SEA-SQUIRTS. 569 



phorescent ascidians, — A. Lucise. These are represented solely by the genus 

 Pyrosoma, which is thus the only member of the family Pyrosomatidai. 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 



