THE SEA SQUIRTS OR ASCIDIANS 



2947 



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 four feet in length, and ten 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 the surface of the giant Pyrosoma, as 

 it lay on deck in a tub at night, and my name came out in a few seconds in letters 

 of fire." 



With the exception of the family just mentioned, and also of a sec- 



118 ond one which constitutes the third order, the present ordinal group 



Ascidians termed the Thaliacea includes the whole of the free swimming 



pelagic representatives of the class. Either simple or compound in 



structure, these ascidians lack both a tail and a notochord in the adult, but have a 



AN INDIVIDUAL OF A CHAIN SALPA. 



(Natural size.) 

 a. inbalent, and b. exhalent, orifice; d. gill; c. e. viscera; f. eye (?);". pedicle of union. 



persistent outer tunic, which may be either feebly or fully developed. In the inner 

 tunic the muscles are arranged in the form of more or less nearly complete circular 

 bands, the contraction of which forms the motive agency of the creatures. The 

 branchial chamber has either two large openings, or a number of smaller gill slits, 

 leading to a single atrial cavity; the latter communicating with the exterior by the 

 exhalent aperture, and the vent opening within it. In all the members of the 

 group an alternation of generations takes place; and this may be further compli- 

 cated by the individuals of a single generation being unlike one another. During 

 one period of existence temporary colonies may be formed, but these never increase 

 by the budding of the constituent units, which eventually separate from one another 

 and disperse. 



The well-known salpae form a suborder Hemimyaria characterized by the 

 formation of temporary colonies in the sexual generation, and represent a family 

 (Salpidce] distinguished by the muscular bands of the inner tunic being incomplete 



