, STRUCTURE OF THE EXCRETORY ORC4ANS OF AMPHIOXUS. 187 
canal; but this deceptive appearance is soon exposed on a 
more critical examination of the preparation. Thick sections 
are especially misleading. ISTo observation made on a section 
more than 5 thick is in the least conclusive. The technical 
difficulties are very great in the study of Amphio.Kus; 
the tissues are brittle, the cells very small and difficult 
to stain satisfactorily. Formol and Flemming’’s fluid, cor- 
rosive-acetic, and picro-sulphuric-formol are all good pi-e- 
servatives. Great care must, however, be taken to avoid 
shrinkage, and for this purpose the method of double 
embedding in celloidin and paraffin is most useful. By far 
the best sections are obtained from pieces of the pharynx 
removed from the fresh animal, and preserved separately. 
One may use either carmine or hmmatoxylin for staining the 
nuclei ; but it is quite essential to add some suitable cyto- 
plasmic stain such as acid fuchsin. For the particular 
])urpose Ave are now concerned with, perhaps some strong 
staining reagent like Mann’s methyl-blue eosin is the best 
for working out minute details under high powers, though 
picro-nigrosin also yields valuable results. 
Turning now to the structure of the nephridium, we find 
the external pore opening at the very top of the atrial cavity, 
on the anterior outer surface of the secondary or tongue bar 
{op., figs. 1, 2, 7, and text-figure). The pore leads into 
a canal which gives off a short posterior limb, and a much 
longer anterior limb. The latter passes forwards to the next 
primary bar, and downwards into the triangular coelomic 
cavity delimited by the ligamentum denticulatnm. In a 
fully developed nephridium both the anterior and posterior 
limbs give off diverticula of varying length, which may 
sometimes branch. These are shown in fig. 1 of Part 1 (7), 
and are seen again in the reconstructions given in this paper 
(figs. 1, 2, 3). 
Let us pass to the conclusive evidence which can only be 
obtained from sections. The wall of the nephridial canal 
contains many nuclei (figs. 7, 13). In some places they are 
so closely packed that they seem to press against each other. 
