Part 36

The large jelly-like Medusæ which in summer are seen floating around our coasts, driving themselves along by alternate contractions and expansions of their umbrella, are frequently infested by little creatures of widely different organisation, Crustaceans belonging to the genera Hyperia and Metoecus. On the beautiful Chrysaora of the southern coast I have seen the Metoecus medusarum, a little shrimp about half-an-inch in length, with enormous lustrous green eyes, which takes up his residence in the cavities of the sub-umbrella,--dwelling in them as in so many spacious and commodious apartments, of which he takes possession, evidently without asking leave of the landlord, or paying him even the compliment of a peppercorn rent. Here he snugly ensconces himself, and feels so much at home, that he is not afraid to leave his dwelling now and then, to take a swim in the free water, returning to his chamber after his exercise; and here he rears his numerous family, which, in the form of tiny white specks, very much unlike their parents in shape, stud the membranes of the jelly-fish.

But, what is stranger still, Mr M'Cready has recently discovered in the harbour of Charleston in North America, a Medusa which is parasitic upon another Medusa. Cunina octonaria does not swim freely in the water, but inhabits the cavity of the bell of Turritopsis nutricula. "Not only does the latter furnish a shelter and dwelling-place for the larvæ during their development; it also serves as their nurse, by allowing the parasites, whilst adhering by their tentacles, to draw nourishment out of its mouth by means of a large proboscis. In point of fact, the relation between them is of so unprecedented a nature, that the author may well be excused for having at first taken the impudent parasite for the gemmiparous progeny of the sheltering Medusa. The youngest state of this parasitic Medusa observed by the author formed a ciliated body of clavate form, adhering to the cavity of the bell by means of the slender stalk in which it terminated. The first change consists in the emission, from the thick end, of two slender flexible tentacles, and in the formation of a central cavity by liquefaction. At this stage of development, the author frequently observed gemmation taking place at the thicker end, sometimes frequently repeated. Subsequently the number of tentacles becomes doubled. These bend together over the clavate extremity, and are then employed, instead of the thin end of the body, in adhering to the cavity of the sheltering Medusa. The thin extremity then acquires a mouth, and may be recognised as a stomachal peduncle, which is employed, as above indicated, in obtaining nourishment. The morphological nature of the proboscis becomes still more distinct when, after the lapse of some little time, an annular fold makes its appearance immediately under the tentacles, which is recognisable from its form, and from the formation in it of (eight) otolithic capsules, as the first indication of the future bell. Simultaneously with the otolithic capsules, four rudimentary tentacles make their appearance between the four tentacles. The Medusa remains in this stage of development for a long time. The bell gradually becomes more freely developed, and at last, by the reduction and entire disappearance of the stomachal peduncle, becomes the most essential part of the Medusa, after it has left its previous dwelling-place in the bell of the Turritopsis. The bell nevertheless retains for some time its earlier lobed form and unequal tentacles."

More remarkable even than this association is the fact that certain true Fishes habitually reside in the stomachs of star-fishes. This circumstance, which had been observed in the Oriental Archipelago by MM. Quoy and Gaimard, and by Dr Bleeker, has recently been confirmed by Dr Doleschall, who has written a very interesting Memoir on it.

This learned naturalist states that the fact of the connexion between the fish and the star-fish is well known to most of the fishermen in Amboyna, and that he was able to obtain a sufficiency of specimens for examination; but as the star-fishes (and with them the fishes) speedily died in confinement, he was unable to make continuous observations upon them in a living state. Of the results of his observations he gives the following summary:--

"The fish stands to the star-fish in a definite relation which cannot be the object of observation. Why the little fish should always seek the stomachal cavity of one and the same species of star-fish, and not that of various species, is a mystery. It is well known that Crustaceans of the genus Pagurus inhabit the empty shells of Mollusca; but we find on the shore the same species of Pagurus in the shells of the most various genera and species.

"I have never met with Oxybeles gracilis, on the contrary, in any other species of star-fish than Culcita discoidea. The fish was described by Bleeker under the above name in 'Natuurkundig Tijdschrift,' vii., p. 162. The author proceeds to state that neither he nor any one else in Amboyna has ever captured the fish under other circumstances, or while swimming freely in the sea; but upon this Dr Bleeker remarks that many of his specimens of Fierasfer Brandesii, and all those of Fierasfer (Oxybeles) gracilis and F. lumbricoides, were obtained by him along with other fishes, and were probably taken while swimming freely in the sea.

