There can be no doubt but that the death of these male rotifers is natural in the fullest sense. The females, however, although they are provided with complete digestive organs, do not escape a similar fate. Their life is longer and more complex than that of the males, and so is subject to many more chances. The females therefore may come to die from starvation or from other external, accidental causes. But, if they are kept in favourable conditions, they may live for about fifteen days, towards the end of which they die naturally, exhibiting the symptoms that I have described in the case of the males (Fig. 17). [Illustration: FIG. 17.—Female _Pleurtrocha haffkini_, which has died a natural death.] Rotifers are not the only animals which undergo natural death in a fashion quite unlike the violent end of Pilidium and Diplogaster. There are other cases amongst invertebrates, but I shall limit myself to describing one that is well ascertained. More than fifty years ago, Dana, the American naturalist, discovered a pelagic marine creature with characters so curious that he gave to it the name _Monstrilla_. It is a little crustacean akin to the _Cyclops_ of lakes. But although the latter is endowed with the organs necessary to capture and digest food, _Monstrilla_ has neither organs of prehension nor a digestive canal. It is a highly muscular animal with organs of sense and reproduction and a nervous system; but it is devoid of apparatus for prolonging life by nutrition. _Monstrilla_ therefore is a creature doomed to natural death. [Illustration: FIG. 18.—_Monstrilla._ (After M. Malaquin.)] The detailed observations of M. Malaquin[88] have supplied full information regarding this strange life-history. _Monstrilla_ passes a portion of its life as a parasite on Annelid worms. In that stage it accumulates the necessary material for the growth of the sexual products (ova and spermatozoa) and for free life in the sea whilst the young are developing. It is not only the males which have no digestive apparatus. The females also lack it, which is the more surprising as they carry about the eggs attached to the body (as is done by many other Crustacea, such as crayfish and lobsters) until the young are ready to hatch (Fig. 18). M. Malaquin thinks that the Monstrillas die of starvation. “As they are without a digestive tube or organs of prehension or mastication,” M. Malaquin says (p. 192), “the Monstrillas, which have no means of nutrition, are doomed to death from inanition after a short pelagic life. This is a logical inference from their structure.” In support of his view, M. Malaquin states that before death the tissues and organs show plain signs of degeneration. “The eyes first show traces of degeneration. The pigment spreads and disappears little by little and then the visual elements fade out.”