CHAPTER II. 4

FREE MESSMATES.

We meet with free messmates in various classes of the animal kingdom.

They sometimes mount on the back of a neighbour, sometimes occupy the

opening of the mouth, the digestive passages, or the exit for the

excreta; at times they place themselves under the shelter of the cloak

of their host, from whom they receive both aid and protection.

Among the vertebrates, there are few except fishes which merit a place

here; it is only amongst these that we meet with species at the mercy

of others, and dependent on acolytes, which are in every respect

inferior to themselves.

An interesting messmate belonging to this first category is a fish of

graceful form, named donzelina, which goes to seek its fortune in the

body of a holothuria. Naturalists have long known it under the name of

Fierasfer. It has a long body like that of an eel, entirely covered

with small scales; and as it is quite compressed, it has been compared

to the sword which conjurors thrust into their oesophagus. They are

found in different seas, and all have similar habits. This fish is

lodged in the digestive tube of his companion, and, without any 5

regard for the hospitality which he receives, he seizes on his portion

of all that enters. The Fierasfer contrives to cause himself to be

served by a neighbour better provided than himself with the means of

fishing.

Dr. Greef, at present Professor at Marbourg, found at Madeira a

holothuria of a foot in length, in which a vigorous Fierasfer lived in

peace. Quoy and Gaimard, in the account of their voyage round the

world, have remarked long since, that the _Fierasfer hornei_ is

found in the _Stichopus tuberculosus_.

The holothuriæ seem to exist under very advantageous conditions in

this respect, since we see Fierasfers, which are themselves tolerable

gluttons, accompanied by Palæmons and Pinnotheres in the same animal.

Professor C. Semper has seen holothuriæ in the Philippine Islands

which bore a considerable resemblance, in this respect, to an hotel

with its table d'hôte.

These singular fishes have been long noticed, but it was not till

recently that their presence in a host so low in the scale as a

holothurian could be explained.

But if naturalists are agreed as to the bond which unites these fishes

to the holothuriæ, they do not agree as to the organs which they

inhabit in their living hotel. Do they lodge in the digestive cavity

of the holothuriæ, or do they inhabit the arborescent respiratory

processes which open at the posterior extremity of the body? Until

recently it was thought that it was in their stomach, but a doubt has

arisen. Professor Semper, who has studied these animals with

particular care at the Philippine Islands, had the curiosity to open

the stomach of some of them, and found there, not the animals taken by 6

the holothuriæ, but the remains of its respiratory processess which

they were in the act of digesting. Is it then merely a messmate? We

must have more information on this point; and if it were not

accidentally that the fierasfer swallowed the walls of the compartment

in which he was lodged, he ought rather to take his place among

parasites. Though it lodges in the respiratory processes, as the

learned professor at Wurtzburg asserts, the fierasfer may also be a

messmate after the fashion of so many others which inhabit the

neighbourhood of the rectum, in order the more conveniently to snap up

those animals which are attracted by the odour.

The fierasfers are not the only fishes which seek assistance from the

holothuriæ; a species lives at Zamboanga, to which the specific name

of Scabra has been given, and in the stomach of which, says Mons.

Johannes Müller, usually lives a myxinoid fish, called _Enchelyophis

vermicularis_. Unfortunately, we are not told in what part of the

stomach it resides; for all is stomach in these animals.

It is less degrading for a fish to ask assistance from one in his own

rank. The Mediterranean offers a curious instance of this. Risso saw

at Nice, at the commencement of this century, the monstrous fish known

under the name of _Beaudroie_ (the angler, or fishing-frog)

lodging in its enormous branchial sac a fish of the family of the

Murenidæ, the _Apterychtus ocellatus_. He is found there

evidently under the condition of a messmate. Although the eels

generally get their living easily, the Angler possesses fishing

implements which are wanting in them, and when both of them are 7

immersed in the ooze, it carries on a fishery sufficiently abundant to

enable it to share the spoil with others. This same angler lives in

the northern seas, and there it harbours an amphipod crustacean, which

until lately has escaped the vigilance of carcinologists. We shall

speak of it further on.

Dr. Collingwood saw a sea anemone in the Chinese Sea, which was not

less than two feet in diameter, and in the interior of which lodges a

very frisky little fish, the name of which he could not tell.

Lieut. de Crispigny has observed a sea anemone (_Actinia

crassicornis_) living on good terms with a malacopterygian fish, the

_Premnas biaculeatus_. This fish penetrates into the interior of the

anemone; the tentacles close round it, and it lives thus for a

considerable time enclosed as in a living tomb. Mons. de Crispigny has

kept these animals alive for more than a year, in order to make

careful observations on them. A fish known by the name of _Oxybeles

lumbricoides_ has been also found in the Indian Seas, which modestly

takes up his quarters in a star-fish (_Asterias discoida_). Another

case of _commensalism_ has been made known to us by Professor

Reinhardt of Copenhagen. A siluroid of Brazil, of the genus

_Platystoma_, a skilful fisherman, thanks to his numerous barbules,

lodges in the cavity of his mouth some very small fishes, which were

for a long time considered as young siluroids; it was supposed that

the mother brought her progeny to maturity in the cavity of the mouth,

as marsupials do in the abdominal pouch, or as some other fishes do.