"Upon the habits of Oxybeles gracilis the author goes on to say that it is certain that this animal passes the greater part of its existence in the stomach of the star-fish, rarely shewing itself outside of this, and then probably at night. That it does come out occasionally, appears from the fact that in two cases the author observed the fish with a portion of its body outside the cavity of the star-fish, and in the act of creeping in.

"The same observations shewed that the fish, in returning to its concealment, passes along the furrow of the lower surface of one of the arms leading to the mouth of the star-fish, which is wide enough, when the tentacles are retracted, to leave room for the passage of the slender body of the Oxybeles. This fact likewise proves that the Oxybeles does not get into the stomach of the Culcita by accident.

"If a living Culcita be cut in two, the fish is seen moving freely in the cavity of its body. If it be taken out, it immediately seeks the shade. If the two halves of the Culcita (still alive) be placed in the water, the fish will soon be seen to draw towards them, in order to get into the cavity of the star-fish. When exposed to the light, it is uneasy, and its iris contracts excessively. The author never found two fishes in the same star-fish.

"In most of the fishes examined by him, the author found the stomach empty; it was full only in one. The contents of the stomach had the appearance of a lump of fat, and consisted of half-digested muscle. Under the microscope, striated muscular fibres could be detected, and the author thinks that they belonged to the muscles of a fish. This circumstance proves that Oxybeles does not feed upon the chyle of the star-fish, but that its nourishment is analogous to that of other fishes. Whether it seizes upon the fishes taken by the star-fish for its own nourishment must be determined by further investigations.

"The author's observations establish--

"1. That Oxybeles gracilis is not a true parasite.

"2. That it passes the greater part of its life in the stomach of Culcita discoidea, as is also indicated by the unusually pale colour of the fish.

"3. That, however, it can come out, either to seek nourishment, or for the purpose of reproduction.

"4. That it returns to the mouth along the furrow on the ventral surface of the arms.

"5. That it is very sensitive to light.

"6. That it feeds upon other animals.

"In fresh water the animals live for about half-an-hour. The pigment upon the peritoneum exhibits under the microscope the most beautiful stellate forms. The fish possesses a swimming-bladder."

Some very curious instances of parasitism occur, in which one kind of creature compels or induces another creature to labour for its special benefit. Indeed, in all cases, the parasite is benefited by the functions of the supporter; but, in the cases I refer to, the slavery is more special and more apparent.

There is a large species of Crab (Dromia) found in the West Indies, which is invariably found covered with a dense mass of sponge. The sponge is found to have grown in such a manner as to fit every prominence and cavity of the crab, exactly as if a plastic material had been moulded on it, yet it is not adherent to it, but is merely held in position by the hindmost pair of feet, which in this genus of crabs, are turned upwards, and apparently serve no other purpose than that of hooks to hold on the sponge in situ.

On our own shores we are familiar with the Hermit crabs making use of various kinds of univalve shells as houses to protect their softer hindparts; but in many of these cases there is a third party in the transaction, which is made to work for the crab's especial advantage. The shell of the mollusk is sometimes covered with a sort of fleshy polype-mass (Hydractinia echinata), which is parasitic on the shell. The shell, however, being tenanted also by the active crab, the polype, as it grows, moulds itself on the crab's body, and thus extends the dimensions of its house, so that it has no necessity either to enlarge its dwelling by the absorption of part of the interior shell-wall, or to leave this shell and search for one of ampler size, as other Hermit-crabs are obliged to do who have not the advantage of so accommodating a fellow-lodger. "One can understand," says Dr Gray, "that the Crab may have the instinct to search for shells, on which the coral [polype] has begun to grow; but this will scarcely explain why we never find the [polype] except on shells in which Hermit-crabs have taken up their residence."

Small Annelids and Crustaceans not unfrequently burrow into the stony walls of corals; but Dr Gray records a much more uncommon case, from the Guilding collection. "It is an expanded coral, which forms a thin surface on the top of another coral, and is furnished with a number of small, depressed, horizontal cases, opening with an oblong mouth. Some of these contain within them a small, free, crustaceous animal, a Cymothoa, which nearly fits the case; and it is evident that, by their moving backwards and forwards on the surface, they have caused the animal of the coral to form one of these cases for the protection of each specimen."

