tapeworm develop further, but the great powers of reproduction prevent the species from dying out. If, now, either a joint or the eggs from one are taken into the gut of a suitable host, the joint and the egg-shells, or the latter only, as the case may be, are digested in the stomach or intestine, so that the larvæ are set free. These (Fig. 137) are spherical, glass-like, of microscopic size, as might be expected, and bear three pairs of hooklets, by means of which they quickly perforate the wall of the gut, and make a way through the tissues of the host. They may also be carried further by the blood-stream. The larva of a particular species settles down finally, not only in some particular kind of host, but also in some definite organ or tissue. It then loses its hooks, and is rapidly transformed into a hollow _bladder-worm_ (measle), which grows till it reaches a definite size, depending on the species. Meanwhile it gradually develops one (_Cysticercus_) or several (_Cœnurus_) ingrowths, or (in _Echinococcus_, Fig. 138) several new bladders are developed inside the bladder-worm, and ingrowths are usually formed within these, similar to those of the simpler kinds of bladder-worm. These ingrowths resemble tapeworm heads in all respects, possessing suckers, and, it may be, a circlet of hooks, but these are _inside_, and not _outside_. Later—either in the animal originally inhabited by the bladder-worm, or after it has been transferred in the flesh of this to the gut of some carnivorous animal—the bladder-worm contracts, so that it can no longer hold the fluid which is present, and the ingrowth is turned inside out, the suckers thus becoming external. The tapeworm head is formed in this way, but the bladder still remains attached to its hinder end (Fig. 139). If a host inhabited by bladder-worms is not devoured by another animal, the bladder-worms, after reaching their full size, remain for a long time in the same stage of development without being able to develop further. But if the host is devoured by another animal, the bladder-worms enter its stomach and intestine, where the bladder is digested, while the tapeworm head remains uninjured. This thrusts out its suckers and attaches itself by means of them to the wall of the intestine, the process of joint-formation already described beginning soon afterwards. The following tapeworms, which are important to the owner of stock, will be briefly dealt with. The =Common= or =Armed Tapeworm of Man= (_Tænia solium_). [Illustration: FIG. 140.—Measles in Pork; the white spot is the head (natural size). ]