2946 THE SEA SQUIRTS OR ASCIDIANS 



Omitting mention of the remaining families of the group just considered, we 

 come to the third and last suborder of the typical ascidians, namely, the phosphor- 

 escent ascidians, A. Luciae. These are represented solely 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 sac thus formed the constituent individuals are imbedded in such a manner 

 that all their inhaleut apertures open on its outer surface, while their exhalent ori- 

 fices are situated within the cylinder; the mouth of the sac forming the common dis- 

 charging 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 bud- 

 ding 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 upward 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 

 A COMPOUND ASCIDIAN (Amaructum) IN the most beautiful phosphorescence of 

 WINTER CONDITION. tropical seas is due, each colony, when 



(Natural size.) 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. 2954. 



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 lumi- 

 nous 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 shown 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 



