AQUEDUCTS THEIR USES. 



169 



the same phenomena as exhibited in the bivalved mollusca, 

 in whom the foot can frequently be made to surpass the 

 shell ; nor is this capacity of temporary and varying increase 

 in size limited to conch- F} 2g 



iferous mollusks, but is 

 possessed also by the 

 naked species. The slugs 

 are more tumid and trans- 

 parent in moist weather 

 than in a drought; and 

 the sea-slugs, whether 

 Gasteropod or Pteropod, 

 retain only their amplest 

 dimensions when im- 

 mersed in water; for 

 when removed thence, 

 they shrink and shrivel 

 down to perhaps a half of 

 their former bulk. This 

 increase in any species is 

 always accompanied with 

 a greater transparency of 

 the body; and only in 

 this state can the tenta- 

 cula and the other exterior organs of the animal be properly 

 displayed, or progression accomplished. 



The mechanism by which the animals effect these relative 

 changes in their bulk is curious and simple. Were you 

 to fill a cup exactly with a dried sponge, you could make it 

 rise above the rim, and expand over it on all sides by 

 pouring some water into the cup. The water does not 

 float the sponge, but is imbibed into its interior by perco- 

 lating through its pores and canals. It is somewhat similar 

 with some of the mollusca. These are framed with a system 

 of canals and cavities, excavated principally in the foot and 

 penetrating it thoroughly, which has a communication with 

 the exterior surface on the one hand, and leads into the 

 visceral cavity on the other. The circumfluent water enter- 

 ing by the external orifices flows into and through these 

 canals, fills them and the cavities with which they commu- 

 nicate, and, as a necessary sequence, the organs are dis- 

 tended, enlarged, and rendered firm and more capable of 

 muscular exertions. When the animal wills to come out 

 from its shell, or to remove to another site, the water is 

 made to flow inwards through the aqueducts ; and when it 

 wishes to reenter or conceal itself, the muscular compres- 