These messmates are perfectly developed and adult, but instead of

living on the produce of their own labour, they prefer to instal

themselves in the mouth of an obliging neighbour, and to take their 8

tithes of the succulent morsels which he swallows. This little fish

has received the name of _Stegophilus insidiatus_. We see that in the

animal world it is not always the great which take advantage of the

little. Still, let us not be deceived; there are fishes in the

latitude of the Island of Ceylon which really hatch their eggs in the

cavity of the mouth, and we have seen some in the museum at Edinburgh,

labelled with the name of _Arius bookei_. Louis Agassiz has made the

same observation on a fish of the Amazon, which has also been

recognised by Jeffreys Wyman. One fish wraps up its eggs in the

fringes of its branchiæ, and protects them till they are hatched;

another lays its eggs in holes hollowed out by itself in the steep

banks of the river, and protects the young ones after they are hatched.

To hatch the eggs in the mouth is not more extraordinary than to hatch

them in any other part of the body. The _Sygnathidæ_ hatch theirs

in a pouch behind the anus; and it is a curious circumstance that the

females do not undertake this duty. The males alone carry their

progeny with them. This recalls to our recollection that curious

example of the birds known under the name of _Phalaropes_, among

which the males only hatch the eggs. The female of the cuckoo abandons

her eggs, and entrusts them to the female of another bird.

The cuckoo suggests to us the mound-making Megapode and the Talegalla

of Latham, both of which inhabit Australia; these birds deposit their

eggs in an enormous mass of leaves or grass, which grows warm by

decomposition, and the temperature of which is great enough to hatch

them. The young ones when they come out of the egg are sufficiently 9

developed to be able to provide for their own wants, and to do without

a mother's care.

To return to our animal messmates: let us notice the result of the

observations of a learned and skilful naturalist who has rendered

great services to ichthyology. Dr. Bleeker has described a still more

remarkable association in the Indian seas; it is that of a crustacean,

the _Cymothoa_, taking advantage of a fish known under the name

of _Stromatea_; too imperfectly organized to fish for itself at

large, but more skilful in snapping up all that comes within its

reach, it makes its home in the buccal cavity of the Stromatea.

But of all crustaceans, the most cruel is the isopod named

_Ichthyoxena_, which hollows out for itself and its female a

large dwelling-place in the coats of the stomach of a cyprinoid fish.

We will return again to these examples.

The _Physaliæ_, those charming living nosegays of the tropical

regions, also give lodging in their cavities, and in the midst of

their long cirrhi, to little adult and perfect fishes, belonging to

the family of the _Scombridæ_, a family to which are attached the

tunny and the mackerel. These sea-butterflies flutter away their

indolent existence at the expense of their host. Voyagers tell us that

they have seen them by dozens concealed in these animated festoons.

Mons. Al. Agassiz has mentioned, in his illustrated catalogue, another

fact, quite as extraordinary, observed in the Bay of Nantucket, in the

United States; it relates to a nocturnal Pelagia (_Dactylometra

quinquecirra_, Ag.) always accompanied, not to say escorted, by a

species of herring. The two neighbours constitute together an 10

association which probably redounds to the advantage of both.

Without quitting our own sea-coast, we find an association of the same

kind between young fishes (_Caranx trachurus_) and a beautiful medusa

(_Chrysaora isocela_). This sea nettle often encloses several young

specimens of Caranx, which we are surprised to see issuing full of

life from the transparent bodies of these polyps. Indeed, it is not

rare to find other fishes in the medusæ. Dr. Gunther, who has arranged

with so much care the rich collection of fishes in the British Museum,

has shown us some specimens of the _Labrax lupus_, and of the

_Gasterosteus_, which had been obtained from the interior of different

medusæ; and these associations have been also remarked by various

distinguished observers, among whom we may mention Messrs. Sars, Rud.

Leuckart, and Peach. The captain of the frigate _Jouan_, when in the

Indian Sea, on October 26th, 1871, in 13° 20' N. lat., and 60° 30' E.

long., that is to say, about 200 leagues to the west of the Laccadive

Islands, saw, in very fine weather, the sea, which was at that time

very calm, covered with medusæ, and the greater part of these were

escorted by many little fishes of the genus _Ostracion_, the species

of which he was unable to ascertain. It is probable that the school of

medusæ set in motion certain animals which are eagerly sought after by

the Ostracions.

The Pilot is a fish of which much has been recorded; fishing for it is

one of the principal recreations of sailors during their long voyages.

Some assure us that it snaps off the bait, without touching the

murderous hook which threatens the shark; and as it never quits its

companion, others have supposed that it lives on the morsels 11

abandoned by it. Neither of these suppositions is correct; and as the

shark does not need its services to point out the danger, we must

content ourselves with mentioning this curious association without

endeavouring to explain it.

In fact, we have had the opportunity of examining many well-preserved

specimens, the stomach of which contained potato parings, the

carapaces of crustaceans, the _débris_ of fishes, marine plants

(fuci), and a piece of _cut_ fish, which had evidently served as

a bait. The pilot does not, therefore, live on the leavings of his

companion, but on his own industry, and doubtless finds some advantage

in piloting his neighbour. Through the great kindness of Dr. Gunther

we have been able to make this interesting examination in the rich

galleries of the British Museum. We desire to take this opportunity of

expressing our gratitude to this learned man and to his illustrious

colleagues, who have the direction of that vast establishment, which

is ever open to those who labour for the advancement of science.

The pilot has sometimes been confounded with a very different fish,

which does not merely remain in the neighbourhood of the shark, but

establishes itself upon him, and moors himself to him by the aid of a

particular apparatus, for a longer or shorter time; we may even say

during the whole of the voyage. This is the Remora.

Is this fish the messmate of the shark to which he is attached? As in

the case of the pilot, an examination alone could decide the question.

We have opened at the British Museum the stomachs of several remoras

of different sizes, and we have been able to ascertain that they also

fish on their own account; their food was composed of morsels of 12

fish which had served as bait, of young fish swallowed whole, and of

some remains of crustacea. The remora is simply anchored to his host,

and asks from him nothing but his passage. He is contented, like the

pilot, to fish in the same waters as the shark which transports him.

