The _cysticercus cellulosæ_ has been already described (page 23). The cystic form of the dog’s tape-worm (_echinococcus_) is a most dangerous parasite for man. When the egg of the dog’s tape-worm is swallowed by man, the embryo escaping from this egg burrows from the alimentary canal, and forms large cysts, chiefly in the liver, but occasionally in the lungs, brain, and other organs. For the removal of these, surgical interference is required. This form of cyst is commonly known as a _hydatid_. It is most frequently seen in Iceland and Australia, though not uncommon in this country. Its frequency depends largely on the number of dogs, and on the facility with which the ova of their tape-worms can gain access to water. The adult _Tape-worms_ are usually derived in man from eating meat containing the cystic form. The _cysticercus_ of the pig produces _Tænia Solium_; that of the ox, the _Tænia Mediocanellata_. These are the two most common forms of tape-worm in man. The minute head of _T. Solium_ has four suckers and a double row of hooklets, 28 in number; while the head of _T. Mediocanellata_ has four suckers but no hooklets. The segments of _T. Solium_ are smaller than of _T. Medioc._, and the structure of the segments of the two is somewhat different. =Preventive Measures.=—In avoiding the various Entozoa described, it is important (1) to _carefully avoid all underdone meat_. The eating of smoked sausages, or of meat which is not cooked throughout, is a common source of tape-worm and of trichinosis. (2) _All vegetables should be thoroughly washed_: this is especially important in the case of water-cress, lettuce, etc., which are eaten raw. (3) If the purity of the _water_ is not ensured, it _should be boiled_ or filtered through a Pasteur-Chamberland filter (page 98), especially in tropical climates, and where many dogs are kept. Dogs should be kept out of the kitchen, lest ova accidentally gain access to articles of food. (4) The possibility of _flies_ and _mosquitoes_ acting as carriers of parasitic disease must be remembered, and precautions taken. CHAPTER XLII. THE RÔLE OF INSECTS IN SPREADING DISEASE. Insects are now known to be important agents, (_a_) as carriers and (_b_) as intermediate hosts of disease-agents.