The burrows of muskrats are very extensive, and consequently injurious to dykes and dams, meadow banks, etc. The entrance is always under water, and thence sloping upwards above the level of the water, so that the muskrat has to dive in going in and out. These creatures are excellent divers and swimmers, and, being nocturnal, are rarely seen unless by those who watch for them at night. Sometimes we alarm one near the mouth of the den, and he darts away across the water, near the bottom, marking his course by a turbid streak in the stream: occasionally we are made aware of the passage of one to some distance down the current, in the same way; but in both cases the action is so rapidly performed, that we should scarcely imagine what was the cause, if not previously informed. Except by burrowing into and spoiling the banks, they are not productive of much evil, their food consisting principally of the roots of aquatic plants, in addition to the shell-fish. The musky odour which gives rise to their common name is caused by glandular organs placed near the tail, filled with a viscid and powerfully musky fluid, whose uses we know but little of, though it is thought to be intended as a guide by which these creatures may discover each other. This inference is strengthened by finding some such contrivance in different races of animals, in various modifications. A great number carry it in pouches similar to those just mentioned. Some, as the musk animal, have the pouch under the belly; the shrew has the glands on the side; the camel on the back of the neck; the crocodile under the throat, etc. At least no other use has ever been assigned for this apparatus, and in all creatures possessing it the arrangement seems to be adapted peculiarly to the habits of the animals. The crocodile, for instance, generally approaches the shore in such a manner as to apply the neck and throat to the soil, while the hinder part of the body is under water. The glands under the throat leave the traces of his presence, therefore, with ease, as they come into contact with the shore. The glandular apparatus on the back of the neck of the male camel, seems to have reference to the general elevation of the olfactory organs of the female; and the dorsal gland of the peccary, no doubt, has some similar relation to the peculiarities of the race.