Sailors, even now, are convinced that if any one of these remoras

should attach itself to the ship, no human power could cause it to

advance, and that it must of necessity stop. It is certain that the

fishermen of the Mozambique Channel take advantage of this faculty, to

fish for turtles and certain large fish. They pass through the tail of

the remora a ring to which a cord is attached, and then send it in

pursuit of the first passer-by which they consider worthy to be

caught. This kind of fishing resembles in some degree the sport of

hawking with falcons.

So extraordinary a being could not fail to attract the attention of

those among the ancients who were students of nature. Pliny assures us

that the remora was used in the preparation of a philtre capable of

extinguishing the flames of love.

There must be many free animal messmates among insects, and

entomologists should make them known; for example, many of them live

with ants, as the _Pselaphidæ_ and _Staphylinidæ_. Certain

hairs of these insects, it is said, secrete a sweet liquid of which

ants partake greedily. If we may believe a skilful observer, Mons.

Lespès, there are some among them, as the Clavigers, which in exchange

for the services which they render are fed by the ants themselves. We

may also mention the larvæ of the _Meloë_, which seem to live as

parasites, and the true nature of which was so long unknown.

The females of the _Meloë_ lay their eggs near the ranunculus and 13

other plants whose flowers are regularly visited by bees. After these

are hatched, the larvæ ascend into the flowers and wait patiently till

a bee takes them on his back, and carries them into the interior of

the hive. This insect was formerly known under the name of the

bee-louse, but this appellation is improper, for the bee is not the

host of the meloë, but simply its beast of burden. According to recent

observations, flies perform the same office for _Chelifers_, and

certain aquatic and land coleoptera for several kinds of acaridæ.

In the class of animal messmates we find also a coleopterous insect

that lodges in a manner similar to the paguri, of which we shall

presently speak. The female of the _Drilus_, a species allied to

glowworms, attacks the snail, and when it has devoured it, instals

itself in the shell, to pass through its metamorphoses; when

necessary, it frequently changes its shell and chooses successively

more spacious lodgings. Like a true Sybarite, the drilus weaves a

curtain of tapestry before the entrance of its habitation, and remains

there peaceably surrounded by the vestment of its youth.

Remarkable examples of free messmates are found more especially among

crustaceans. It is well known that this class includes lobsters,

crabs, prawns, and those legions of small animals which serve as the

police of the sea-shore, purifying the waters of the ocean of all

organic matters, which otherwise would corrupt them. They do not, like

insects, shine with variegated colours; their forms are hardy and

varied, and they are often pleasing on account of the singularity of

their movements. Professor Verrill has recently studied some of 14

these creatures, and has clearly shown how interesting they are, not

only to naturalists, but to people in general.

Crustaceans and worms furnish the greatest number of paupers and

infirm individuals; and a great many of them need the continual

assistance of their neighbours to enable them to get their living.

While other animals advance towards perfection as they grow older, it

is far different with many crustaceans, and we should be tempted to

refer to the vegetable kingdom many of them at the very period when

they are approaching the adult condition. Cuvier placed all the class

of cirrhipedes among the mollusca, and the lernæans among the worms.

Many of these animals which are but indifferently adapted to live

without help from others, have recourse to benevolent neighbours; from

one they seek only shelter, from another a part of his booty, from a

third both an asylum and protection. They are often reduced to a mere

skin; everything else has disappeared, and there remains no proper

organ except that which is necessary for the reproduction of the

species. Corpulent, blind, impotent, legless cripples, their existence

is more precarious than that of those miserable mutilated beings found

in our cities; they only live on the blood of the neighbour which

gives them an asylum. Yet when they first quit the egg they are all

free; they frisk, they swim with the rapidity of lightning, and at the

close of life we find them deformed, and crouched in some living

refuge, as if a foul leprosy had atrophied within them all the organs

which served as a means of communication with the outer world.

Parasites and messmates, furnished at first with the same kind of

limbs and the same habits, can sometimes only be distinguished from 15

each other when we have made our observations on them in their first

swaddling clothes. The child has given a clue to the history of the

old man.

We will not examine these animals in all the details of their private

life, and yet we are strongly tempted to confess to our readers some

of the indiscreet acts of which we have been guilty, in watching them

while changing their dress. Notwithstanding their shyness and their

desire to escape observation during the moulting period, we have more

than once made observations on them while quitting their garment which

has become too small. The old tunic generally splits down the back,

and falls off all in one piece as it gives the animal egress. The

crustacean is extended quite soft and supple by the side of its rigid

carapace.

Of all the free crustacean messmates, one of the most interesting,

though among the smallest of them, is a tiny crab, about as large as a

young spider, which lives in mussels, and which has been often

accused, though evidently wrongfully, as the cause of the

indisposition so well known by those who are fond of this mollusc.

Very many of them have been seen within the last few years, and yet

accidents have been very few. The mussels themselves are guilty; they

produce on some persons an injurious effect, through _idiosyncracy_.

We have at least a word to serve as an explanation, and at present we

must content ourselves with it.

