Chapter II: 4 (2)

There are, perhaps, many other crustaceans which, placed among messmates, like the _Pandarus_ and others, would have a right to claim a further inquiry. It is a fact that they are never seen except on the skin of their host, where they are always visible, preserve their colours entire, and never change their costume for the undress of a parasite. The _Pandari_ live especially on the _Squalidæ_. Some which are found in our seas are of rare elegance of form. We must, perhaps, place among messmates the crustacean which Siebold found in the Adriatic, at Pola, on the belly of the worm _Sabella ventilabrum_, and it is not impossible that the _Staurosoma_ observed by Will on an actinia, should have its place here rather than among the parasites.

A Rotifer without vibratory ciliæ, the _Balatro calvus_ of Claparède, lives as an epizoon on the same annelids which lodge the Albertia in their interior. The Darwinists, observes Claparède, will not fail to remark the presence of these Rotifers of the genus Albertia in the interior of the animal, and of the genus Balatro on the exterior. The parasite Balatro, like a shadow, never quits his Mecænas, says the learned naturalist of Geneva; who has observed it on the _limicolous Oligochæts_ of the Seime, in the Canton of Geneva.

The _Nebalia_ of Geoffroy is an interesting crustacean, abundant on the coast of Brittany. This charming animal gives lodging habitually to a messmate which Mons. Hesse considered as an animal allied to the 36 _Histriobdellæ_, but which is only an imperfectly described Rotator. We believe that it is the same animal to which Professor Grube has given the name of _Seison nebalia_. It appears to assume the aspect of the Histriobdellæ, and may perhaps be adduced as an example of mimicry.

The molluscs, whatever their name may imply, are those which show the most independence among all the inferior ranks of animals; not only are they contented with the slowness of their pace and the wretchedness of their food, but they only very rarely seek help from their neighbours. It is not, however, uncommon to find some living among corals, which have even been designated coralligenous molluscs. There exists a group of Gasteropods, the Eulimæ, which lodge in certain Echinoderms, and in every respect deserve to be classed among messmates; it was a long time before the relation which exists between them and the animals which shelter them had been thoroughly appreciated. Dr. Gräffe found one species, the _Eulima brevicula_, on the _Archaster typicus_ of the Uvea Islands, in the Pacific Ocean. The molluscs, known by the name of _Stylifer_, have the same mode of life; they have been observed in the Asteriæ, the Ophiuræ, the Comatulæ, and even in the Holothuriæ; and as they inhabit the digestive cavity of these animals, it was believed that they frequented them as parasites. This was the opinion expressed first by d'Orbigny, and adopted by most naturalists. Professor Semper found some in the skin of a holothurian (_Stichopus variegatus_), which he considered incapable of nourishing themselves otherwise than at the expense of their host. However this may be, these molluscs, ranged alternately among the _Phasianellæ_, the _Turritellæ_, the _Cerithia_, the _Pyramidellæ_, the _Scalariæ_, 37 the _Rissoairia_, or in a distinct family, seem to belong rather to messmates than to parasites. We meet with Stylifers at the entrance of the mouth (Montacuta); more frequently they prefer, like the Fierasfers, to lodge themselves deeply in the digestive cavity in the midst of the _débris_ of the prey. The Melania (_M. Cambessedesii_, Risso), which Delle Chiaie found in the Bay of Naples, on the foot of some comatulæ, belongs probably to this group of molluscs.

Among the gasteropod molluscs which are not able to maintain themselves, we may mention another, a curious parasite, which instals itself in one of the rays of a star-fish, and whose presence is revealed by a swelling which is not produced in the other rays. This mollusc has received the name of _Stylina_.

The molluscs which are the most remarkable from the point of view from which we are now considering them, are the _Entoconchæ_; they live in Enchinoderms, and it was thought for a while that we could see in them an example of the transformation of one class into another. Some years since J. Müller found in a Synapta from the Adriatic, tubes with male and female organs, without any other apparatus, and in these tubes appeared eggs, whence this great physiologist saw molluscs proceed, with a helicoid shell, similar to that of a small natica; he gave them the name of _Entoconcha mirabilis_. Professor Semper has since discovered another species of these, which he has dedicated to the illustrious physiologist of Berlin, and which he found attached to the cloacal sac of the _Holothuria edulis_.

