47 



V. SALPIDiE. 



Very different from the simple, the social, or true com- 

 pound Ascidians, are the animals of the genus Salpa. They 

 are free, and habitually swim in the waters of the ocean. 

 In form they resemble short but rather wide tubes, often of 

 considerable size. The tube is composed of the test or tunic, 

 semi-cartilaginous or gelatinous in structure, seeming as if 

 carved in crystal, lined with the mantle, which in this 

 tribe is adherent throughout. Each end of the tube is 

 open, often terminating in a conical more or less produced 

 process. Within we find two narrow, oblique, unequal, 

 leaf-like branchiae, attached to the anterior and posterior 

 walls of the respiratory cavity. The branchial orifice is 

 protected by a valve. Near one extremity is the principal 

 visceral mass or nucleus, conspicuous owing to the brilliant 

 orange, brown, or reddish hues of the liver. Not unfre- 

 quently we find SalpeB making their way through the waters 

 deprived of their nuclei by birds or fishes, retaining their 

 vitality for a considerable time, and exercising their muscu- 

 lar powers when the organs of digestion, circulation, and 

 reproduction have been torn away. Peculiar crustaceans 

 make use of the cavity of the Salpa as a dwelling-place and 

 carriage ; and the number of minute phosphorescent animals 

 which lodge themselves within it is often so great, as to 

 mislead the observer into the belief that it is the mollusk 

 itself which gives out phosphorescent flashes. 



A great interest is attached to the natural history of the 

 Salpee, on account of their singular mode of reproduction, 

 discovered by the German naturalist Chamisso, and the 

 extraordinary generalisation to which that discovery in a 

 great measure gave rise. Previous observers had noticed 

 that these animals were sometimes found solitary, at others 



