GENERAL CONSIDERATIONS. 



571 



mesial side of the sub-chordal coelomic sac, and open into the atrial 

 cavity at the uppermost angle where the wall that covers the outer 

 side of the gill-bars bends outward to 

 form, together with the wall of the 

 sub-chordal coelomic sac, the thin 

 partition-wall known as the ligamen- 

 tum denticulatum (Id). This latter, in 

 the primary gill-bars, extends further 

 down than in the secondary (Fig. 312, 

 / and IT). The nephridial canals 

 recur regularly in the pharyngeal 

 region and open on the tongue-bars 

 (Fig. 312). In each canal (nk) an 

 anterior ascending branch can be 

 distinguished from a short posterior 

 branch. Each of these branches 

 opens at its end into the sub-chordal 

 coelomic cavity (nc, funnel), and in 

 the course of the canal three or four 

 other apertures occur. Round these 

 apertures (we) the cells of the walls of 

 the sub-chordal coelom are peculiarly 

 modified. Highly refractive spherical cells are found in the body- 

 cavity connected with the apertures by means of delicate filaments 

 (thread-cells). Round the nephridial canals, the upper ends of the 

 branchial vessels, by anastomosing, form a vascular network which 

 we may call the glomerulus (Fig. 311, gl). 



BOVEEI regarded these canals as homologues of the pronephros of the 

 Craniata, and the atrial cavity of Amphioxus as the homologue of the 

 pronephric duct of the Vertebrata. For further details on these points we 

 must refer the reader to the works of this author. Nothing is as yet known 

 of the development of these renal canals. We are also in the dark as to their 

 relation to the larval organ mentioned above and described by HATSCHEK as 

 the nephridium, but it should be mentioned that the figure of this organ given 

 by HATSCHEK (No. 14) shows a certain similarity to the canals discovered by 

 BOVERI. Objections have recently been raised to BOVEBI'S generalisations by 

 SKMON and VAN WIJHE (No. 22). 



General Considerations. 



With regard to the systematic position of Amphioxus, we adopt 

 the view now accepted by most zoologists that this animal is to be 

 regarded as the representative of a very primitive group which served 



FIG. 310. Transverse section 

 through the genital rudiment of 

 the stage represented in Fig. 309- 

 (after BOVERI). bm, ventral 

 muscle ; g, blood-vessel ; gd, 

 genital gland ; w, fold separating 

 myocoele and gouadic pouch. 