The true relation between these molluscs and the holothurians remains 38 to be discovered, and how the entoconchæ become at last simple sexual tubes. At present we must admit that it is the result of a retrogressive development like that of the peltogasters, which, like them, lose all the attributes of their class. They ought, perhaps, to be placed farther on, among parasites.

Some years since, some molluscs were observed which have compromised more or less the dignity of their class. Gräffe cites a species of the genus _Cypræa_, which one would certainly not expect to find in this category; it lives among the Viti Islands, in the compartments of the _Milithæa ochracea_. We have referred to it before. Naturalists have given the name of Melithæa to a very beautiful polyp which forms colonies of two or three metres in height. Mons. Steenstrup, with that perspicacity which discerns the most complex phenomena, has also described _Purpuræ_ which live as messmates with the Antipathes and the Madrepores. Quite recently, indeed, Mr. Stimpson has observed in the port of Charleston, a gasteropod mollusc, similar to a Planorbis (_Cochlioelepsis parasitus_) which lives as a messmate in the body of an annelid (_Ocoetes lupina_).

It is not the same with a mollusc called _Magilus_, which naturalists considered for a long time to be the calcareous tube of an annelid. All conchologists know the shell of the Magili, so valued by collectors. This gasteropod when young takes up its lodgings in the substance of a madrepore which grows more quickly than he, and in order not to die, stifled in this living wall, he constructs a calcareous tube similar to the shell, of which it appears to be the continuation, and which allows it to procure for itself water, air, 39 and food. The animal, protected by the madrepore, can do without its calcareous mantle, and only shows the end of the tube at the outside. It is this organ which sustains the struggle against the exuberant growth of the polyp, since it is by means of it that the mollusc obtains nourishment. The Magilus is like an oyster which is living in contact with a bank of mussels, with this difference, that the oyster almost always succumbs, while the magilus is always victorious in the struggle. We might also cite as well as the Magili, some _Vermeti_, certain _Crepidulæ_ and _Hipponices_, which struggle with the same success against those which pilot or receive them.

As there exist parasites which only depend on others during their youth, so there are messmates which are completely independent when fully grown. Jacobson, of Copenhagen, wrote, in or about 1830, a memoir to show that the young bivalves which are found in the external branchial processes of the Anadontæ are parasites, and he proposed for them the name of _Glochidium_. Blainville and Duméril were charged to make a report on this memoir, which the author had sent to the Académie des Sciences. But his opinion had not many supporters, and it is now thoroughly known that the young anodonts differ considerably in their early and their full-grown state. During their stay in the branchial tubes, each young animal carries a long cable which descends from the middle of the foot, and serves to attach the anodont to the body of a fish, and yet permits it to move to a certain distance.[1] In fact the young anodonts have, not like the other acephala, 40 vibratory wheels in order to move themselves; they are conveyed in this manner by their neighbours. There are also messmate acephala, as the _Modiolaria marmorata_, which lodge on the mantle of ascidians. Professor Semper found attached to the skin of a _Synapta similis_, a mollusc which possesses a peculiarity rare among these animals, that of carrying its shell in the interior and not on the outside.

There are few animals so infested with parasites as the Ascidians in general. Not only does their surface sometimes become a _microcosm_, as the name of one Mediterranean species indicates, but even in the substance of their testa lodge _Crenellæ_ and other molluscs and polyps, which choose by preference to place their dwelling there. There are also Annelids which hollow out galleries in their interior, Lernæans which establish themselves in their respiratory cavity, Nematodes, Pycnogonidæ, Ophiuræ, and many others besides. Mons. Alfred Giard has described several Amphipods and Isopods which establish themselves on Tunicates. One cannot say that there is always such a complete agreement between animals of such different kinds, for Mons. Alfred Giard gives examples of grave disagreements which he has seen break out, and which have caused the death of several among them.

Another association is that of a gasteropod with one of the acephala. In the environs of Caracas lives an Ampullaria (_Crocostoma_) which lodges in the umbilicus of its shell another mollusc, the only fluviatile species of those countries, called the _Sphaerium modioliforme_. We have every reason to suppose that the Sphaerium lives on good terms with the Ampullaria, since they are usually found associated.

