mites. It possesses two pairs of jointed limbs, and certain style-like organs pertaining to the mouth. There is thus the clearest evidence that linguatulina is a degraded animal. It is the degenerate descendant of a free-living and apparently four-legged—or it may be eight legged—ancestor; and its further history seems to afford a clew to the causes of its retrogression. For the four-legged larvæ of linguatulina escape, while still within the egg, from the nose of the dog or sheep host which has harbored their parents. Received along with food into the body of the hare or rabbit, the larval being liberates itself. From the rabbit's digestive system it bores its way through the tissues to the liver, thus reminding one strongly of the similar migrations of the embryo tapeworm. In the liver further changes ensue. Frequent moltings become the order of the day, and at length they assume a worm-like aspect and remain thus, still imperfect, until, by transference to the body of dog, wolf, or sheep, and by passage to the frontal sinuses, they acquire perfection of their life-functions. If the history of these beings teaches us anything concerning their past, it points to a free and active state as their original condition, and to the probable acquirement, first, of a lodgment in the digestive system of one animal as a relatively simple parasite; and, secondly, of a further modification of habit, transferring at once its perfection and completed degradation to the forehead cavities of a second host.
But the conditions which make for the degeneracy of an animal are, as we have seen in the case of the barnacles, not always associated with a parasitic habit. Mere fixation, as we have observed, secures the disappearance of useless organs, such as organs of motion and sense-organs, which, being possessed by the young form, clearly indicate that the ancestry of the animals in question has at any rate been capable of leading to better things than the descendants represent in their existent persons. The sea-squirts, or ascidians, besides serving as a text for the derivation of vertebrates, and for abnormal ways in the animal chemistry which imitates the plant's work, have been selected as fruitful objects of discussion by those biologists who find in the idea of degeneration an explanation of knotty points in natural history. For the same voice that proclaims the fact that a sea-squirt—which is a mere rooted bag with a double neck (Fig. 16)—begins life as a free-swimming, tadpole-like larva (Fig. IT, 5), tells us in the same breath that there must have been retrogression and degeneration from an active condition to produce the sac-like adult state. The assertion that the youthful sea-squirt, moreover, possesses first a rod-like body—called the notochord (Fig. 17, n)—only found besides in the young of vertebrate animals, is also to be taken as implying the superiority of ascidian infancy to sea-squirt maturity. And, when it is added that the elderly squirt wants the sense-organs and nervous cord which the larva possesses, it may well be argued that sheer degeneracy of habit and structure can alone account for the sweeping transformations
