138 



GASTEROPODOUS MOLLUSCA. 



Slugs, properly so called, belong to this class of Gastero- 

 poda. There is here an obvious advance in organization 

 above the bivalves, even in the external characters of the 

 an iraal. The body is more symmetrical j there is a greater 

 distinction of parts, an obvious head, an evident tail ; 

 and, save that the body is without legs, we have often 

 a considerable outward resemblance to some vertebrate 

 animal, in the form of the body and in the expression 

 of the countenance. For here is a well-formed face, sur- 

 mounted by two or four tentacula, commonly called 

 horns, which either, as in the Snail, carry each an eye 

 at its summit, or, as is the case in most of the marine 

 kinds, have an eye on a prominence at the base. When 

 we look at the internal structure of these animals, the 

 advance in organiza- 

 tion is still more 

 clearly shown. The 

 organs of digestion 

 and of circulation are 

 formed on a very per- 

 fect type, and the 

 nervous system is not 

 only amply develop- 

 ed, but there is a 

 well-defined nervous 

 centre, or brain. The 

 mouth, in many spe- 

 cies, is furnished with 

 sharp and strong 

 teeth; in others, the process of digestion is facilitated 

 by strong, bony gizzards, which bruise the food in its 



