STUDIES IN ANIMAL LIFE. 77 



one common plan of structure prevails, and the 

 presence of a backbone is the index by which to 

 recognize this plan. 



The Triton has been wriggling grotesquely in 

 our grasp while we have made him our text, and, 

 now he is restored to his vase, plunges to the bot- 

 tom with great satisfaction at his escape. This wa- 

 ter-snail, crawling slowly up the side' of the vase, 

 and cleaning it of the green growth of microscopic 

 plants, which he devours, shall be our representa- 

 tive of the second great division — the MoLLUSCA. 

 I can not suggest any obvious character so distinct- 

 ive as a backbone by which the word moUusk may 

 at once call up an idea of the type which prevails 

 in the group. It won't do to say " shellfish," be- 

 cause many mollusks have no shells, and many ani- 

 mals which have shells are not mollusks. The 

 name was originally bestowed on account of the 

 softness of the animals. But they are not softer 

 than worms, and much less so than jellyfish. You 

 may know that snails and slugs, oysters and cuttle- 

 fish, are mollusks ; but if you want some one char- 

 acter by which the type may be remembered, you 

 must fix on the imperfect ' symmetry of the mol- 

 lusk's organs. I say imperfect symmetry, because it 

 is an error, though a common one, to speak of the 

 mollusk's body not being bilateral — ^that is to say, of 

 its not being composed of two symmetrical halves. 

 A vertebrate animal may be divided lengthwise, 

 and each half will closely resemble the other ; the 