The Bryozoaria, the animal mosses, establish themselves on all solid 41 bodies at the bottom of the sea, like true mosses on stones or on trees. One species, a _Membranipora_, is usually found on the common mussel. These animals are of small size, group themselves in colonies on the surface of shells and of polyparies, or even on crustaceans, and form by their union a fine kind of lace, the dazzling whiteness of which often comes out sharply on the varying and glittering colour of the shell. This is because each animal lodges in a cell which is not larger than the head of a pin, and all the cells of a colony are grouped together with the symmetrical regularity of the façade of a Gothic building.

Many Bryozoaria live in such a manner that it is impossible to say whether they are messmates, or have installed themselves by chance in a hiding-place for which they have no predilection. A charming bryozoon is developed in abundance on the carapace and the claws of the _Arcturus Baffini_, on the coast of Greenland, and propagates itself with extreme rapidity. On a single Arcturus we have found, scattered over its claws by the side of each other, Balani, Spirorbes, Sertulariæ, and vast colonies of Membranipora. One can see, merely by this example, the great zoological riches of the polar seas.

Certain annelids off the coasts of Normandy and Bretagne are the abodes of a bryozoary known under the name of _Pedicellina_, or _Loxosoma_. This interesting animal, which my fellow-labourer, Mons. Hesse, took for a Trematode, and whose drawings had led me into error, lives like others at liberty while young, and soon fixes itself to a Clymenian, in order to pass as a messmate the later period of its life. We have called it _Cyclatella annelidicola_, because of 42 its residence in a Clymenian annelid. Claparède and Keferstein have observed a species, the _Loxosoma singulare_, on a capitellian annelid, of the genus _Notomastus_, at St. Vaast-la-Hogue, on the coast of Normandy. After this, Claparède found another species, the _Loxosoma Kefersteinii_, in the bay of Naples, on an _Acamarchis_, a bryozoarian mollusc. Mons. Kowalewsky has observed in the Bay of Naples the _Loxosoma Napolitanum_.

We found some years ago the Pedicellinæ in so great abundance in the oyster beds of Ostend, that the baskets and other things floating on the water were literally covered with them. We have several times since endeavoured to procure them again, but it was in vain to search in the same places where they were formerly so abundant: we have not been able to discover a single one.

The class of worms includes not only parasites, it contains also, as we shall see, true messmates; we find some on crustaceans, on molluscs, on animals of their own class, on Echinoderms, and on Polyps.

One of the most curious of these worms is the _Myzostoma_, whose true nature has just been revealed by the excellent researches of Mons. Mecznikow. These myzostomes resemble trematode worms, but they have symmetrical appendages, and are covered with vibratory ciliæ. They live on the comatulæ, and run upon these echinoderms with remarkable rapidity. They have not hitherto been found elsewhere; they are evidently no more parasites than the last mentioned, and their place is among free messmates. Two great annelids are found, the one, the _Nereis bilineata_, by the side of Paguri in the same shell, the other, the _Nereis succinea_, according to Grube, in the tubes or galleries 43 of the Teredines. These dangerous acolytes introduce themselves furtively into the retreat of their host; and, always on the watch, they obtain at all times, and in every place, a certain prey, and a hiding-place from which they can take their share of their neighbour's goods. Another nereis, observed by Delle Chiaie, _Nereis tethycola_, lives in the cavities of a sponge, the _Tethya pyrifera_, which is visited by so many messmates and parasites, that it becomes a kind of hotel, where every one establishes himself at his ease. Risso also mentions a _Lysidice erythrocephala_ which lives in sponges.

In the same class is found an Amphinoma, a beautiful red-blooded worm, which proudly wears a plume of red branchiæ on its head, and which Fritz Müller observed on the coast of Brazil, begging assistance from a poor _Lepas anatifera_. Many Polynoës live upon other annelids; the _Harmothoë Malmgreni_ on the sheath of the _Choetopterus insignis_, the _Antinoe nobilis_ on the case of the _Terebella nebulosa_. Prof. Ray Lankester has lately communicated some observations on this subject to the Linnæan Society of London, and Dr. M'Intosh mentions some new species leading the same kind of life on the coast of Scotland.