Under what conditions do those crabs, called by naturalists

Pinnotheres, and which we do not find elsewhere, inhabit mussels? Are

they parasites, pseudo-parasites, or messmates? It is not a taste for

voyaging which tempts them, but the desire of having always a secure 16

retreat in every place. The pinnothere is a brigand who causes himself

to be followed by the cavern which he inhabits, and which opens only

at a well-known watchword. The association redounds to the advantage

of both; the remains of food which the pinnothere abandons are seized

upon by the mollusc. It is the rich man who instals himself in the

dwelling of the poor, and causes him to participate in all the

advantages of his position. The pinnotheres are, in our opinion, true

messmates. They take their food in the same waters as their

fellow-lodger, and the crumbs of the rapacious crabs are doubtless not

lost in the mouth of the peaceful mussel. There is no doubt that these

little plunderers are good lodgers, and if the mussels furnish them

with an excellent hiding-place and a safe lodging, they themselves

profit largely by the leavings of the feast which fall from their

pincers. Little as they are, these crabs are well furnished with

tackle, and advantageously placed to carry on their fishery in every

season. Concealed in the bottom of their living dwelling-place (a den

which the mussel transports at will) they choose admirably the moment

and the place to rush out to the attack, and always fall on their

enemy unawares. Some of these pinnotheres live in all seas, and

inhabit a great number of bivalve molluscs. The northern seas contain

a large species of Modiola (_Modiola Papuana_) which is especially

found in deep and almost inaccessible parts, and which always encloses

a couple of pinnotheres about the size of a hazel-nut. We have opened

hundreds of these modiolæ, and we have never met with any without

their crabs. We have long since deposited some specimens of these 17

pinnotheres in the galleries of the Natural History Museum at Paris.

The large mussel, which furnishes fine pearls (_Avicula

margaritifera_), lodges also pinnotheres of a particular species by

the side of another messmate more allied to a lobster than a crab. It

is not even impossible that these crustaceans, with other messmates or

parasites, contribute to the formation of pearls, since these gems, so

highly prized in the fashionable world, are only the result of

vitiated secretions, and are usually the result of wounds.

We also meet with a little crab (_Ostracotheres tridacnæ_, Ruppel) in

the acephalous mollusc, whose immense shell sometimes serves as a

vessel for holy water; and it lives doubtless in many other bivalves

which have not yet been examined.

Dr. Léon Vaillant has written a very interesting memoir on the

Tridacnæ, and informs us that the crab takes shelter in their

branchial chamber. Therefore, since the molluscs live only on

vegetable substances, while the Ostracotheres feed entirely on animal

matter, Mons. Vaillant supposes that the latter take their choice of

the food as it enters, and seize on its passage that which suits them

best. Mr. Peters, during his abode on the coast of Mozambique, studied

a great many of these acephala and pearl-mussels, and found their

interior inhabited by three crustacean decapods, a pinnothere, and two

macrouræ allied to the _Pontonia_, to which he has given the name of

_Conchodytes_; the _Conchodytes tridacnæ_ inhabits the _Tridacna

squamosa_; the _Conchodytes meleagrinæ_, as its specific name

indicates, lives in the shell of the pearl-mussel.

Professor Semper has recently observed pinnotheres in holothurians at 18

the Philippine Isles, and Mons. Alphonse M. Edwards has described some

from New Caledonia (_P. Fischerii_); so that these little crabs,

the friends of the molluscs, are known in both hemispheres.

Do not these conditions seem to authorize the conclusion that the same

thought has presided over the appearance of all living creatures; that

they have all come into existence, not according to the chance

arrangement of surrounding media, but according to the laws

established from the very origin of all things?

The shell which lodges both these pinnotheres, in the Mediterranean as

well as the Atlantic, is a large acephalous mollusc, known under the

name of _Jambonneau_ (a small ham or gammon), and which, according to

Aristotle, harbours two different kinds of messmates. This illustrious

natural philosopher also described a Pontonia (_Pontonia custos_,

Guérin--_P. Pyrrhena_, M. Edw.) about an inch and a half long, of a

pale rose colour, more or less transparent, and which lives with its

companion, the pinnothere, in the cavity of the _Pinna marina_. This

is the same animal which a naturalist of the last century named the

_Cancer custos_.

We have wished to ascertain whether Pliny knew these crustaceans. He

has spoken of them in the following terms:--"The Chama is a clumsy

animal without eyes, which opens its valves and attracts other fishes,

which enter without mistrust, and begin to take their pastime in their

new abode. The pinnothere seeing his dwelling invaded by strangers,

pinches his host, who immediately closes his valves, and kills one

after another these presumptuous visitors, that he may eat them at his

leisure."

Cuvier did not believe that the pinnothere brought any food to the 19

mollusc, since the latter, in his opinion, lives entirely on

sea-water.

Other zoologists regard the pinnothere as an intruder whom chance has

brought into this mysterious position. Others again consider mussels

as acquaintances possessed of a very curious disposition, and that

having no eyes, they have interested in their fate this little crab,

which is perfectly provided with eyesight. In fact, in common with

other crustaceans of his species, he carries on each side of his

carapace, at the end of a movable stalk, a charming little globe,

provided with some hundreds of eyes, which he can direct upon his

prey, as the astronomer turns his telescope on any point of the

firmament. These later naturalists consider, in fact, their crab as a

living journal which supplies his host with the news of the day.

Rumphius, a Dutchman, the first who described the animal of the

nautilus, also understood the habits of pinnotheres. In his "Amboinche

Rariteit Kamer," published in 1741, he says that these crustaceans

inhabit always two kinds of shellfish, the _Pinna_ and the _Chama

squamata_. According to him, when these molluscs have attained their

growth, one pinnothere (one only at least in the Chama) lives in their

interior and does not abandon its lodging till the death of its host.

Rumphius regards this crustacean as a faithful guardian, fulfilling

the duties of a door-keeper. In 1638 he found actually two sorts of

keepers: by the side of a Brachyuron, carrying an embossed buckler,

slender in front, he discovered a Macrouron of the length of his

finger-nail, of a yellowish orange colour, semi-transparent, with

white and very slender claws. It is without doubt the same animal 20

that Mons. Peters, of Berlin, found on the coast of Mozambique, and of

which we have spoken before.

