The parasitic disease-producing protozoa, on the other hand, are of softer substance, often have the habit of twisting themselves in a corkscrew-like manner, and usually are provided with an undulating membrane or frill, as well as with one or with two whip-like swimming processes (the latter are present also and are often numerous in the actively swimming phases of bacteria), and have a more complicated life-history. They divide, as a rule, longitudinally and not transversely, and pass from one "host" to a second, where they assume distinct forms--males and females, which conjugate and break up (each conjugated or fused pair) into a mass of very numerous, excessively minute, young. The disease-producing protozoa of this kind are frequently parasitic in the blood of man and animals, and were only recently recognised, after the disease-producing bacteria of many kinds had been thoroughly studied. These animal microbes are often spoken of as "blood-flagellates" or haemo-flagellata, and the larger kinds are called "Trypanosomes," or "screw-form parasites," or whilst a series of more minute ones are called "Piroplasma," or "pear-shaped parasites." Many, but not all, are found during a certain period of their life, actually inside the corpuscles of the blood. The fact that many of these blood-flagellates (if not all) have, besides their life in the blood of one species of animal, a second period of existence in the juices or the gut of another animal, has made it very difficult to trace their migrations, since in the second phase of their history their appearance differs considerably from that which they presented in the first. And often they exist in one kind of animal without doing any harm, and are only poisonous when introduced by insects into the blood of other kinds of animals! There is, further, another set of disease-causing protozoan parasites which are similar to the amoeba or proteus-animalcule, and a third, which belong to the group of "ciliated infusoria." They are not so minute as the preceding set, and are not usually referred to as "microbes." They inhabit the intestine of man and animals, and cause, in some instances, dysentery. These two later kinds of protozoan parasites I will at the moment leave out of consideration, as well as the "coccidia," which multiply in the tissue-cells of animals--for instance, rabbits and mice--and cause an unhealthy growth and excessive multiplication of the cells of the tissues, which in some respects resembles that seen in the terrible disease known as cancer. Indeed, it is held by many investigators that some such parasite--though not yet discovered--is the cause of cancer.