Grube found at Trieste, in a star-fish (_Astropecten aurantiacus_), between its rows of suckers, a _Polynoë malleata_, with its stomach attached to the animal; and Delle Chiaie has lately observed on an asteria, a _Nereis squamosa_ by the side of a _Nereis flexuosa_. Mons. Grube thinks that the nereis of Delle Chiaie is no other than the _Polynoë malleata_. Lobsters are often covered with very small tubicular worms, which invade the whole carapace, and which, as true 44 messmates, give themselves up to the caprices of their host. These are a kind of _Spirorbis_, which, under the form of small spiral tubes, instal themselves, by preference, on the limbs, the antennæ, or the claws.

Mr. A. Agassiz has seen on the coast of the United States, a Beroë (_Mnemiopsis Leidyi_) which gives lodging in its interior to worms which somewhat resemble the Hirudinidæ, and which doubtless live there as messmates. Mr. A. Agassiz has remarked to me another example of commensalism. On the coast of the territory of Washington, as far as California, is found a worm of the genus _Lepidonotus_, which always lives near the mouth of a star-fish, the _Asteracanthion ochraceus_ of Brandt; sometimes as many as five are found together on a single individual, and are placed on different parts of the ambulacral rays. Mr. Pourtalis and Mr. Verril have observed annelids lodged in the polypidoms of the Stylaster.

There are few fish on which are not found _Caligi_, charming crustaceans which please the eye by their attenuated shape and their graceful movements. On these Caligi, which sometimes literally cover the skin of cod-fish coming from the north, we often find a curious trematode, the _Udonella_, which resembles one of the small hirudinidæ. Should this worm be placed among messmates? What is the part which it plays? We are persuaded that it is the same as that of the histriobdellæ under the tail of lobsters, that is to say, that it clears off the eggs of caligi which do not arrive at perfection, but perish in the course of their evolution.

Roussel de Vauzème has mentioned another worm, a nematode, to which 45 he has given the name of _Odontobius_, and which lives on the palatal membranes (the whalebones) of the southern whale. It is evidently a messmate. It can get nothing from the whalebones, but it snaps up on their passage in the interstices of the baleen, small animals of all kinds which swarm in these waters. When we open the _Pylidium girans_, we often find in the interior of its digestive cavity a larva, which was once thought to be descended from it, but instead of being allied to the Pylidium, this larva comes from a nemertian known by the name of _Alardus caudatus_. The young nemertian never abandons his host until it approaches the period of puberty, and then all the individuals living under the same conditions emancipate themselves at once, to pass the rest of their days free and roving like their mother.

Worms which have less freedom, like the Distomians, are sometimes both messmates and parasites. We find a remarkable example of this in the _Distomum ocreatum_ of the Baltic. According to the observations of Willemoes-Suhm, this trematode passes its cercarial life freely in the sea, and instead of encysting itself in the body of a neighbour, it attaches itself to a copepod crustacean, the whole of the inside of which it devours, in order to clothe itself afterwards with the carapace of its victim. It is under the cover of its prey that it passes into the herring, and completes its sexual evolution.

Mons. Ulianin has recently found another Distome (_Distomum ventricosum_) which passes its cercarial life in freedom in the bay of Sebastopol, and completes its evolution in the fishes of the Black Sea. J. Müller has long since found Cercaria living freely in the Mediterranean.

We ourselves, some years ago, while making some researches among the 46 Turbellaria, found among the eggs of some ordinary crabs of our coasts (_Carcinus mænas_), an interesting worm which we named _Polia involuta_, but which Prof. Kolliker appears to have known before, and designated by the name of _Nemertes carcinophilus_. It is not known whether it plays the same part as the Histriobdellæ and the Udonellæ. Delle Chiaie, as well as Prof. Frey and Prof. Leuckart, make mention of another nemertian which inhabits the _Ascidia mamillata_. Among the nemertians, we may allude to the _Anoplodium parasita_, which lives in the _Holothuria tubulosa_, and the _Anoplodium Schneiderii_, inhabiting the intestines of the _Stichopus variegatus_.

