178 THE STORY OF LIFE IN THE SEAS. 



symmetrical, and we might jump to the con- 

 clusion that this was due to a corresponding 

 symmetry of the internal organs. 



Such a conclusion would, however, be an 

 erroneous one, for the results of the careful 

 anatomical study of these Molluscs proves most 

 definitely, that although a false symmetry of the 

 organs is often shown, there is a general sup- 

 pression of the organs of one side of the body. 

 A study of the development of these animals 

 also shows, that in the early stages of their life, 

 the shell is not symmetrical like that of the 

 adult, but twisted into a spiral like that of a 

 Whelk or a Periwinkle. These facts indicate 

 that the surface-swimming Molluscs have passed 

 through a stage in their evolution when their 

 bodies were twisted up into a spiral shell, and 

 that the false symmetry, which they exhibit 

 in the adult condition, is an adaptation to their 

 peculiar habits of life. The study of the group 

 of Gastropods alone then does not give us any 

 evidence in favour of the view that the surface- 

 swimming Fauna is primitive ; in fact, it proves 

 almost conclusively that its share in the Fauna 

 has been contributed from the shallow- water 

 districts. 



The group of the Tunicates affords similar 

 evidence. There is no good reason for believing 

 that the Salps and Pyrosoma which drift about 

 in the surface waters are more primitive than 

 the fixed Sea-squirts of the rocks and Sea-weeds, 

 in fact, the view is gaining ground, as our 

 knowledge increases, that all the free Tunicates 

 must have passed through a sessile ancestry. 



