290 INVERTEBRATE MORPHOLOGY. 



and presumably receive the blood which they contain more 

 or less directly from the aorta and distribute it to the lacuuar 

 spaces of the foot. 



The mouth lies on the ventral surface, in front of the 

 anterior end of the foot, and leads into a pharynx provided 

 with a well-developed radula characterized by a somewhat 

 complex arrangement of the teeth. Into the oesophagus 

 a pair of glands opens in Chiton whose secretion contains an 

 amylolytic ferment, and in addition a pair of small glands 

 open into the mouth-cavity. The oesophagus communicates 

 with a sac-like stomach, into which open the ducts of the 

 paired digestive gland, and the intestine, being considerably 

 longer than the body, is thrown into numerous coils, and 

 terminates by a short rectum which opens at the posterior 

 extremity of the body. 



The nervous system is characterized by the diffuse 

 arrangement of the nerve-cells, no well-defined ganglia oc- 

 curring on the principal nerve-cords. These consist of a 

 strong circumcesophageal ring (Fig. 130), the upper part of 

 which gives off numerous nerves and evidently corresponds 

 to the cerebral ganglia of other Mollusca, while the lower 

 part, corresponding to the pedal ganglia, gives rise to two 

 nerve-cords (pc), the pedal nerves, which pursue a parallel 

 course throughout the foot, giving off a number of nerves 

 laterally and being connected by a number of somewhat irreg- 

 ularly arranged transverse commissures, which almost suggest 

 a metameric arrangement. From the sides of the circum- 

 oesophageal ring two other strong nerves, the pleuro-visceral 

 nerve-cords, arise and pass backwards along the sides of the 

 body, uniting with each other posteriorly above the terminal 

 portion of the digestive tract. These cords (pi], like the cir- 

 cumoesophageal ring, present no distinct gaugliouic enlarge- 

 ments, but contain the elements of the pleural, visceral, and 

 parietal ganglia, sending off numerous nerves to the branchiae, 

 the mantle, and probably also to the heart and nephridia. 



In addition to these principal nerve-cords others of smaller' 

 size also arise from the circumo3sophageal ring. One pair of 

 these pass to a pair of ganglia, the buccal ganglia, lying 

 beneath the buccal mass and send nerves to the oesophagus 



