102 SHELL GALLEEY. 
breaks off in the form of a chain and swims away, other chains being 
detached in succession. A chain is formed of individuals arranged 
in two rows, the individuals in each row being alternate (not opposite). 
Each individual of a chain differs from the solitary individual in 
shape, arrangement of muscle bands, etc., but especially in having re- 
productive organs. The chain Salpid is hermaphrodite ; the embryo 
develops into a solitary asexual Salpa which produces the chains by 
budding. The wonderful life history of Salpa was discovered by the 
poet Chamisso during a voyage round the world in 1819. He 
observes : " A Salpa mother is not like its daughter or its own mother, 
but resembles its sister, its granddaughter, and its grandmother." 
Here we have an example of " alternation of generations," a sexual 
generation (chain form) giving rise to an asexual generation (solitary 
form), which latter produces the sexual generation.* 
Most of the species of Salpa have double names owing to the 
chain and solitary forms having been regarded as distinct species 
before they were known to be phases in the life history of one and 
the same species. Salpa runcinata-fusiformis, solitary form (Fig. 
22 B), is barrel-shaped, truncated at each end, with terminal orifices, 
and with nine muscle-bands on the dorsal surface, some of which 
converge towards each other. An individual of a chain (Fig. 22 A) 
is fusiform, with six muscle-bands, and with the orifices not terminal, 
but at each end of the dorsal surface. 
The solitary form of S. africana-maxima is barrel-shaped, 
with truncated ends and terminal orifices, and with nine broad 
parallel muscle-bands. The chain form is conical at one end, with 
six bands, and with orifices on the dorsal surface. The exhibited 
specimen of the chain form, which is in an early stage of growth, 
contains 202 individuals. The solitary and chain individuals of 
Salpa costata-tilesii attain a length of six to eight inches. The 
solitary form has eighteen muscle-bands and two large spines at the 
posterior end. The individual of the chain has five muscle-bands. 
A chain of three individuals is exhibited. 
Salpa pinnata produces a circular chain ; the exhibited specimen 
of the solitary form shows a small chain about to be detached ; a 
circular chain of six individuals is also exhibited. Species of Salpa 
abound in all seas, but specimens from Naples have alone been 
exhibited on account of their good preservation. 
* It should be mentioned that one high authority, Prof. W. K. Brooks, does not 
regard the life history of Salpa as an example of alternation of generations, but 
considers the solitary Salpa to be, not asexual, but a female which produces a chain 
of males ; but it is impossible to enter into a difficult question of controversy here. 