According to Mr. A. Agassiz, a species of Planarian (_Planaria angulata_, Mull.), lives as a free messmate on the lower surface of the Limulus, and prefers to establish itself near the base of the tail. Mons. Max Schultze recognized last year this same messmate on a limulus, which had died at Cologne in the large aquarium, and which had been sent to him for his anatomical studies. He showed at the congress of German naturalists at Wiesbaden, in 1873, the drawing which he had made of this animal, which he thought new to science. We may remark in passing, that he arrived, by means of his anatomical observations on Limuli, at the same result as did my son by his embryogenic observations, namely, that these supposed crustaceans are to be regarded as aquatic scorpions. Mr. Leidy also makes mention of Planarian parasites (_Bdellura_), with a sucker at the extremity of the body; and Mons. Giard noticed a blue one on the body of a Botryllus.

But of all the Turbellaria, the genus which appears to us the most 47 interesting is the Temnophila, which Gay first observed on crabs at Chili, and which Professor Semper afterwards found on the crabs of the Philippine Islands. Gay and Phillipi found colonies of these animals on the body, the claws, and more especially the abdomen, of the _OEglea_. This messmate resembles a trematode by its form and by its posterior sucker, but by its entire character, and especially by its sexual organs, it belongs to the _Turbellariæ_. Mons. Blanchard calls it _Temnophila Chilensis_. Professor Semper saw at the Philippine Islands these Temnophilæ on river crabs, at five thousand feet above the level of the sea.

The _Cydippe densa_, a charming polyp of the Gulf of Naples, lodges in its gastro-vascular apparatus larvæ of annelids, which may as well be considered parasites as messmates. We owe to Panceri the first observations on these worms, of which two genera, _Alciopina_ and _Rhynconerulla_, seem to live in the same manner in their youth. A naturalist, whose loss is profoundly deplored by the scientific world, Claparède, occupied himself with observations on these annelids during the last years of his life. It appears that these worms are so common in these polyps, that four have been found at once in the same animal.

The Spoon-worm, named by OErsted, _Sipunculus concharum_, ought doubtless to find its place here. An oligochete worm, _Hemidasys agaso_, from the Gulf of Naples, lives on the _Nereilepas caudata_, and Claparède did not think it unworthy of his attention. The surest means of finding it, says this philosopher, is to look for it on this annelid; and our much regretted fellow-labourer at Geneva did not 48 abandon this messmate before he had completely studied it. Let us remark in passing, that Professor Grube published in 1831, at Königsberg, a special work on the abodes of annelids in general.

Cases of commensalism among the Echinodermata are still more rare. These animals are sufficiently provided with organs, both with respect to their food and their skin, not to require the assistance of their neighbours. We cannot rank as a phenomenon of commensalism, the conduct of the young Comatulæ, which fasten themselves, as Mr. A. Agassiz informs me, to the basal cirrhi of the adult echinoderms, and there form a little colony of young Pentacrinites.

We only know one Ophiurus (_Ophiocnemis obscura_), which lives as a messmate on a comatula, and consequently seeks assistance from an animal of its own rank. Another kind of Ophiuride (_Asteromorpha lævis_, Lym.) fixes itself on a _Gorgonella Guadelupensis_ of Barbadoes. Everything induces us to suppose that we shall find more than one species of echinoderm, which will take its place among these when their mode of life has been studied with greater care. Professor Lütken has just proved this by quite recently making known another _Ophiothela_, which lives in the straits of Formosa, and seems to be the messmate of an Isidian polyp, known under the name of _Parisis loxa_. Another species (_Oph. mirabilis_) from Panama, infests certain Gorgoniæ and sponges; a third is found in the Fiji Islands on the _Melitodes virgata_; a fourth at the Isle of France on Gorgoniæ; and a fifth at Japan on the _Mopsella Japonica_. There is also another in the Pacific Ocean, but its companion is not known.

Professor Mobius, as well as Dr. F. Martens, has noticed a _Hemicuryale 49 pustulata_ on a polyp of Jamaica, known under the name of _Verrucella Guadelupensis_. This is a curious instance of mimicry.

