MOLLUSCA — GASTROPODS 237 



Into the buccal cavity open the salivary glands which secrete 

 a simple mucus with little or no digestive action. Food passes 

 from the buccal cavity through the oesophagus to the stomach, 

 a simple enlargement of the digestive canal. The liver is a soft 

 brownish mass, more or less completely surrounding the stomach 

 and forming a large part of the coiled mass of viscera. It is the 

 chief organ of digestion, secreting a ferment which it pours into 

 the stomach. From the stomach the food enters the intestine 

 through whose walls the digested food is absorbed and from whose 

 posterior extremity the waste products are discharged into the 

 mantle cavity, whence they are borne outside the body by the 

 respiratory current of water as it leaves the gills. Busycon is 

 carnivorous. 



The main organ of the circulatory system is the dorsal heart 

 inclosed in the pericardium at the base of the gill ; it consists 

 of two chambers, one auricle and one ventricle. From the heart 

 the blood passes through arteries, open sinuses, and veins, thus 

 distributing food and oxygen all along its course. The blood of 

 Busycon is colorless. The chief organ of excretion is the kid- 

 ney, — essentially a sac lined with secretory eipthelium, which 

 discharges the waste products which it collects into the mantle 

 cavity, whence it is washed out of the shell. 



The nervous system consists of paired cerebral and pleural 

 ganglia situated over the oesophagus, paired pedal ganglia sit- 

 uated in the foot and paired visceral and abdominal ganglia. 

 Nerve connectives unite all these ganglia ; those uniting the 

 pleural and visceral ganglia are crossed, producing an 8-shaped 

 loop. 



Sensory cells are present over the whole surface of the mantle, 

 but in certain areas, mostly in the anterior regions of the body, 

 they become specialized into definite sense organs. The sense 

 of touch is especially resident in the lower surface of the foot, 

 which is covered by a network of fine nerves, and also in the 

 tactile tentacles. The tentacles function also in the sense of 

 smell. 



