566 MOLLUSCA 



the forward end of the intestine is usually a tubular pocket, which opens 

 into the stomach in some forms, containing a crystalline rod of unknown 

 function. The heart consists of a muscular ventricle and a pair of 

 auricles which receive the blood which has just passed through the gills. 

 The kidneys are paired, each opening into the pericardium and also 

 into the mantle cavity. Additional excretory glands are the organs 

 of Keber, which are a pair of glandular organs arising either from 

 the walls of the auricle or the pericardium; they are wanting in 

 the Protobranchiata. Three pairs of widely separated nerve ganglia 

 characterize most bivalves, the cerebral ganglia, just above the mouth, 

 the pedal ganglia in the foot, and the visceral ganglia at the hinder end 

 of the visceral mass; the cerebral ganglia are joined with each of the 

 other pair by a pair of connectives. 



In correlation with their sluggish habits, the special sense organs 

 of the bivalves are poorly developed. A pair of osphradia, which are 

 supposed to be olfactory organs, are present at the base of the gills near 

 the posterior adductor muscle. Tactile cells are often present, especially 

 upon the siphons and the edge of the mantle. Eyes are usually absent 

 but are present in a number of forms. Pecten possesses highly developed 

 eyes, which are on projections at the edge of the mantle. Area has com- 

 pound eyes, similar to those of arthropods. A pair of lithocysts occurs 

 in the foot. 



The gonads are branched organs which open into the mantle cavity 

 at the base of the gills. In the Protobranchiata and certain other primi- 

 tive forms the gonads open into the kidneys, but in most bivalves dis- 

 tinct genital ducts are present. The sexes are usually separate, but 

 hermaphroditism is riot uncommon. Ova and sperm are extruded into 

 the water, where fertilization takes place. Special egg capsules are 

 not formed ; in the Unionidae, however, the eggs are taken into the water 

 tubes usually of the outer and in the Cyrenidae into those of the inner 

 gills, where they are protected until they hatch. 



Distribution and Habits. Pelecypods are all aquatic, and mostly 

 marine animals which live in great part in shallow water along the mar- 

 gins of the continents. They vary in size between the gigantic Tridacna 

 gigas of the Pacific Ocean, the shells of which may be a meter and more 

 long and weigh 250 kilograms, and the minute fresh water Pisidium or 

 Spharium, which may measure only a few millimeters in length. Most 

 of them creep slowly about on the wedge-shaped foot, some can spring 

 (Cardium), while some move by squirting water from the siphons 

 (Ensis). Pecten swims rapidly by clapping the shells together. The 

 food consists of minute animals and plants, which are drawn into the 

 mouth by the cilia of the oral palps. Very many bury themselves in 



