PHYLUM MOLLUSCA 



291 



Mantle 

 margin 



Foot 



Shell 



Anus 



Muscle 



Figure 180. Top, photograph of a segmented mollusk, Neopilina galatheae, a newly found 

 primitive mollusk which represents a class that probably existed about 450,000,000 years ago; 

 it is truly a "living fossil." The largest specimen collected measured about 37 mm. in length, 

 33 mm. in width, and 14 mm. in height. (Photo courtesy of Henning Lemche, Zoological 

 Museum, University of Copenhagen, who published the first report on Neopilina.) Bottom, 

 drawing from a sketch by Henning Lemche of the most primitive mollusk. 



vents evaporation. Locomotion in snails is 

 very interesting. A slime gland at the for- 

 ward end of the foot deposits a film of 

 mucus on which the snail moves by means 

 of wavelike contractions of the foot muscles. 

 It thus lays its own pavement ahead of 



itself, which is always the same, whether 

 the path is rough or smooth, uphill or down- 

 hill. Progress is therefore always about the 

 same; it may be two inches per minute, 10 

 feet per hour, and 240 feet per day, provided 

 the animal keeps going continuously. 



