INTRODUCTION. 



XXI 



the surface moist and acutely sensitive to touch and smell. 

 There is no internal bony skeleton assisting in the movements 

 of the animal ; all action is produced by a beautiful contractile 

 and protrusive muscular system, one part as it were pulling 

 against and acting on the other. 



In the Mollusca the primitive plan of structure was a body 

 symmetrical to a median vertical plane, the alimentary canal 

 running from the mouth at one end to the anal aperture at the 

 other. This body had a lower ventral or neural face, an upper 

 dorsal or haemal face, and a right and left side. Only in a few of 

 the Mollusca has this symmetry of the body been retained undis- 

 turbed, as in the very ancient classes Amphineura, Scaphopoda, 

 and Pelecypoda, and the haemal face is not produced into a 

 visceral sac; but in the great majority of mollusks such a 

 visceral sac is found. In the Gasteropoda the ventral face 

 gives rise to a muscular foot. The structure of this foot is 

 important; it shows considerable modification, which renders it of 

 much value in classification, according as it has been modified for 

 digging, swimming, or creeping as in the land forms. 



The dorsal face is generally produced at its margin into a free 

 fold, called the mantle or pallium, derived from the primitive 



Explanation of Pig. i. 



A. Diagrammatic view of right side, to show the position of the generative 



organs within the body-cavity and that of the rudimentary shell. About 

 nat. size. 



B. Similar view, to show the position of the nervous ganglia and a few of the 



nerves radiating from it ; also to show the general plan of the muscular 

 system, a tew of the main muscles at the position of their attachment 

 points, the buccal mass and alimentary system with the salivary glands, 

 and the position of the branchial chamber and pericardium. About riat. 

 size. 



O. The cerebral and pedal ganglia with some of the various nerves, together 

 with the main muscles of the buccal mass and eye-tentacles. Viewed 

 from above, laid out. Drawn under microscope. X 4'5. 



D. The same, drawn by eye, showing the nerves to middle of the foot (6') and 



position of the buccal mass and oesophagus. Viewed from above. 



E. The left eye-tentacle and oral tentacle, severed from the part in C. Viewed 



from the inside, showing the small retractor basal muscles (m). 



E E. Eight eye-tentacle. 

 L E. Left eye-tentacle. 

 OT. Oral tentacles. 

 S G. Salivary glands. 

 sd. Salivary ducts. 

 rwE. Eetractor muscles of eye- 

 tentacles. 



rm B. Do. of buccal mass. 

 r. Eenal organs. 

 i. Intestine covered by salivary 



gland. 

 j. Jaw. 

 h. Heart. 

 ant.ar. Anterior artery. 



*/. Side of foot. 



B. Buccal mass. 



ce. (Esophagus. 

 m. Muscles. 



1. Cerebral ganglia. 



2. Pedal ganglia. 



3. Nerve to base of left eye- 



tentacle. 



4. Nerves to lips. 



5. Do. to posterior right side of 



foot. 



6, 6 r . Do. to basal edge of foot. 

 7. The position of the otocysts. 



