Is it inanition that is the cause of death? I do not think so, because up to the time of death the tissues appear to be unmodified. In the case of the females I have sometimes seen phenomena of inanition. In old and exhausted cultures the starved females become thin, flattened and quite transparent, and the tissues lose their granular appearance. No such changes are visible in the dying males, the tissues of which, on the contrary, retain a normal aspect.
The most probable explanation is that death comes from poisoning by the secretions of the tissues themselves. The large size of the organs of excretion indicates that in the course of metabolism waste matter is produced some of which is got rid of. If, after a time, the secretions are insufficiently eliminated, the tissues must be poisoned. As death is preceded by a spasm of uncoordinated movement, it appears as if the fatal intoxication of the males affected the nervous system first. The vibrating cilia and the muscles are attacked later.
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]).
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
Fig. 18.—Monstrilla. (After M. Malaquin.) life by nutrition. Monstrilla therefore is a creature doomed to natural death.