242 AN INTRODUCTION TO THE STUDY OF FOSSILS 



gill and nephridium on the shortened side, and typically their 

 atrophy and disappearance. In many gastropods this twisting 

 of the body involves likewise the nerve connectives, which thus 

 become crossed. 



In the pulmonates, — the air-breathing snails, which are 

 mainly terrestrial or of a fresh-water habitat, there are usually 

 no gills. The mantle cavity becomes completely closed except 

 for a small opening, and functions as a lung. 



The water for respiration passes in through a fold of the mantle 

 lying in the anterior notch of the shell, or through the canal, 

 where these are present, and out above, past the anus. If a 

 posterior canal is present it passes out through this. 



The heart consists usually of but two chambers, an auricle 

 and a ventricle ; rarely, as in Haliotis, are there two auricles 

 and a ventricle. The blood is usually colorless. It is red in 

 most species of Planorbis in which haemoglobin is diffused in 

 the plasma, and bluish in Polygyra albolahris owing to the 

 presence of haemocyanin, an albuminoid containing copper. 



The heart of adult gastropods beats between 30 and 100 

 times a minute, but during hibernation not more than twice a 

 minute. In young gastropods the number of beats may reach 



ISO- 

 Experiment indicates that touch and s^nell are the most 



acute senses of gastropods. It is almost exclusively by the sense 



of smell that food is found. This sense varies in acuteness in 



different gastropods. Its usual range does not exceed one inch, 



though some carnivorous marine forms {e.g. Nassa) can thus 



detect objects at a distance of more than six feet. It has been 



shown that Polygyra albolabris can locate food when covered 



from sight, even at a distance of 18 inches. Natica heros will 



go in a straight Hne to a dead crab. 



A large percentage of the gastropods are herbivorous. Of 



the marine forms the vegetable feeders have as a rule the outer 



lip of the shell aperture entire (exceptions are Natica, the Scala- 



riidae, etc.), while the carnivorous forms have this lip notched or 



