On the Teeth of the Ox, Sheep, and Pig. 
217 
while many of them are furnished witli a distinct dental apparatus 
for the like purpose. We may select as examples of the latter 
the RoTiFERA, those cre.iturcs, the peculiar movements of whose 
cilia in the collection of their food have ohtained for them the 
common name of the wheel animalcules. 
We may ascend the scale to a far greater height, and still find 
creatures without respiratory or circulatory organs. Take as an 
illustration the Entozoa — those parasitic creatures that live within 
the bodies of other animals. Select from among these the liver- 
fluke, the well-known entozoon which abounds in the gall-ducts 
of sheep affected with the rot, and here we have, as a type of the 
class, a creature whose systems of digestion and generation are 
perfect, without any traces of those of respiration and circulation. 
Self-support seems in the entozoa to be only secondary ; the 
great end of their existence appears to be the extension of their 
species. They live for this. Their own sustenance, and the per- 
fection of their ova, are alike derived from the elements taken 
in through their digestive organs. Hence the entire creature 
seems to be but a generative and a digestive system mingled 
together, and confined within a certain boundary by an external 
skin, which gives form and outline to its body. To come to 
animals still higher in the scale, did the occasion require, it 
could be shown how system after system of organs is superadded 
to those alluded to, until at last we arrive at the Vertebrata, 
where all are perfected. 
Teeth, or the organs for the bruising and comminution of the 
food, will be found to occupy different situations among vertebrate 
animals. One rule, however, obtains throughout, namely, that in 
these ci'eatures they are always placed anterior to the true digestive 
organ, the stomach. In the Invertebrata, on the contrary, we have 
many examples of teeth being situated within the stomach. The 
most familiar of these are the crab and the lobster. The " lady of 
the lobster" is the true dental apparatus of this creature. The 
food, on being swallowed, is carried at once to the stomach to be 
subjected simultaneously to the action of these gastric teeth and 
of the digestive fluid. Here it is bruised, ground, and finely 
divided, that it may at the same time be the more easily digested. 
In birds we have a provision, somewhat analogous to these 
gastric teeth of the Crustacea, in the development of a 
peculiar organ, termed the gizzard. In birds there are no 
teeth properly so considered, although the beaks of many serve 
a similar office. The carnivorous birds, which tear their food, 
and the granivorous, as the parrot, which bruise the seeds of 
plants, offer us some of the best illustrations of this fact. In the 
grain leeding birds the gizzard reaches its highest development. 
In some of these the bills are serrated, and in the mixed feeders 