A little crab is known to live near the coast of Peru (_Fabia

Chilensis_, Dana), which exists under somewhat different conditions.

He chooses, not a bivalve mollusc, but a sea-urchin (_Euriechinus

imbecillus_, Verrill), and lodges in the intestine, near its

termination, so as to seize as they pass by all those living creatures

which are attracted by the odour. Doubtless, the delicacy of our sense

of smell is disgusted by such a mode of seeking food; but this

predilection may have a reason with which we are not acquainted. There

are a considerable number of other species which live under similar

conditions.

On the coast of Brazil, my son found two couples of crabs in the tube

of a very long annelid, narrow at the ends, and wide in the middle.

The tube was too small at the end to allow them to escape. These

crustaceans had, no doubt, penetrated thither before they had attained

their full size.

A crab of the family of the Maidæ conceals itself in the substance of

a polypidom very common in the Viti Islands, in company with a

gasteropod mollusc, and both of them assume the exact colour of the

polypidom. This is a new kind of _mimicry_. This crab is known by the

name of _Pisa Styx_, the gasteropod is a _Cypræa_, the polyp is the

_Melithea ochracea_. A decapod crustacean, the _Galathea

spinirostris_, seeks for a _Comatula_, the colour of which it exactly

imitates, and with which it lives on the most friendly terms.

The holothuriæ, of which we have already spoken, appear to afford an

abode to many animals: independently of the _Fierasfer_, the 21

_Holothuria scabra_ of the Philippine Islands regularly lodges in

its interior a couple, and sometimes, though rarely, a greater number

of pinnotheres belonging to two distinct species. They choose this

domicile at an early period, and must be highly delighted with this

obscure abode, since they are seen no more, and when they have once

entered never quit this living cavern. This observation is due to

Professor Semper, who has made us acquainted with so many curious

facts of the China Sea and the Pacific Ocean. In the midst of the

slender branches of a coral of the Sandwich Islands, the _Pæcilopora

cæspitosa_ of Dana, there lives a little crab (_Hopalocarcinus

marsupialis_, Stimpson), which is at last completely enclosed by

the vegetation of the coral. It only keeps up sufficient communication

with the exterior to enable it to procure food. The coral, however,

furnishes it nothing but a resting-place in the midst of its tissues.

Among the Philippine Islands, also, a brachyurous crustacean lives in

the branchial cavity of one of the _Haliotidæ_, and another on the

body of a holothuria. On the coasts of Brazil, F. Müller, during his

abode at Desterro, saw some _Porcellanæ_ inhabiting star-fish, not as

parasites, as had been supposed, but as true messmates. A crustacean

possessed of but little generosity is the _Lithoscaptus_ of Mons.

Milne-Edwards. Provided with beak and claws for the purpose of attack,

it instals itself, sad to say, in the pantry of a medusa, and instead

of making use of its own weapons, takes advantage of the perfidious

nematocysts of its acolyte, in order to live quietly at his expense.

Under the name of _Asellus medusæ_, Sir J. G. Dalyell has made 22

us acquainted with another messmate of the medusæ which greatly

resembles an _Idothea_.

Another kind of commensalism is that of the Dromiæ. These crabs are of

the ordinary size, and lodge, from their earliest youth, under a

growing family of polyps, which increases with them. This colony has

for its principal foundation a living Alcyonium, which covers the

carapace, and as it develops, adapts itself perfectly to all the

inequalities of the cephalothorax; one might consider it an integral

part of the crab. Sertulariæ, Corynes, Algæ, develop themselves on

this Alcyonium, and the Dromia, masked by this living rock which it

carries on its shoulders like the fabled Atlas, marches gravely in

pursuit of her prey. She has no fear of arousing the attention of her

enemies. The greatest vigilance cannot prevent the sudden attack of

these dangerous neighbours. There is in the Mediterranean a species

which sometimes comes to our coast. They are also known in the Indian

Seas and in the Northern Pacific. Rumphius named the dromia _Cancer

lanosus_; it is, said he, a crab which carries grass or moss on its

back. It is also mentioned by Renard. Dana has observed a sea-anemone

covering a crab in the same manner as the Alcyonium does the dromia,

and which is not less dangerous. The mode of life of this anemone has

procured for it the name of _Cancrisocia expansa_. In the north

of California, a crab (_Cryptolithoides typicus_) covers itself

in the same manner with a living cloak which hides it from view, and

under cover of which it surprises those whom it attacks. It has

already cleared the ground of its prey before any alarm has been given

to the neighbourhood.

We should perhaps speak here of an association of another kind, the 23

nature of which it is difficult to ascertain; I refer to the little

crab, the Turtle Crab of Brown, which is met with in the open sea on

the carapace of turtles, and sometimes on sea-weeds. It may be

supposed that it takes advantage of the carapace of its neighbour, in

order to transport itself at little expense into different latitudes,

and it is asserted that the sight of this crustacean gave confidence

to Christopher Columbus, eighteen days before the discovery of the New

World. Besides this animal, a whole society chooses this movable

habitation: in addition to the cirrhipedes we also find the

_Tanaïs_, which is not, however, condemned to live there always.

The macrourous decapods are more rarely found as messmates, but still

a Palæmon is sometimes seen on the body of an Actinia, according to

Semper, and another in the branchial cavity of a Pagurus. But that

which is more generally known, is the presence in the _Euplectella

aspergillum_ of the palæmon which lodges in this fairy palace. It is

probable that the Euplectella of the Atlantic, recently observed near

the Cape Verd Islands by the naturalists on board the _Challenger_,

also conceals this crustacean in its interior. We may also allude here

to the _Hypoconcha tabulosa_, a crab whose carapace is too soft to

allow it to venture out undefended, and which covers itself with the

shell of a bivalve mollusc.

