158 



THE HARMONIES OF NATURE. 



body, under the deep fold of the mantle ; in others they form 

 a fringe round the margin of the body, between the edge of the 

 mantle and the foot, while in all the spiral univalve molluscs 

 whose shells enrich our cabinets, they are shaped like the teeth 

 of a comb, and placed in a large hollow chamber in the animal, 

 communicating with the surface by a wide slit, through which 

 the water finds free access to the gills. 



The digestive apparatus of the gasteropoda offers many 

 points of considerable interest. The mouth, in many species, 



is furnished with sharp 

 and strong teeth, as for 

 instance in the tritonia, 

 where it is armed with 

 two cutting blades (6 6), 

 resembling in every re- 

 spect a pair of strong 

 curved shears, which, set in 

 motion by powerful mus- 

 cular fibres, are so effi- 

 cacious that few animal structures can resist their edge. The 

 lips which are placed in front of these teeth are strong and 

 very flexible, forming a muscular tube, by means of which the 

 food is seized and brought within the power of its formidable 

 jaws, and then the divided morsels, being seized by the horny 

 teeth which invest the tongue (d), are conveyed into the 

 oesophagus. 



In others the process of digestion is facilitated by strong 

 bony gizzards, which bruise the food in its passage into the 

 stomach ; and in others the tongue is armed with spinous pro- 

 cesses obviously intended 

 to assist in the preparation 

 of the food. That of the 

 limpet, which is three 

 times the length of the 

 body, is supported by two 

 cartilaginous pieces (6 6) 



A, limpet's tongue. B, portion magnified, placed On each side of its 



root. From these arise strong and short muscles, which 

 wield the organ. The surface of this singular tongue, a mag- 

 nified view of which is given at B, is armed with minute though 



