GENERAL INTRODUCTORY 13 



(Plate III., Fig. 13), 00 : o : 00 (Plate III., 

 Fig. 14), etc. 



The shapes of the teeth are so constant in the 

 several molluscs that they assist not only in the 

 determination of families and genera, but, with few 

 exceptions, in that of species also. 



The forms of the teeth are, further, some index to 

 the diet of the animals, the purely herbivorous having 

 short, broad-pointed teeth, the carnivorous sharp- 

 pointed teeth, which in those feeding on living 

 animals are barbed to retain their prey, while in the 

 Cones the teeth are not only barbed, but perforated 

 and connected with poison glands. 



In habit the Mollusca are far from active, only 

 some of the Cuttlefishes being capable of spasmodic 

 rapid motion, so much so that the term " sluggish," 

 borrowed from them, best describes them. 



The exact relationship of the Mollusca is difficult 

 to determine, though they belong to the same 

 division of the animal kingdom as the Chsetopoda, 

 Gephyrea, Rotifera, Bryozoa, and Brachiopoda ; but 

 on the whole it is considered that the most archaic 

 molluscs, the Chitons, come nearest to the free 

 Polychetes, such as the gaudy Sea-Mouse (Aphrodite), 

 so common on our coasts. 



