280 CLASSIFICATION OF ANIMALS 



surface. The mantle lines the entire inner surface of the shell. Some- 

 times the edges of the mantle are partly united. There are always two 

 points, however, where they are not united. One is at the posterior border 

 and one is ventral anterior, opposite the foot. The posterior opening is 

 frequently double and the edges of the mantle are then often extended 

 so as to form a pair of tubes or siphons, or one double siphon. Through 

 the ventral siphon the water enters the mantle cavity and escapes by the 

 dorsal siphon. 



606. The flattened body is suspended from the dorsal border of the 

 mantle lobes. At its anterior and posterior ends are two strong muscles 

 which connect the two valves of the shell. These are the adductors which 

 close the shell. The gills are suspended from the dorsal border of the body 

 in the mantle cavity. Each gill consists of two series of parallel vertical 

 bars or filaments which are connected by short longitudinal and trans- 

 verse bars. The gill is therefore a sort of double grating. On either 

 side of the mouth are two triangular lappets, the labial palps. The 

 surface of the gills and palps is covered with cilia which induce the currents 

 in the water for respiration and feeding. The minute particles of food 

 are carried toward the mouth along the edge of the mantle and thence 

 between the palps. The mouth is a simple opening into the short 

 oesophagus. The stomach is rather large and receives several ducts 

 from the large digestive gland. From the stomach the intestine makes 

 a number of loops and passes out of the visceral mass dorsally and pos- 

 teriorly to a point above and behind the posterior adductor muscle into 

 the mantle cavity. 



607. The heart lies on the dorsal side of the visceral mass and consists 

 of a ventricle and two lateral auricles. It is enclosed in a pericardial 

 chamber which represents the body cavity. A nephridial funnel opens 

 into the pericardial chamber on each side and this connects with the 

 kidneys, or organs of Bojanus. The kidneys open into the mantle cavity 

 on the side of the visceral mass. The sexes are usually separate. The 

 gonads are large paired organs embedded in the visceral mass and opening 

 with or near the kidney openings. 



608. The nervous system consists of a cerebral ganglion which lies 

 above the oesophagus, a visceral ganglion below the posterior adductor 

 muscle, and a pedal ganglion embedded in the foot. These ganglia are 

 connected by pairs of nerves. Sometimes there is also a separate pleural 

 ganglion. The special sense organs are not well developed. There is a 

 double statocyst near the pedal ganglion. Eyes are seldom found on 

 the body but in a number of forms they occur on the edge of the mantle 



