Animals are occasionally found which are devoid of some organ necessary for prolonged life. The absence of a digestive tract in an animal that lives in an environment rich in dissolved nutritive material (as for instance tapeworms living in the intestinal tract) is not surprising. But when creatures of the sea or of fresh water have no digestive tract, their life can be maintained only at the expense of nutritive material stored within them during embryonic life. The death which comes eventually is truly natural. The best cases, that is to say those which can be studied most completely, of such natural death occur amongst the Rotifera. These are minute creatures of fresh or sea water, at one time confused with the Infusoria, but possessed of a much more complex organisation. They have a well-developed digestive tube, organs of excretion, nervous system, and organs of sense. The animals are diœcious; in each species both males and females exist. Whilst the females have the complete structure of the species, the males are much reduced, and are devoid of a digestive canal. The cuticle is fairly stout, and they are unable to absorb dissolved nutriment through it; as they have no organs of digestion, their life must be short. To study in detail the life and death of these creatures, I selected a species sent to me by M. Haffkine. So far as I can judge, the species in question is a hitherto unknown member of the genus _Pleurotrocha_, and I propose for it the name _Pleurotrocha haffkini_. This rotifer is convenient to study as it thrives in vessels containing fresh-water to which some bread-crumb has been added (in the proportion of a gram of bread to 500 grams of water). The sexes of the little rotifer can be distinguished from the earliest age, for eggs that are to become females are much larger than those from which males develop. It is easy to isolate the male eggs and to follow the life-history up to the moment of natural death. The whole course of life from the laying of the egg until death lasts only about three days, and is probably the shortest duration of life in the animal kingdom. Although some Ephemeridæ live only a few hours in the adult state, their total life-cycle is much longer than that of the rotifers, as the larval stages last for months or even for years. [Illustration: FIG. 16.—Male _Pleurotrocha haffkini_.]