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 

 adductor muscles. 



Respiration consist in tak- 

 ing the water through the inhal- 

 ing siphon, passing it over and 

 between the gills and out of the 

 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. 



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 



FIG. 141. Diagrams of a Freshwater Mussel. 

 B. Beak. 

 L. Ligament. 

 H.H. Hinge. 

 A. Adductor scars. 

 S. Shell. 



F. Foot. 



Br. Branchial openings. 

 A. Anal opening. 

 The Arrows indicate direction of the animal and the currents. 



ZI9 



