GENERAL CHARACTERS OF MOLLTJSCA. 



47 



surface having frequently a glandular character. This general muscular 

 envelope is the only locomotive organ possessed by a large portion of the 

 Mollusca ; but its contractile properties are usually greatest at some particular 

 spot, where it is thickened into a sort of disk, by the alternate contraction 

 and extension of which the animal can slowly propel itself; this is well seen, 

 by causing a Snail or Slug to crawl over a piece of glass, so that the under 

 surface of the disk may be seen whilst it is in operation. The general cha- 

 racter of their locomotion, however, is well expressed by the term sluggish; 

 and there are scarcely any among the typical Mollusca, whose activity is such 

 as to demand for them any higher appellation. 



14. The general development of their organs of Nutrition, however, is 



Fig. 3. 



TV 



V 



Aplysia cut open, showing the viscera ; a, the upper part of oesophagus ; 6, penis ; e, c, salivary glands ; 

 d, superior or cephalic ganglion ; e, e, inferior or subcesophageal ganglia; /, termination of oesophagus ; 

 g, g, first stomach ; h, third stomach ; i, second stomach ; k, intestine ; I, I, I, liver ; m, posterior ganglion ; 

 n, aorta; o, hepatic artery; p, ventricle of heart; q, auricle; r, s, branchiee ; i,testis; u, lower part of 

 intestine ; f, ovary ; w, anus. 