Among the various associations of this kind, none is more remarkable

than that of the soldier-crabs, so abundant on our coasts, and called

by the names of _Bernard the Hermit_ and _Kakerlot_ by the Ostend

fishermen. It is well known that these crabs are decapod crustaceans,

very like miniature lobsters, which lodge in deserted shells, and 24

change their dwelling-place as they grow larger. The young ones are

content with very little habitations.

The shells which give them shelter are such as have been shed, which

they find at the bottom of the sea, and in which they conceal their

weakness and their misery. These animals have an abdomen too soft to

bear the dangers which they meet with in their warfare, and that they

may be less exposed to the claws of their numerous enemies, they take

shelter in a shell which serves at the same time both as a dwelling

and a buckler. Armed cap-à-pie, the soldier-crabs march boldly on the

enemy, and know no danger, since they always have a secure retreat.

But this animal does not live alone in this asylum. He is not so much

of an anchorite as he appears to be, for by his side an annelid

usually instals himself as a messmate, which forms with the Pagurus

one of the most terrible associations that are known. This annelid is

a long worm, like all the nereids, whose supple and undulating body is

armed along its sides with arrows, lances, pikes, and poniards, the

wounds of which are always dangerous. It is a living panoply which

glides furtively into the enemy's camp without giving the alarm.

When a pagurus is on the march it resembles a nest of pirates, who

never cease their exploits till all has been ravaged around them. This

shell is so innocent in its appearance, that it introduces itself

everywhere without provoking the least suspicion. It is usually

covered with a colony of Hydractiniæ, and in the interior,

Peltogasters, Lyriopes, and other crustaceans often establish 25

themselves. The paguri are not messmates of an ordinary kind, for they

inhabit only a deserted shell. They are spread over all seas. They are

found in the Mediterranean, the Northern Sea, on the coasts of the

Pacific, of New Zealand, and of the East Indian islands: thirty

species and even more have been inserted in the catalogue of

crustaceans.

Naturalists have given the name of _Cenobitæ_ to some pagurians

inhabiting the seas of warmer latitudes; these have an abdomen like

the pagurus, antennæ like the _Birgus_, and like it they inhabit

shells. The _Cenobita Diogenes_ is a species found in the Antilles.

Other pagurians, the _Birgi_, grow very large, and conceal their

abdomen no longer in a shell, but in the crevices of the rocks, as

lobsters do at the moulting time, to protect their body while deprived

of their defensive armour. In the East Indies they remain on land, and

even climb into trees. They have so much strength in their pincers,

that Rumphius relates of one of these crustaceans, that, while

stretched on a branch of a tree, it raised a goat by the ears.

Side by side with the pagurians which instal themselves in a shell

with thick and completely opaque walls, we recognize crustaceans of

the order of amphipods, the _Phronimæ_, which choose for themselves

not an abandoned hovel, but a veritable crystal palace, and take

possession of it without inquiring whether or no it is inhabited. The

daylight penetrates through the walls of their dwellings, and it can

scarcely be discerned in the water whether or no their body is

protected by a covering. They usually take the dwelling of a Salpa, a

Beroë, or a Pyrosoma, and from within this lodging they give 26

themselves up to the pleasures of fishing.

The _Phronima sedentaria_ which lodges with the salpa seems to be

scattered over the warm seas of both hemispheres. For the honour of

the species, the females alone seek the assistance of their

neighbours, without at the same time abandoning their characteristic

robe. The sexes differ little from each other except in size, in the

abdomen, and in the antennæ. Maury has described certain amphipod

crustaceans which also inhabit the Salpæ.

Another phronima described by Professor Claus, the _Phronima

elongata_, lives in the same manner; but instead of occupying a

living house, it generally seeks an empty lodging, in which it

establishes itself like a pagurus.

The "Bernard the Hermit" of the Marseillaise fishermen, the _Pyades_,

becomes the messmate of an anemone which Dugès has called _Actinia

parasitica_. According to the observations of the learned professor at

Montpelier, the mouth of this anemone is always situated opposite to

that of the crustacean, to take advantage of the morsels which escape

from his pincers. Both of them profit by this association; and the

opening of the shell is prolonged by a horny expansion furnished by

the foot of the actinia.

On the coast of England lives another soldier-crab (_Pagurus

Prideauxii_), which has as its principal messmate a sea anemone

called _Adamsia_, which Mons. Greeff found at the island of

Madeira. This pagurus is especially remarkable for the good

understanding which exists between himself and his acolyte--he is a

model Amphitryon. Lieut.-Col. Stuart Wortley has watched it in its

private life, and thus relates the result of his observations: this 27

animal after he has fished, never fails to offer the best morsels to

his neighbour, and often during the day, ascertains if it is not

hungry. But more especially when he is about to change his dwelling,

does he redouble his care and his attention. He manoeuvres with all

the delicacy of which he is capable, to make the anemone change its

shell; he assists it in detaching itself, and if by chance the new

dwelling is not to its taste, it seeks another until the _Adamsia_ is

perfectly satisfied. This association is not confined to the union of

a decapod with a nereid and an actinia; a curious cirrhipede often

establishes itself on the body of the pagurus, and on the outside of

the shell we generally find a colony of polyps, of a rose or yellow

colour, which extend like a living carpet round this habitation.

Thirty-six years ago we have given the name of _Hydractinia_ to these

polyps, which were till then entirely unknown to naturalists, and

which form habitually a double overcoat for the paguri, if I may

employ the expression of my learned colleague, Mons. Ch. Desmoulins.

