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

STRUCTURE 01? THE EXCRETORY OHG-ANS OV AMmfOXUS. 199
Part 4.—The Development of the Left Series of Nephridia in
the Larva.
For many years I have been trying to trace the development
of the nephridia in Amphioxus. In 1902 I collected a
large amount of material from the Pantano at Faro; but illhealth
prevented iny working out the development on the
living larva,, and I failed to do so on the preserved specimens.
It was not till last year that I was again able, in
Helgoland, to study the living larva, and succeeded in
tracing some stages of the development of the excretory
organs. In the meantime Legros had been studying the
same subject in Naples, and published anonymously a preliminary
notice of his results a short time ago (16). x
In the present paper I shall not discuss in detail the first
origin of the nephridia, but restrict myself to a description of
the stages found in the larva with from ten to fifteen gill-slits
of the left hand series, and no trace of the right ha,nd series.
These are the only stages which I have been able to study
sufficiently in the living state.
Fig. 30 gives a left side view of a young larva with eleven
slits. The anterior gill-slits are still well on the right side,
but the hinder slits are in or near the middle line. The
future dorsal edge of each slit may, of course, at this stage
be more ventral than the future ventral edge. The nephridia
are seen as small rounded sacs near the posterior ventral
corner of each slit. Every slit from the first to the last has
such a. nephridium. At this early stage, thei'e is no atrium,
the slits have an internal margin of thick branchial epithelium,
which is thrown into characteristic folds when the
branchial muscles contract, while the external margin of the
slit is formed by a thin fold of the body wall, acting as a sort
1
Through the kindness of M. Legros I have had the opportunity of
examining his sections, and I cannot agree with his conclusions as to
the origin of the nephridia from the ccelomic epithelium, nor as to the
presence of internal openings. But I believe he has modified his views
considerably on these points since the publication of the note.