MOLLUSCA. 11 



portant generic and specific characters may be derived 

 from a careful examination of this organ. In the Bullidee 

 the rachis of the tongue is unarmed, and the trituration of 

 the food is effected by means of calcareous plates in a 

 muscular gizzard. 



The organs of touch in the Mollusca are in the form of 

 soft fleshy tentacles without joints. Usually in the Gas- 

 teropods there are two arising from the sides of the head, 

 sometimes free, as in the snails, sometimes united with the 

 eye-pedicles, as in the Murices, and sometimes partially 

 free, as in the Strombs. In the Tunicaries and Bivalves 

 there are numerous tentacles placed around the breathing 

 apertures and margins of the mantle. Additional pairs of 

 tentacles at the sides of the mouth, called buccal append- 

 ages, and also prolongations of the lips, termed labial 

 appendages, are often supplied, as in the AplysiidcB and 

 Onchidiidce, and occasionally the exposed surface of the 

 mantle exhibits fleshy tentacular extensions, which serve 

 to apprize the owners of danger. In the Pteropods and 

 Cephalopods long tentacular arms are extended round 

 the head for the more accurate perception of touch, and 

 in the Trochidte and Turbinid^ the sides of the foot are 

 furnished with tentacular filaments for the same purpose. 



The /oot is a muscular organ usually developed in pro- 

 portion to the locomotive powers enjoyed by the animals ; 

 it is wanting in the oyster, which is fixed, but in the snail 

 it is long and broad. By means of a muscular foot the 

 PholadideE and Solenida. burrow in the mud, the Limn^us 

 swims at the surface of the water, and with their grooved 

 and linear foot the Nudibranchs clasp the stems of floating 

 fuci. In the Atalants it is compressed laterally into a 

 swimming fin-like organ ; and in the Dolium it is some- 

 times greatly expanded by the introduction of water. 