The manner in which this result is obtained is thus explained--"The animals which form their habitation in corals, appear to begin their domicile in the same way as the barnacles before referred to; they take advantage of the soft and yielding nature of the animals which form the corals, &c., and taking up a lodgement in their body, all they have to do is to keep a clear passage in it, either by the moving backwards and forwards, the exertion of their limbs, or the ingress or egress of water to and from their bodies, and in time, as the coral is secreted by the animal, it will form a wall round them; but if, by any accident, the parasite animal should not keep a passage from the coral to the surface of the body of the animal clear, which it must be constantly induced to do, since by this means it procures food, the coral animal will in a very short time close over it and bury it alive in the mass of the coral; and this, from the number of these animals, of all sizes and in different stages of growth, which are to be found in the substance of the large and massive corals, must often be occurring. Thus the Italian romance is often literally fulfilled in nature."

Certain birds are parasitic, in this sense, that they compel or induce other birds to perform the labour of incubation and of rearing their young. The Rhea or Ostrich of South America is parasitical on its own species; the females laying each several eggs in the nests of several other females, and the male ostrich taking all the cares of incubation. More familiar examples, however, occur in our own Cuckoo, and in the Cowpen birds (Molothrus pecoris and M. niger) of North and South America. "These fasten themselves," as has been remarked by Mr Swainson, "on another living animal, whose animal heat brings their young into life, whose food they live upon, and whose death would cause theirs during the period of infancy."

The habit, at least in the case of the European Cuckoo, is so well known, that I need not do more than merely allude to the fact, that the female seeks for the nests of other insect-eating birds, always much smaller than itself, and deposits its own eggs,--a single egg in each; that this stranger egg is hatched by the foster-mother with all care, and the young bird is nurtured with all tenderness even at the expense of its own proper eggs and young, which in general are sacrificed in the course of the process. Every schoolboy knows these facts, but few perhaps have ever suspected the existence of a romantic feeling of love and fealty in the little bird towards the cuckoo herself, prompting the rendering of the service required as a coveted honour. Yet a naturalist has communicated to Mr Yarrell some facts which certainly look this way; and because they are indubitably the very romance of natural history, I cite them, leaving my readers to judge of their value.

"As you have contributed," writes Mr W. C. Newby of Stockton, "so much to the information and amusement of the numerous class of readers who take an interest in subjects of natural history, I consider it my duty to communicate first to you, what appears to me a new fact in the habits and character of that general favourite the cuckoo.

"An egg of this bird was brought to me on the 6th instant, which had been taken from the nest of the yellow bunting, at a short distance from this town, and the boy who got the egg gave the following account, which, I think, may be relied on. When bird-nesting the previous Saturday, he found a nest of the gold spink (a local name for the yellow bunting) with the young birds just hatched. On visiting the nest the following day, he flushed the old bird, having seen her sitting on it, but the young birds were all excluded, and were lying dead near; and to his surprise, a single egg--the one he brought to me--occupied the place of the callow brood. He took away the egg (which is now in my possession) so that it is impossible to corroborate the statement in any degree. The above circumstance was first named to me by Tom Green, a well-known character and naturalist in this town, whom I have always found to be accurate in his observations on birds, and by him I was referred to the boy. On my objecting to Green that the accident appeared incredible, because unnatural, and contrary to strong parental instinct, he replied, "Ay, sir, but little birds are mightily ta'en up with a cuckoo; they'll awmost dee out for them;" and he related the following fact which came under his own observation. When out with his gun, collecting birds to stuff, (animal-preserving being one of his many trades), he shot at and wounded a cuckoo, which, after flying some distance, fell upon a hedge with its wings outstretched: the attendant bird, which in this case was one of the pipits, continued in the flight of its patron after the shot, and when Green approached, he found it sitting on the body of the dead cuckoo.

"It has been supposed by some, that small birds follow the cuckoo for the sake of annoyance, mistaking it for a sparrow-hawk, to give public notice of a pirate abroad, and to warn all peaceful subjects of the air against a common danger. But this is clearly not so, for the flight and cries clearly distinguish the feelings in the two cases. The attendance on the cuckoo is at a distance, silent and respectful; but in the other we have a sort of hue and cry raised, as it were, against a felon, and which is kept up from place to place, if not to the shame, at least to the discomfiture of the culprit.