P. H. GOSSE, _del._ LEIGHTON, BROS.
BOTRYLLI.
]
Each bright speck in these radiating star-like systems is an animal
essentially of the Ascidian form, with the following peculiarities.
All have budded from one primary individual, which was produced as a
tiny tadpole, from an egg: the manner and direction in which the buds
were put forth determining the starry arrangement. There is a common
gelatinous envelope, in which the whole are imbedded, and which ever
extends as the individuals and systems multiply, and which seems to
have the power of developing isolated individuals which have not budded
in the ordinary way, but which then produce others by budding, and
so become the commencing points of other systems. In each individual
the siphonal orifices are remote from each other, the receiving one
being placed on the circumference of the ring or system, while the
ejecting one is placed at the opposite end, opening, in common with
the discharging siphons of all of that system, into a central main
orifice, which rises out of the level with a circular rim, and forms
the dark centre of the system.
In the open ocean there are forms of _Tunicata_ which are not attached,
but swim freely; and, what is surprising, even compound forms are
thus found, progressing by a combined action. One of the most curious
is the genus _Pyrosoma_, which consists of long-bodied Ascidians, so
united in rings as to constitute a long, free cylindrical tube, closed
at one end and open at the other. By the rhythmical contractions and
dilatations of the multitude, this great cylinder slowly swims through
the open sea. But the most interesting circumstance in its history is
that it is intensely luminous, lighting up the midnight ocean with
flashes of vivid light, or seen gliding through the dark water like
glowing sticks of fire. So it is described by some voyagers, perhaps
with a little exaggeration, for Mr. Bennett, to whom we are indebted
for many valuable remarks on oceanic zoology, speaks in more subdued
tones of it. His account of this and other pelagic phosphorescence is
very interesting:--