's function. " Nach meiner Meinung/' says van Wijhe, " ist das organ nicht anderes als ein Rudiment des vordereu Darmendes, welches beim Embryo in das Flimmersackchen (linke Entoderrnsackchen) ausmiindete." It is to G-oldschnidt that we are indebted for the first description of solenocytes in the nepliridium of Hatschek (4), placing its homology with the posterior nephridia beyond doubfc. His account seems, however, to be based on imperfect material, and he falls into the error of ascribing to the canal an internal opening such as Boveri had described in the paired nephridia. I have recently had the opportunity of studying this interesting organ in adult and larval specimens in Helgoland, 3 and am thus able to give a more complete description of it. 1 I am unable to agree with the view of either Hatschek or MacBride as to the origin of this nephridium. -' I gladly seize this opportunity of thanking Prof. Heincke, Prof. Havtlaub, and the staff of the Konigl. Biologische Anstalt for the kind way in which they received me in Helgoland.
STRUCTURE 0.1;' THE EXCRETORY ORGANS Ob' AMl'EIOXUS. 197 The nephridium of Hatschek reaches its maximum development in the adult, where it is indeed the largest nephridinm in the body, some 2 mm. in length. Lying on the left side, below and parallel to the notochord, it opens just behind the velum into the pharynx, 1 and runs forward a long distance to a point just in front oE the ciliated groove (Raderorgan). Here it ends blindly, and along its course are given off short blind diverticula (figs. 27, 42, 43, 44). Solenocytes are set on the dorsal and lateral sui'faces of the organ along almost its whole length, being especially numerous on the diverticula (fig. 28). Altogether au enormous number of solenocytes are present on this nephridinm in the adult Amphioxus. The canal runs along the floor of a narrow cavity beside the aoi-ta (figs. 42—44). It is to the wall of this cavity that the solenocytes are attached, and it appears to be of coclomic nature; at all events it is in open communication with the myocoele of the first myotome in larval stages (figs. 25, 26, 33). In the adult, however, it is closed off, and the lining epithelium seems to be very irregularly developed, forming no distinct layer of cells (fig. 28). In the larva of about 13 gill-slits, of the left series only (fig. 33), the nephridium can be well seen by transparency as a short tube opening behind into the pharynx (fig. 24). Its dorsal surface is entirely beset with solenocytes in several closely packed rows (fig. 29). An optical section of the organ at this stage is represented in fig. 38, showing clearly the way in which the tubes of the solenocytes pierce the thin dorsal wall. We may summarise as follows the observations recorded above :—The nephridium of Hatschek is a true nephridium, similar in structure to the posterior paired nephridia. In the adult, where it reaches its maximum development, it extends along the left aorta from in front of the ciliated 1 On one occasion only I have found an opening from the canal into tlie hinder region of the buccal cavity itself, as well as the posterior opening into the pharynx.
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On the Structure of the Excretory Organs of Amphioxus. Part 2.—The ...

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