In the Mediterranean lives the _Perella di mare_ of the Italian

fishermen, the _Reclus marin_ of the Marseillaise; this Alcyonium

ought, by its manner of life, to be placed near the Hydractiniæ, and

has been carefully studied by Mons. Ch. Desmoulins. It is the

_Alcyonium_ (_Suberites_) _domuncula_ of Lamarck and Lamouroux.

The abdomen of these paguri is not only sheltered in a shell, but

habitually visited by isopod crustaceans, described under the names of

_Athelea_, _Prosthetes_, and _Phryxus_, which have entirely lost the

livery of their order.

In the same association we also find the _Liriope_, a little isopod 28

crustacean, of which much has been said, but which for a long time

obstinately resisted all attempt at observation.

This latter personage is an isopod crustacean, of moderate size, which

chooses the Peltogaster as a place of abode, after having undergone a

very curious regressive metamorphosis. In fact, the young lyriope has

at first its little feet like other isopods, but in the adult state,

the female loses her antennæ, and changes her buccal as well as her

branchial appendages, so as to assume a different appearance. Several

naturalists have already endeavoured to give the life-history of this

singular Bopyrian. The illustrious Rathke of Königsberg discovered it;

Professor Lilljeborg, of the University of Upsal, gave the first

account of it; and finally Professor Steenstrup of Copenhagen made

known its true origin. In short, the Lyriopes are Bopyrian Isopods,

living on cirrhipedes (Sacculinideæ) as real messmates, if not as

parasites; the male preserves his dignity and his prestige, but the

female strips herself of all the attributes of her sex, and descends

to the lowest degree of servitude.

Faujas de Saint-Fond has mentioned a fossil hermit-crab as found in

the mountain, St. Pierre de Maestricht; but he called by this name a

crustacean of the genus _Callianassa_ and not a pagurus. These

_Callianassæ_ are always completely isolated in the chalk, and it is

probable that they have no other domicile than the sand or ooze at the

bottom of the sea, in which they hollow out galleries for themselves.

Lobsters act in the same manner after moulting. The _Gebiæ_ live like

the Callianassæ, hidden in the mud. The _Limnaria lignorum_ and the 29

_Chelura terebrans_ dig out a retreat for themselves in wood, like the

Teredines.

We have just seen that the higher crustaceans, with their well-mounted

eyes, their enormous antennæ, and their formidable pincers, are not

all of them the great lords they pretend to be; more than one of them

has to hold out its hand and to accept humbly the assistance of its

neighbours.

In the group of isopod crustaceans we find many necessitous beings,

which, too proud to ask for food, are contented to take their place on

some fish which is a good swimmer, which they abandon as soon as their

interest demands it; if their host conducts them to regions that do

not suit them, or if they have otherwise to complain of him, they give

him up, and begin their maritime peregrinations with a fresh

colleague. They always preserve all their fishing tackle and their

sailing gear, and the female does not change her dress any more than

the male. We have to notice that these crustaceans often identify

themselves so entirely with their host that they seem to be a portion

of him, and even to assume his peculiar colour. This is not a sign of

servility, but a means of passing unobserved, and of escaping from the

sight of the enemy that is watching them. Naturalists have given the

name of _Anilocræ_ to some of these free messmates.

Any one who has remained for some time on the coast of Brittany,

especially at Concarneau, and who does not look with indifference on

the many superb fishes which are taken every day, cannot fail to have

been struck with the presence of a rather large crustacean,

which clings to the sides of several kinds of _Labra_, especially 30

the smaller species. This crustacean is an Anilocrian so common that

we can scarcely imagine it to have escaped the attention of any

naturalist. Nevertheless, no work makes mention of the regular

attendance on the Labra by the Anilocra, which bears, we know not why,

the specific name of _Mediterranean_. Rondelet was probably acquainted

with it, when he spoke of the fish-lice, which do not derive their

birth from these fishes, but from the sea mud. We often see males by

the side of females on the same individual.

Some years ago a school of large cetaceans, known under the name of

Grindewhalls or Globicephalæ were pursued in the Mediterranean, and

those which were captured contained in the cavity of their nostrils,

isopods closely allied to the _Cirolana spinipes_, if not identical

with it. Till then the isopods had only been found on sea fishes;

fresh-water fish are not, however, entirely exempt; in fact, a species

of OEga (_OEga interrupta_ of Martens) has just been found on the skin

of a fresh-water fish of Borneo, the _Notopterus hypselonotus_. This

same genus includes a species (_OEga spongiophila_) which lives in the

magnificent sponge, the _Euplectella_. We know also a certain number

of isopods which prefer the interior of their neighbour's body, and

instal themselves in the cavity of the mouth, either to fish at the

same time as their host, or to seize the food on its passage; others

are of such a cruel nature, that they make no scruple to establish

themselves in the stomach of a peaceable white fish. Without injuring

any important organ, they penetrate in couples between the intestines,

and, concealed in this retreat, they seize by the narrow entrance door, 31

which they keep half open, all the little animals which are

sufficiently bold to pass by. The cruelty of these beings knows no

bounds. To instal themselves conveniently, they pierce the body of

their host, skilfully open his stomach, and live there as Sybarites;

their lodging is in future assured to them, and their fate is bound up

with that of their host. Dr. Herklots, who has unfortunately been

recently lost to science, communicated in 1869, to the Academy of the

Netherlands, a very interesting memoir on two crustaceans of a new

species, the _Epichtys giganteus_, which lives on a fish of the Indian

Archipelago, and the _Ichthyoxenus Jellinghausii_, which lodges in a

fresh-water fish of the Island of Java. It is to the latter that we

refer here, and it seems that in this species we are approaching the

limits at which commensalism commences.

