102 SHELL GALLEKY. 



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 rundnata-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-maxma 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 cosfata-filesii 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-bauds. 

 A chain of three individuals is exhibited. 



Salpta 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 geneiations, but 

 considers the solitary Salpa to be, not asexual, but a female which produces a chain 

 of Imales ; but it is impossible to enter into a difficult question of controversy here. 