The class of polyps includes several species which seek for assistance from others, and are classed among messmates. One of the most remarkable is the Gigantic Medusa, which can extend its arms downwards to a hundred and twenty feet, and bears the name of _Cyanea arctica_; the disc is seven feet and a half in diameter, and when the animal is on the surface of the water, the fringes, which surround the cavity at its mouth, occasionally afford lodging in the midst of them to a species of actinia, which lives there as messmate. Sometimes three, and even four or five, are found on a single Cyanæa. This also is an observation due to Mr. A. Agassiz, which he has published in his interesting work, "Sea-side Studies." Prof. Haeckel supposed that the _Geryoniæ_ produce _OEginidæ_ by means of buds; but it appears that the learned professor was mistaken as to the nature of these buds; that instead of being produced one from the other, they have, according to Steenstrup, a completely different genealogy, being only united by conditions of good-fellowship. They may be truly called messmates.

Mons. Lacaze-Duthiers, who went to the coast of Africa to study corals, met with a young polyp which requires the assistance of another polyp in its early condition. This animal, to which he has given the name of _Gerardia Lamarckii_, lives on one of the Gorgoniæ, which it invades and stifles, as the lianas strangle the tree over which they spread themselves. But these same Gerardiæ can also develop themselves on the eggs of the _Plagiostoma_, and are then 50 capable of living separately. In the substance of this polyp lives a crustacean, the nature of which Mons. Lacaze-Duthiers has not yet made known.

The superb sponge, _Euplectella aspergillum_, the elegant structure of which cannot be sufficiently admired, is, unlike the Alcyonium of the Dromia, rooted to the soil, but nevertheless gives shelter to three kinds of crustaceans: Pinnotheres, Palemonidæ, and Isopods. These supposed plants have been known for many years under the Spanish name of _Regadera_, or the English "Venus' Flower-basket;" they were first brought from Japan, and afterwards from the Moluccas, and more recently from the Philippine Islands. In almost all the individuals which Professor Semper was able to study in those parts, were found the same crustaceans. These _Euplectellæ_ have just been met with to the south-west of Cape St. Vincent, by Wyville Thomson, who has brought up some from a depth of 1090 fathoms, while on board the _Challenger_. This skilful professor has discovered another sponge to the north-west of Scotland, at a depth of 460 fathoms; it bears the name of _Holtenia Carpenteri_; and I have in my possession a fine specimen which I owe to his generosity, and keep as a _souvenir_ of the delightful hospitality which he extended to me at the Edinburgh meeting.

There are also sponges which construct a dwelling in the abode of their neighbour. We find, among others, a small sponge known under the name of _Clione_, which establishes itself in the substance of the shell of oysters, and hollows out galleries as the teredo does in wood. Mr. Albany Hancock found twelve species of Clione on a single 51 Tridacna. They are evidently not parasites, and I am not sure if their place is properly among messmates. The oyster, and more especially the _Ostrea hippopus_, lodges three or four different sorts in its shell. These Cliones possess siliceous spicules, by means of which they hollow out galleries in the substance of shells. Mr. Hancock has published a monograph of this genus, in which he recognizes twenty-four species collected from different shells, and two other species, which he refers to the genus _Thoasa_.

The cliones are real lodgers which lead us to the _Saxicavæ_, the _Pholades_, and the _Teredines_; they seek their lodging in rocks or in wood; these lead directly to the sea-urchins, which also hollow out lodgings in rocks, but without penetrating deeply. Professor Allman has just observed a very remarkable case of commensalism between a sponge and one of the tubulariæ. The crown of the tubularia is extended at the entrance of the canals of the sponge; and the association is so complete, that the Edinburgh professor imagined that he had before his eyes a true sponge with the arms of a tubularia.

In the lowest ranks of the animal scale, there are certain kinds of animalcules, which establish themselves on the bodies of obliging neighbours, and take advantage of their fins in order to swim at their expense. Thus we often find the bodies of certain crustaceans covered with a forest of vorticellæ and other infusoria. They cause themselves to be towed like cirrhipedes, but they do not change their toilet like them, so that it cannot be said that they put on the livery of servitude. The kind of life led by several of these animalculæ is as 52 yet little known.

Mons. Leydig has found in the stomach of the _Hydatina Senta_ a messmate which much resembles an Euglena, and still more the _Distigma tenax_, Ehr.

[1] I owe this observation to Dr. W. S. Kent, who showed me, in
London, anodonts attached in this manner to sticklebacks.