162 ANIMAL ACTIVITIES. 
whose walls the interchange of gases common in all 
breathing takes place. 
Reproduction. The eggs of the fresh-water mussel 
pass from the ovaries to the cavity of the outer gill, 
where they hatch. The number of young is enor- 
mous. They remain for a time within these gills and 
then pass out to grow or to be destroyed, as the case 
may be. 
Discovery. The senses of the mussel are not acute 
and there are no special organs of sense. Touch is 
more acute in the foot and at the margins of the 
mantles, as might be expected, these being almost the 
only exposed parts. There is a so-called ear-sac in 
the foot, but it is not probable that the mussel can hear. 
The nervous system consists of three pairs of ganglia 
connected by nerves. One pair form a sort of brain 
(the supraesophageal ganglia), one pair lie directly 
under the posterior adductor muscle (the wisceral 
gangla), and the third pair are imbedded in the tissue 
where the foot joins the body (pedal ganglia). 
Motion. The opening and closing of the shell at 
will is brought about by the action of an elastic hinge- 
ligament anda pair of adductor muscles. When the 
muscles contract the shell is closed; when they relax 
the hinge-ligament acts like a door-spring and forces 
the valves apart. 
The foot is protracted partly by muscular activity 
and partly by increasing its size by pumping its vascular 
vessels full of blood. When left lying on its side in 
the aquarium the animal rises to an erect position by 
sinking its foot into the sand. It plows its way through 
the mud or sand by muscular contractions of this foot. 
The Garden-slug. Obtain some garden-slugs, or, 
better still, some of the larger slugs found in damp 
cellars or green-houses. Keep them in a box with 
plenty of moisture and feed them with cabbage-leaves 
and bread. Watch the mode of locomotion. Allow 
the animal to crawl on a piece of glass and watch the 
