42 TERRESTRIAL AIR-BREATHING MOLLUSKS. 



value for the purpose of classification, and on the bibliography of the 

 Bubject. In rewriting this article for the present volume, I have con- 

 sidered it best to redraw all the figures for the sake of greater accuracy, 

 as well as artistic merit. 



General Remarks. 



As many of my readers are quite unfamiliar with the subject, espe- 

 cially most of those who have so largely contributed specimens for ex- 

 amination, I will describe in detail the position of the organs and th* 3 

 method adopted for their study. 



On holding up against the light an individual of Mesodon thyroides 

 in one hand, and offering to him with the other some food (a piece of 

 lettuce or carrot is always acceptable), one can readily see with the 

 naked eye the two organs here treated of. Above the external opening 

 of the mouth, through the tranparent tissue of the head, is seen a small, 

 arched, reddish, free instrument, which appears to rise and fall as if 

 used in cutting off morsels of food. This is the jaiv. 



On the floor of the mouth is the lingual membrane, occupying about 

 the position of the human tongue. Its color is too nearly the same as 

 that of the head to afford any strong contrast, but, with close atten- 

 tion, it will be detected by its glistening silvery appearance, as it works 

 backward and forward. Its use seems to be to rasp the food and also to 

 force it back into the oesophagus. 



More detailed description, fully illustrated by figures, of the position 

 of these two organs, will be found in the chapters on Special Anatomy 

 in Volume I. (See also below, Chapter IV.) 



Method op Extraction. 



On opening the head of Mesodon thyroides from above, one readily 

 notices at the extreme anterior part, close against the outer integument, 

 a prominent oval body. 1 This is called the buccal mass. It is easily 

 cut away from the animal, and will be found to contain both jaw and 

 lingual membrane. These can be removed by fine scissors or knives 

 from the buccal mass in the larger species, but in the smaller species 

 the method usually employed is putting the whole buccal mass in a 



1 I must earnestly beg my readers to be deterred from this examination by no imagi- 

 nary difficulties. It is the simplest and easiest process. Indeed, the same may be said of 

 examination of the complete anatomy. All that is required is to carry it on under water. 

 The various organs are then readily separated. 



