632 ZOOLOGY. 



or the free-moving young of higher animals, which are 

 carried into the mouth in currents of water or swallowed 

 bodily with sand or mud. 



Among the worms true organs of mastication for the first 

 time appear in the Rotatoria (Fig. 122), where the food, such 

 as infusoria, etc., is crushed and is partly comminuted by 

 the well-marked horny or chitinous pieces attached to the 

 mastax. In most other low worms the mouth is unarmed. 

 In the leeches there are three, usually in the annelids two, 

 denticulated or serrate, chitinous flattened bodies situated 

 in the extensible pharynx of these worms, and suited for 

 seizing and crushing their prey. 



In the higher mollusks, such as the snails (Ceplialoplwra) 

 and cuttles, besides broad thin pharyngeal teeth, compara- 

 ble with those mentioned as existing in the worms, is the lin- 

 gual ribbon already described (p. 276, Fig. 215.), and admira- 

 bly adapted for sawing or slicing sea- weeds and cutting 

 and boring into hard shells, acting somewhat like a lapi- 

 dary's wheel ; this organ, however, is limited in its action, 

 and in the cuttles the jaws, which are like a parrot's beak, 

 do the work of tearing and biting the animals serving as 

 food, which are seized and held in place by the suckered 

 arms. 



In the crustaceans and insects we have an approach to 

 true jaws, but here they work laterally, not up and down or 

 vertically, as in the vertebrate jaws ; the mandibles of these 

 animals are modified feet, and the teeth on their edges are 

 simply irregularities or sharp processes adapting the mandi- 

 bles for tearing and comminuting the food. It is generally 

 stated that the numerous teeth lining the crop of Crustacea 

 and insects (Fig. 282) serve to further comminute the food 

 after being partially crushed by the mandibles, but it is now 

 supposed that these numerous points also act collectively as 

 a strainer to keep the larger particles of food from passing 

 into the chyle-stomach until finely crushed. 



The king-crab burrows in the mud for worms (Nereids, 

 etc. ) ; these may be found almost entire in the intestine, 

 having only been torn here and there and partly crushed by 

 the spines of the base of the foot-jaws, which thus serve the 