The _Cymothoes_ constitute another category of very interesting

Isopods; they lodge with their female in the cavity of a fish's mouth.

Dr. Bleeker, who has so successfully explored the Indian seas,

obtained more than twenty species of these; but unfortunately he has

not made a note of the fishes which harbour them. He has, however,

made one exception with regard to a fish from the roadstead of

Pondicherry, which is two feet long, and is called a Bat. It is known

to naturalists under the name of _Stromatea Nigra_; its flesh is much

esteemed, and it carried in its mouth a Cymothoe called by Dr. Bleeker

_Cymothoe Stromatei_. A cymothoe has also been observed in the mouth

of an Indian Chetodon. De Kay found one in a Rhombus in the United

States, and De Saussure saw another at Cuba; and lately, Mons. Lafont

discovered one in the Bay of Arcachon, on the _Boops_, and on the 32

_Trachina vipera_. These cymothoes are about fifteen millimetres in

length, and often fill all the cavity of the mouth. The most curious

of all is that which is found in the mouth of the flying-fish, a kind

of herring with elongated fins, which it uses as wings to rise into

the air, when too closely pursued in the water. My son, when examining

these fishes, in his passage from Cape Verd to Rio de Janeiro, found

in the cavity of their mouth an enormous female, firmly wedged in the

branchial arches, with its head inclined outwards, and the male, which

was rather smaller, installed at her side. Their dwelling thus by

pairs, as well as the entire conformation of the animal, plainly shows

that these crustaceans make themselves at home, and live as true

messmates. Cunningham has given them the name of _Ceratothoa exoceti_.

A short time since, these Cymothoes were only known on marine fishes,

but it appears from recent observations, that fresh-water fish are far

from being exempt from them. Mons. Gertsfeld has recently noticed some

on the _Cyprinus lacustris_ of the river Amour, and another in the Rio

Cadea in Brazil, on a _Chromida_. Other isopods also resort to fishes,

and to animals of their own class, but they live as true parasites,

and change their form as soon as they have chosen a resting-place. We

shall return to this subject again. Some which are very common on

prawns, are known under the name of Bopyrus.

An interesting division of amphipods have received the name of

_Hyperinæ_. These crustaceans generally swim with facility, but walk

with difficulty. They therefore usually have recourse to fishes, or

even to medusæ, in order to gain support. We find on our own coasts

the _Hyperina Latreillii_, lodged in the superb _Rhizostoma_, which 33

regularly appears in the later season of the year on the coast of

Ostend; and a long time since, in 1776, O. F. Müller gave to a species

of this genus the name of _Hyperina medusarum_. Mr. Alexander Agassiz

once found a _Hyperina_ on the disc of an _Aurelia_. The medusa, when

extended, forms for them a balloon with its parachute, which supports

and conveys them with greater or less rapidity. Professor Möbius has

but lately remarked the presence of _Hyperina galba_, Mont., in the

_Stomobrachium octocostatum_, Sars, a small species of medusa which

appears in the Bay of Kiel in October and November. This naturalist

supposes that these messmates at first inhabited the _Medusa aurita_,

and then migrated into this species.

Besides these, there are _Gammari_, which, according to Semper, live

in the _Avicula meleagrina_ (pearl mussel), and are perhaps the

principal manufacturers of fine pearls. The immense buccal cavity of

the fishing-frog (_Lophius piscatorius_) is the abode in the

Mediterranean of an _Apterychta_, and in the Northern Ocean of a

curious amphipod of the ordinary size of the _Gammarus_, which takes a

voyage without expense, and with no fear of wanting provisions. My son

discovered it at Ostend, and proposes the name of _Lophiocola_ to

distinguish it. The Gammari give lodging themselves to a great

quantity of parasites, which they must introduce into the bodies of

those to whom they serve as food. It has been long known that whales

have lice, to which naturalists have given the name of Cyami. They are

found on the whales of both hemispheres, and on some other cetaceans.

It is very remarkable that they are seen on the true whales of 34

the north and of the temperate regions, on the _Megaptera_, and on

several _Catodonta_, and that none are found in the _Balenoptera_. Mr.

Dall has just noticed some on the singular _Grey Whale_ of California.

In general, we may say that each cetacean which harbours them, has its

own species. Are they parasites or messmates? If we are to believe

Roussel de Vauzème, they feed on the skin itself of the whale, the

remains of which, it is said, are found in their stomach. According to

this naturalist, the parts of the mouth are not adapted for suction,

and the stomach contains ruminating apparatus. We think that a fresh

examination is necessary before this question can be determined. The

_Cyami_ seem to us to live on the whale, as the _Arguli_ and the

_Caligi_ do on fish; and if these living creatures derive their

nourishment only from the mucous products secreted by the skin, we may

ask whether they ought not to be classed in a separate category, for

they ought not to figure on the list of paupers. We have found the

orifice of the _Tubicinella_ covered with cyami of every age, and

their abundance in this place seems to indicate that their food was

not supplied to them by the skin of their host. Mons. Ch. Lutken has

recently published a very interesting monograph on these curious

animals; according to him the _Cyamus rhytinæ_, which was thought to

proceed from a piece of the skin of a _Stellerus_, appears to have

been found on the skin of a whale.

The Picnogonons, the nature as well as the kind of life of which has

been so long time problematical, deserve to be ranked among messmates,

at least during their youth; in fact, after being hatched, they live on

the _Corynes_, the _Hydractiniæ_, and other polyps, while at a later 35

period they frequent molluscs or higher classes; Allman mentions the

case of a _Phoxichilidium coccineum_ lodged in a _Syncoryne_.

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.