II Natural Death in the Animal World
Different origins of natural death in animals—Examples of natural death associated with violent acts—Examples of natural death in animals without digestive organs—Natural death in the two sexes—Hypothesis as to the cause of natural death in animals
The cases of natural death amongst animals differ from those found in the vegetable world by their greater variety and complexity. As M. Massart has shown for plants, so also natural death must have become established independently in different groups of animals. In some cases, the characters presented are strange and almost paradoxical.
It is usual to contrast natural death with violent death on account of the difference between the two. None the less, natural death may occur in the animal kingdom, that is to say death resulting directly from the constitution, and yet in intimate association with violent acts. I will give some examples.
Small, helmet-shaped organisms, transparent and graceful, are common on the surface of the sea. These have been described by zoologists under the name Pilidium. The organisation is simple. The body wall is a delicate pellicle, through which, on the lower surface, a mouth leads into a capacious stomach. Continual movements of waving cilia direct small particles of food through the mouth to the digestive stomach. As there are no organs of reproduction, it was assumed that these creatures were not adults, but floating larvæ of some marine animal, and, after a good deal of trouble, it was found that the Pilidia were the young stages of ribbon-shaped worms of the group of Nemertines. At a definite stage in the life-history, a fœtus begins to develop round about the stomach of the Pilidium, and eventually completely encloses it and detaches it by violent muscular contractions. The end of the story is that the fœtus abandons the body of the Pilidium carrying off with it the stomach, an organ necessary to the maintenance of life. The remnant of the Pilidium swims about in the sea-water, but soon dies as the result of the mortal wound caused by the removal of the digestive organs.
The act by which the Nemertine separates from its mother is violent, and yet the death of the Pilidium must be regarded as natural. It is the result of agencies within the body and not, as in most cases of accidental death, of violence from without.
The group of Nematode worms contains many common intestinal parasites of man, such as Ascaris, Trichina, Trichocephalus, Oxyuris, &c., but also others that live free in soil or water or in such fluids as vinegar. They are protected by a strong cuticle, and some of them are viviparous, that is to say, instead of laying eggs they give birth to young worms already well grown and capable of independent activity. Amongst the human Nematode parasites, the Trichinæ give birth to swarms of small larvæ which easily escape from the body of the mother by the female generative aperture. In the case of some free-living Nematodes, however, the female aperture is too small to give passage to the rather stout larvæ. More than forty years ago, when I was investigating the life-history87 of one of these Nematodes (Diplogaster tridentatus) I was struck by the fact that the larvæ could leave the body of the mother only by violence and after they had devoured most of its substance. These larvæ develop from eggs produced within the maternal body. As the external reproductive aperture of the female is minute, the larvæ cannot escape through it, but wander amongst the tissues tearing and absorbing them. The mother soon dies, and although her death is violent, it must be included in the category of natural death.
From the teleological point of view it might be said that Pilidium and Diplogaster cease to live because they have fulfilled their function of giving rise to a Nemertine or young Nematodes. Their natural death would thus be predestined. There is no ground for such an interpretation. On the other hand, it is certain that this death, coming after the birth of the new generation, is in no way against the preservation of the species in which the extraordinary natural death by violence occurs. If the female orifice of Diplogaster were slightly larger, the larvæ would emerge without difficulty and without causing the death of the mother which none the less would have fulfilled her purpose.
All the cases of natural death amongst animals are not so brutal as those of the Pilidium and the Nemertine worms. In many instances the death is peaceful. As very frequently it is difficult to establish definitely that the death is natural, I shall select clear cases.
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.
The little males (Fig. 16) begin to swim soon after hatching, the wheel-apparatus and the musculature being vigorous. They seek out the females, as their reproductive organs are mature almost at the moment of hatching. The transparent body, which is devoid of digestive apparatus, swarms with mobile spermatozoa. As soon as the male has seized a female, he discharges the contents of his body. It might be supposed that such an evacuation would cause a violent perturbation of the system leading to the death of the organism. There is no question of this however. The males are able to live for twenty-four hours after having accomplished their function, and the period represents a third of their total duration of life. Moreover, I have isolated males from females without any prolongation of their lives. In one experiment, I isolated two males and placed a third in company with two females. It was the third specimen that lived longest.
The natural death of the males is foreshadowed by a weakening of the movements; although the muscles and cilia remain mobile, the whole animal moves only spasmodically; sometimes the muscles of the head contract, sometimes those of the tail, but no locomotion occurs. Occasionally there is a violent effort of ciliary motion as if the attempt were being made to overcome the immobility of the body. Such a condition lasts for several hours and is followed by death. The spermatozoa inside the body retain activity last of all.
Towards the crisis, bacteria, which abound in the medium occupied by the rotifers, begin to attack the males. Some cluster round the head, others round the tail, although none of them can effect entrance to the body. The death of the males cannot be attributed to microbial infection, but comes from some intrinsic cause.
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).
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.
The detailed observations of M. Malaquin88 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.”
“Finally, individuals, usually females, show complete degeneration. A female taken in a fine-meshed net showed no trace of organs in the head; the eyes, the brain and the intestinal tract had disappeared almost completely. The antennæ were reduced to stumps consisting of the lowest joint and a portion of the second. These were clear indications of the senility that precedes death” (p. 194).
Such evidence not only supports the hypothesis that the natural death of Monstrilla is due to inanition, but is opposed to a similar interpretation being applied to the case of male rotifers, in which death is not preceded by wasting of the organs. The death of some insects, which comes rapidly after the adult stage has been reached, cannot readily be attributed to starvation. In the strange butterflies known as psychids (Solenobia) some of the females lay eggs without having been fertilised,89 and their life in the adult condition lasts only a day. On the other hand, other females of the same butterfly are fertilised before laying their eggs and in this case survive for more than a week although they take no food. The rapid death of the first-mentioned set cannot be attributed to inanition.
In some Ephemeridæ, which supply good cases of natural death, the end comes after a few hours of adult life without any sign of degeneration of the organs. As in others (Chloë), life lasts for several days without food having been taken, it is clear that inanition is not the cause of the swift arrival of death in the first set. It is much more probable that the natural death is due to an auto-intoxication which takes effect at different intervals of time in different circumstances.90
In the higher animals such as vertebrates the conditions are less favourable than in the case of insects for the investigation of the causes of natural death. Vertebrates have always well-developed organs of digestion and so live a relatively longer time and encounter a greater number of chances of accident, with the result that in most cases death comes from external accidental causes. Vertebrates usually perish from hunger or cold, or are devoured by their enemies or killed by the attacks of parasites or diseases. There remains only the human race amongst the more highly developed animals, in which to study the onset of natural death. And in the human race cases which may be designated as natural are extremely rare.