The nocturnal migration into the lymphatic vessels, and thence into the blood of the embryo of the _F. Bancrofti_, is an adaptation to the nocturnal habits of a particular mosquito (_culex pipiens_ or _ciliaris_). When the mosquito bites an infested person, his proboscis removes some embryo filariæ, which are quickly transferred to its stomach. Some of these escape digestion, develop within the mosquito, and when the mosquito dies in water they bore their way out, and are subsequently swallowed by man. It is essential, therefore, in order to prevent this disease to boil or efficiently filter all drinking water, and to prevent the access of mosquitoes to water. Persons infested with filariæ should sleep inside mosquito nets, in order that they may not, when bitten by mosquitoes, spread the disease. =Tape-worms= are found infesting the alimentary canal of man. Each has a double phase of existence. In the first, the characteristic head, or _scolex_, along with a bladder-like body, lies embedded in the solid tissues of an animal; in the second, the _strobilus_ or tape-worm, occupies the alimentary canal of another animal. The tape-worm consists of a number of flat segments, each of which is capable of producing a large number of eggs, from each of which a six-hooked embryo is developed. The segments escape from the alimentary canal, and their ova are discharged and scattered broad-cast. These eggs are swallowed by another animal, the hooked embryo escapes from its case, migrates into the solid tissues, and there produces a scolex. When the host is eaten by another animal or by man, the scolex enters the alimentary canal, loses its bladder-like body, and developes a chain of segments. It follows from the above that two distinct hosts are necessary to complete the cycle of existence of these creatures, one being commonly a herbivorous, and the other a carnivorous animal. Thus:— = Cystic Form.= =Tape-worm Form.= _Cysticercus Cellulosæ in _becomes_ _Tænia Soluim in the alimentary the muscles of the pig_ canal of man._ _Cysticercus Bovis in the „ _Tænia Mediocanellata in the muscles of the ox_ alimentary canal of man._ _Cænurus Cerebralis of the „ _Tænia Cænurus in the alimentary sheep’s brain_ canal of the dog._ _Echinococcus of man, etc._ „ _Tænia Echinococcus in the alimentary canal of the dog._