The adult parasites of the intestine are scarcely visible to the naked eye; the females are 3 to 4 mm. long and more numerous than the males, which measure 1.4 to 1.6 mm. The eggs are very numerous, a single female containing at one time 1200, and probably producing ten times as many during her life. The embryos are hatched out within the uterus, and the larvae leave the body of the mother through the generative pore. The minute larvae bore through the intestinal walls of their host, and then, either burrowing in the tissues or swept along in the stream of blood or lymph, make their way all over the body, and come to rest most usually in the muscles, but occasionally in other parts. When the larva reaches its resting-place, it either pierces the sarcolemma and establishes itself within the substance of the muscle-fibre, or it comes to rest between and not in the fibres. Here its presence sets up the formation of a spindle-shaped cyst which usually contains but one larva, though any number up to seven have been found in one cyst. Within this the larva may remain dormant for years, the walls of the cyst gradually undergoing a fatty or calcareous degeneration. Almost any muscle may be affected; those most usually infested being the muscles of the diaphragm, of the shoulder-blade, and of the lumbar region; the larvae have also been found in the heart. The ends of the muscles near their points of attachment are always the most thoroughly infested. [Illustration: FIG. 72.—_Trichina spiralis_ Owen, encysted in muscle. _a_, Calcareous deposit. Highly magnified. (From Leuckart.)] The number of the encapsuled larvae in one host is enormous. Leuckart counted between 12,000 and 15,000 in a gramme of muscle, which would give a total of thirty to forty million parasites in one host; other estimates place the total even higher. {147}When trichinised meat is eaten, unless it has been thoroughly cooked, the cysts are dissolved and the larvae are set free. Within three or four days they become sexually mature and their ova begin to segment. The males after a time leave the body with the excreta and perish, whilst the larvae of the new brood make their way into the tissues of the host. Man usually acquires trichinosis by eating uncooked or improperly-cooked pork, and the disease is so widely spread and of such a serious nature that most civilised countries have adopted rigorous methods for the detection of trichinised meat. The pigs either acquire the disease by eating uncooked swine's flesh, which is frequently given them in the form of offal, or by devouring rats, which are very susceptible to the disease. IV. FAMILY FILARIIDAE. Mouth with two lips, or without lips. Six oral papillae often present, and sometimes a horny oral capsule. Four pre-anal pairs of papillae, and sometimes an unpaired one as well. Two unequal spicula or a single one. Genera: _Filaria_, _Ichthyonema_, _Hystrichis_, _Spiroptera_, _Dispharagus_, and others.