CARNIVOROUS MOLLUSCS 



99 



this is used to eat away the shells of the molluscs, &c., attacked 

 by such creatures. This is a good example of the numerous cases 

 where our knowledge of the habits of common animals is woe- 

 fully incomplete, and is just the kind of problem which might be 



attacked with fair chance 

 of success by local natu- 

 ral history clubs, given 

 patience and the power 

 of accurately observing 

 details. Most profes- 

 sional zoologists, unfor- 

 tunately, are too much 

 taken up with labora- 

 tory work to attend to 

 such matters, though 

 the study of animal ha- 

 bits is perhaps the most 

 interesting branch of 

 their subject. 



Fig. 382. Heteropods 



A, Atlanta: i, snout; 2, foot; 3, operculum; 4, shell. B, Pterotrachea: i, gills; 2, foot; 3, snout. 

 These creatures swim upside down as represented. 



HETEROPODS. Among predaceous Gastropods are the free- 

 swimming marine forms known as Heteropods (fig. 382), in which 

 the foot is more or less completely transformed into a fin, by 

 means of which the animal swims with its upper surface directed 

 downwards. Like animals of pelagic habit generally they are 

 of almost glass-like transparency, and may or may not be 

 provided with a shell. Atlanta, for example, has a spiral shell, 

 Carinaria a cap-shaped one, and Pterotrachea none at all. They 

 possess a long snout, in the end of which the rasping organ is 



