88 LIVING LIGHTS. 



The Salpa spinosa, a familiar form upon our coasts, is quite 

 cylindrical, often a little flattened above and below, and 

 seemingly moulded in glass, so beautiful is its structure upon 

 examination. As small and common as they are, they have 

 created much discussion. Some observers deem their devel- 

 opment one of the most remarkable instances of the alterna- 

 tion of generations. Chamisso, the German poet-naturalist, 

 explains the relationship as follows : " A Salpa mother is not 

 like its daughter or its own mother, but resembles its sister, 

 its granddaughter, and its grandmother." Dr. W. K. Brooks 

 has given much attention to these forms in this country; 

 and, from his point of view, the alternation of generations 

 would be impossible. 



The Salpce give little signs of animation. " The only con- 

 spicuous vital action," says Professor Owen, " is the rhyth- 

 mical contraction and expansion of the mantle, in which the 

 elasticity of the outer tunic antagonizes the contraction of 

 the inner one. During expansion, the sea-water enters by the 

 posterior aperture, and is expelled, in contraction, by the an- 

 terior one ; its exit by the opposite end being prevented by a 

 valve. The re-action of the jet, which is commonly forced 

 out of a contracted tube, occasions a retrograde movement 

 of the animal." As they move along, on dark nights, they 

 present the appearance of fiery serpents or luminous ribbons 

 (Plate XVI., Fig. 1), winding their way over the sea, a 

 most striking spectacle. 



The light of Salpce observed by Giglioli was confined to 

 the so-called nucleus, but was not constant; indeed, some 

 were luminous and some were not. This was particularly 

 evident in the month of September, when the exploring-ship 

 "Magenta" passed through a bed of these little creatures, 



