the
sea was absolutely crowded with Salpae, in all stages of growth, and of
size very convenient for examination." He was able to verify the
general truth of Chamisso's statement. The aggregate form of Salpa
always gives rise to the solitary salps, and the solitary salps always
give rise to chains of the aggregate salps. But the process of
reproduction he shewed to be quite different in the two cases. The
solitary salp produces in its interior a little stolon or diverticulum
which contains an outgrowth from the circulatory system, and this
stolon gradually becomes pinched off into the members of the chain of
the aggregate form. The salps of the aggregate form are therefore
merely buds from the solitary form, and are not produced in the
ordinary way, by sexual generation. On the other hand, each salp of
the chain has within it a true egg-cell. This is fertilised by a male
cell, and within the body of the parent, nourished by the blood of the
parent, grows up into the solitary form. There is then an alternation
of generations, but there are not two sexual generations. The sexual
generation of chain salps gives rise to forms which reproduce by buds.
From this conclusion, with which all later observers have agreed,
Huxley went on to his theory of individuality. Different names had
been given to the two forms, but Huxley declared that neither form was
a true zooelogical individual; they were only parts of individuals or
organs, and the true individual was the complete cycle involving both
forms.

In addition to determining the interesting method of reproduction,
Huxley made an elaborate investigation of the structure of Salpa. On
one occasion only the _Rattlesnake_ came across a quantity of an
allied Ascidian, Pyrosoma, which had received its name from its
phosphorescence.

"The sky was clear but moonless, and the sea calm; and a more
beautiful sight can hardly be imagined than that presented from
the deck of the ship as she drifted, hour after hour, through
this shoal of miniature pillars of fire gleaming out of the dark
sea, with an ever-waning, ever brightening, soft bluish light, as
far as the eye could reach on every side. The Pyrosomata floated
deep, and it was only with difficulty that some were procured for
examination and placed in a bucketful of sea-water. The
phosphorescence was intermittent, periods of darkness alternating
with periods of brilliancy. The light