12. The sexual form lives for a short time in the intestine of a Vertebrate, and produces larvae which bore through the intestinal wall and become encapsuled in the tissues: example, _Trichina spiralis_. 13. The sexual animal lives in the trachea of birds; the ova containing embryos are coughed up and are taken into other birds with food. They quit the egg-shell and wander into the air-sacs, and finally into the trachea: example, _Syngamus_. 14. There are two larval forms; the first lives in water, the second in the lungs of Amphibia, whence they wander into the intestine and become sexually mature: example, _Nematoxys longicauda_ in _Triton alpestris_. PARASITISM. 1. EFFECT OF PARASITISM ON THE PARASITE.—The usual effect of parasitism on the parasitic organism is that the various organs necessary for a free life tend to degenerate, whilst there is a multiplication and development of organs of adhesion, by means of which the parasite maintains its hold on its host. There is further an immense increase in the powers of reproduction, which may take the form of an increase in the number of fertilised eggs produced, or the parasite may at some time of its life reproduce asexually, by budding, or fission, or parthenogetically. Of the various classes of animals which are more or less parasitic, the Nematodes show less difference between the free-living and parasitic members of the group than obtains in any other class. With few exceptions, such as _Sphaerularia_, _Allantonema_, and one or two others, the parasitic forms have undergone but little degeneration. It is true that they have no eyes such as the free forms often possess, but in other respects, such as in the nervous, muscular, and digestive systems, they do not show any marked retrogression; further, the mouth-armature is developed in many free forms, and is not confined to the parasites. The group has developed no methods of asexual reproduction by budding or fission, such as are found in Platyhelminthes; and the cases of an alternation of generations in which a sexual form alternates with a parthenogenetic form, are rare, _e.g._ {162}_Rhabdonema nigrovenosum_; and it seems possible that even when parthenogenesis has been described, further observation may show that the parthenogenetic stage is really a protandrous hermaphrodite, in which case the alternation of generations in Nematodes, _i.e._ the hermaphrodite alternating with the dioecious form, is a case of _heterogamy_ or the alternation of two sexual generations. On the other hand, parasitic Nematodes produce enormous numbers of eggs. Van Beneden states that 60,000,000 have been computed in a single Nematode, and this multiplication of ova is absolutely necessary, for the chance of the embryo reaching the right host, in which alone it can develop, is always a small one.