GENERAL INTRODUCTORY. 13 
(Plate IIl., Fig. 13), © : 0: © (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 Chetopoda, 
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. 
