MOLLUSC A. 273 



snails, they are united in the same individual. All mol- 

 luscs, with very few exceptions, lay eggs, from which the 

 next generation is produced. 



Molluscs are divided in different ways by different au- 

 thorities. For our purposes we may recognize four divi- 

 sions or classes: Placophora, Gasteropoda, Acephala, and 

 Cephalopoda. 



CLASS I. PLACOPHORA. 



Here belong a few forms known as Chitons. They are 

 separated from all other molluscs by many points of inter- 

 nal structure, while externally they may always be recog- 

 nized by having a dorsal shell composed of eight transverse 

 plates, which overlap from in front backwards, like the 

 shingles on a roof. All are marine. 



CLASS II GASTEROPODA. 



The Gasteropods receive their name from the fact that 

 the foot usually forms a large sole or creeping disc extend- 

 ing along the ventral side of the body. There is a distinct 

 head, which usually bears sensory tentacles, and the eyes are 

 commonly placed at the bases or on the tips of one pair of 

 these structures. In some cases, as in most land-snails, 

 these tentacles can be pulled back into the body. 



In the majority of forms gills are developed in the mantle- 

 chamber. In a few there is a pair of these organs, but in 

 many one gill disappears, while in other species both true 

 gills entirely disappear, and are either replaced by secondary 

 gills developed on the back or in other regions; or the 

 mantle-chamber may be richly lined with blood-vessels and 

 thus be converted into an organ (lung) for breathing air. 

 This is the case in all of our common land-snails. 



