
MOLLUSCS, VERMES AND HYDROZOA 

are the labial palps, two pairs of 
triangular flaps and between the 
gills and the hinge is the peri- 
cardium containing a_ three- 
chambered heart, and below this 
the organ which serves as the 
kidneys. The liver is a com- 
pound digestive gland over the 
posterior portion of the gills and 
under this is the stomach, con- 
nected with the mouth by the 
oesophagus. The digestive 
canal consists of an irregular 




——— 


coiled tube which passes through 
the pericardium to the posterior 
end of the body. The sim- 
ple nervous system consists of 
ganglia at the mouth, foot and 
FIG. 141. Diagrams of a Freshwater Mussel. adductor muscles. 
Beak. 
Vey ineaee Respiration consist in tak- 
.H. Hinge. 
B 
re 
H ; 
pas ae ing the water through the inhal 
S. Shell. ing siphon, passing it over and 
F 
B 
A Foot. 2 
r. Branchial openings. between the gills and out of the 
ie Re body by the exhaling siphon. 
These water currents reach the 
other organs, aerate the blood, convey food to the stomach and carry off 
the waste materials of the system. 
The Arrows indicate direction of the animal and the currents. 
Reproduction varies with the genera, but with most of the freshwater 
species is the following. The eggs are developed in the ovaries, pass to 
the gills, where they lodge and hatch. The minute valves are connected 
by an elastic hinge, the foot only partly developed, as from it extends a 
number of slender filaments, the byssus. When ejected by the parent, 
they swim by opening and closing the valves until they come into contact 
with a fish to which they attach themselves and there undergo a metamor- 
phosis. The byssus disappears, the foot is fully developed, the interior 
organs undergo material changes and the valves assume the shape of the 
adult. Then the mussel quits its host and falling to the bottom takes up 
the habits of its kind. 
The hereafter described molluscs are those of fair size which will be 
of interest to the aquariist, the breeder of aquarium fishes, and the student 
of natural history in the home aquarium. The nomenclature is that now 
219 
