SEA-SQUIRTS. 
569 
phorescent ascidians,—A. Lucias. These are represented solety by the genus 
Pyrosoma, which is thus the only member of the family Pyrosomatidce. These 
ascidians are free-swimming pelagic forms, reproducing by buds in such a manner 
as to form colonies in the shape of a sac; such colonies sometimes attaining huge 
dimensions. In the sack thus formed the constituent individuals are embedded in 
such a manner that all their inhalent apertures open on its outer surface, while 
their exhalent orifices are situated within the cylinder; the mouth of the sac 
forming the common discharging aperture. The apertures of the units are not 
lobed; and the outer tunic is gelatinous and transparent, containing no hard 
spicules, but provided with numerous minute cells. The branchial chamber is 
well-developed, and the tentacles are simple. The first four individuals of the 
colony grow in the form of buds from a rudimentary sexually-developed larva; 
the subsequent increase taking place by budding from a ventral posterior stolon. 
The genus is represented only by four species, in one of which (P. elegans ) the 
individuals form regular oblique rows in the walls of the sac, while in the other 
three they are arranged irregularly. The largest of all is P. spinosum, from the 
Atlantic, in which the total length of the colony may be upwards of four feet; this 
species being distinguished by the surface of the sac being provided only with 
short sharp spines, instead of with large processes of the tunic. It is to these 
ascidians that the most beautiful phosphorescence of tropical seas is due, each colony, 
when stimulated by a touch or shake of the water, giving forth a brilliant ball of 
bluish light, which lasts for several seconds, as the organism floats along beneath 
the surface, and then suddenly disappears. A colony is figured on p. 576. 
Describing the luminosity produced by these ascidians, Bennett states that on 
one occasion in the Australian seas, when he reached the deck, he observed a “ broad 
and extensive sheet of phosphorescence, extending in a direction from east to west, 
as far as the eye could reach. The luminosity was confined to the range of animals 
in this shoal, for there was no similar light in any other direction. I immediately 
cast the towing-net over the stern of the ship, as we approached nearer the 
luminous streak, to ascertain the cause of this extraordinary and so limited 
phenomenon. The ship soon cleaved through the brilliant mass, from which, 
by the disturbance, strong flashes of light were emitted; and the shoal, judging 
from the time the vessel took in passing through the mass, may have been a mile 
in breadth. The passage of the vessel through them increased the light around to 
a far stronger degree, illuminating the ship. On taking in the towing-net, it was 
found half filled with Pyrosoma, which shone with a pale, greenish light; and 
there were also a few shell-fish in the net at the same time. After the mass had 
been passed through, the light was still seen astern, until it became invisible in the 
distance.” Frequently the phosphorescence is intermittent, periods of luminosity 
alternating with intervals of darkness. Moseley writes that during the voyage 
of the Challenger, “ a giant Pyrosoma was caught by us in the deep-sea trawl. 
It was like a great sac, with its walls of jelly about an inch in thickness. It was 
4 feet in length, and 10 inches in diameter. When a Pyrosoma is stimulated by 
having its surface touched, the phosphorescent light breaks out at first at the 
point stimulated, and then spreads over the surface of the colony as the stimulus 
is transmitted to the surrounding animals. I wrote my name with my finger on 
