Because it has a bill like a duck and paws webbed similar to that bird, but legs and body like those of a quadruped, covered with thick, close hair, with a broad tail to steer by. It is believed to lay eggs; it bears a claw on the inside of its foot, with a tube therein, through which it emits a poisonous fluid into the wounds which the claw inflicts.*
* Cunningham's Sketches of New South Wales. - See also Quadrupeds, page 58 of the present volume.
Because it may answer the purposes of teeth to masticate their food; the gizzard being composed of thick and tough muscular substance, small in size, but more powerful in its action than the strongest jaw-bone. It consists of four distinct muscles - a large hemispherical pair at the sides, and a small pair at the two ends of the cavity. By their alternate action these muscles produce two effects; the one, a constant friction on the contents of the cavity, the other, a pressure upon them. These muscles are lined with a cuticle which is extremely callous, and which often becomes cartilaginous, and even horny. Reaumur and Spallanzani compelled geese and other birds to swallow needles, lancets, and other hard substances; in a few hours after which, the birds were killed and examined; - the needles and lancets were uniformly found broken off and blunted, without the slightest injury having been sustained by the stomach. Swallowing pebbles also aids the action of the gizzard upon the food, the stones in some measure serving the purpose of teeth. Mr. Hunter observed that the size of the pebbles is always in proportion to that of the gizzards. In the gizzard of a turkey he counted two hundred; in that of the goose, a thousand. - ( See page 84)
Because of the varieties of food on which they subsist. Thus, in birds of prey, as kites, hawks, owls, it only acts upon animal matter, and does not dissolve vegetables. In other birds, and in all animals feeding on grass, it dissolves vegetable matter, as grass, but will not touch flesh of any kind. This has been ascertained by making them swallow balls with meat in them, and several holes drilled through, to let the gastric juice reach the meat: no effect was produced upon it.
We may farther observe that there is a most curious and beautiful correspondence between this juice in the stomach of different animals, and the other parts of their bodies, connected with the important operations of eating and digesting their food. The use of the juice is plainly to convert what they eat into a fluid, from which, by various other processes, all their parts, blood, bones, muscles, etc. are afterwards formed. But the food is first of all to be obtained, and then prepared by bruising, for the action of the juice. Now, birds of prey have instruments, their claws and beak, for tearing and devouring their food, (which is animals of different kinds) but those instruments are useless for picking up and crushing seeds ; accordingly they have a gastric juice which dissolves the animals they eat; while birds which have only a beak fit for pecking, drinking, and eating seeds, have a juice that dissolves seeds, and not flesh. Nay more, it is found, that the seeds must be bruised before the juice will dissolve them : - this is found by making the experiment, in a vessel with the juice ; and accordingly, the birds have a gizzard, and animals which graze have flat teeth, which grind and bruise their food before the gastric juice is to act upon it.