THE VERMES AND MOLLUSCA OF THE DEEP SEA
It has not been my intention in this volume to confine my attention to
the truly abysmal forms, but rather to consider all those animals living
in deep water that show any characters strikingly different from their
relatives living in shallow water.
The term deep water is, after all, only a relative one.
To one accustomed only to shore collecting, ten fathoms is deep water,
while on the other hand, to such naturalists as those on board the
‘Challenger,’ who are accustomed to dredge in all seas, nothing under
1,000 fathoms is considered deep water.
We must bear in mind, however, that at a depth of only 200 fathoms, the
conditions of life are very different to those of the shore waters. We
find a very great diminution in the amount of light, for instance, that
can penetrate through sea water teeming with floating organisms of all
kinds to reach the fauna attached to the bottom at such a depth. The
diminution in the amount of light must mean a diminution in the rapidity
of growth of chlorophyll-bearing plants, and consequently a diminution
in the food supplies of animals drawn from that source.
We might expect then to find, even in such shallow water as this, some
forms of particular interest. It is true that the greater part of the
fauna is made up of ordinary shallow-water forms that have migrated
quite recently, and perhaps only temporarily, into the depths, but we
expect to find, and actually do find, the outposts of a new fauna.
These remarks lead me to the consideration of one or two very remarkable
animals that have recently been brought to light.
In that strange assembly of animals which, for want of a better word,
the authorities call the Vermes, there are three groups whose relations
to one another and to the other groups of Vermes have been and still
remain a puzzle to naturalists.
These three groups are the Gephyrea, the Polyzoa, and the Brachiopoda.
In external form they are as different from one another as possible.
The Gephyrea are solitary worm-like forms burrowing in the sand or
perforating rocks; the Polyzoa are minute creatures that frequently
build up by budding large colonies which assume in some cases dendritic
forms like corals, and the Brachiopoda are protected by thick bivalve
shells simulating in a striking manner the shells of the
Lamellibranchiate mollusca.
But external form is not the only character that can be relied upon for
purposes of classification. The general and minute anatomy, together
with the story of the development of these animals, teach us that they
are in some way closely related.
It is not within the scope of this book to enter into the discussion of
what these relations are; suffice it to say that the controversy has
within recent years to a great extent turned upon the position in our
classification of three interesting genera. These are Phoronis,
Rhabdopleura and Cephalodiscus.
Phoronis occurs only in shallow water, Rhabdopleura has been found in
water from 40 to 200 fathoms deep off the Shetlands and on the Norwegian
coasts, while Cephalodiscus was discovered by the ‘Challenger’ at a
depth of 245 fathoms off Magellan Straits.
Rhabdopleura forms colonies consisting of branched tubes growing upon
the tests of Ascidians, on sea-weeds, corals, or other objects fixed to
the sea bottom. In the open, free extremity of each of the branches may
be found the polypide attached to a filament or stalk which connects it
with the other polypides of the colony (fig. 12).
[Illustration: FIG. 12.—A portion of a colony of _Rhabdopleura normani_.
(After Lankester.)]
Each polypide is provided with a single pair of large pinnate arms,
resembling the arms of a Brachiopod, and a broad muscular epistome by
means of which it is able to creep up or down the tube.
The affinities of this interesting creature are by no means sufficiently
well understood. It is one of those forms that, without being, strictly
speaking, a connecting link between large and well-known groups of
animals, indicates to us some of the lines of evolution that these
groups may have passed through; and, in so far as it does this, it has
its value and importance.
Cephalodiscus, though related to Rhabdopleura in the presence of a
structure corresponding to the arms, and a broad epistome, seems to be
more closely connected with such a form as Balanoglossus in the presence
of a single pair of gill-slits, a small rudimentary notochord and the
position of the central nervous system.[2]
Footnote 2:
A rudimentary notochord projecting forward from the buccal cavity into
the epistome has quite recently been discovered in Rhabdopleura.
[Illustration: FIG. 13.—A single polypide of _Rhabdopleura normani_. M,
mouth; B, epistome; S, polypide stalk. (After Lankester.)]
Whatever position these genera may ultimately occupy in our systems of
classification, there can be little doubt that much valuable information
will be obtained by a further study of their structure and
development—information that will probably shed much light on the
relationships to one another of the many groups of Vermes. Their
occurrence in water of moderate depths only indicates perhaps that they
are gradually being crowded out from the more favourable localities of
shallow water, and are tending towards extinction on the one hand, or a
deep-sea habitat on the other.
The Brachiopoda need not detain us long. Some species are capable of
existing at a great variety of depths without any observable
modification of shape or characters. Thus _Terebratulina caput
serpentis_ has the extraordinary bathymetrical distribution of 0–1,180
fathoms, and _Terebratula vitrea_ 5–1,456 fathoms. _Atretia_ is the only
genus peculiar to deep water. It is a noteworthy fact in connection with
this order that the two genera, _Lingula_ and _Glottidia_, which compose
the sub-order Ecardines, are both confined to shallow water. Now the
Ecardines are anatomically, at any rate, the most primitive of the
Brachiopoda, and Lingula has the most ancient geological history of any
living genus of the animal kingdom, shells almost identical with those
of the living species being found abundantly in the Cambrian strata. Why
it is that Lingula has been able to maintain itself almost unchanged
through all the countless generations that have elapsed since Cambrian
times, and can now flourish amid the desperate struggle for existence in
the shallow waters of the tropics, while its companions, the corals,
mollusks, arthropods, &c., have changed or passed away, is one of those
problems in natural history that seem to us impossible of solution. The
time may come when we shall be able to appreciate better than we do now
the complicated relations between animals and their environment, and
then perhaps the peculiar fitness of Lingula will be made manifest; but
at present we can but mention the fact as a fact, and leave the solution
of the problem to the future.
The order Gephyrea is probably another very ancient group of animals,
although in the absence of any hard calcareous, siliceous, or horny
skeleton the geological record can give us no confirmation of their
antiquity. As with the Brachiopods so with the Gephyrea, some of the
species have a very wide bathymetrical distribution. _Sipunculus nudus_,
for example, the commonest and best known of all the Gephyrea, extends
from quite shallow water to a depth of over 1,500 fathoms. As Selenka
has pointed out, it is those Gephyreans that live in holes in stones, or
in shells such as _Phascolion_ and _Phascolosoma_, that are more
frequently found at the greater depths; but, apart from this, there are
no characters that exclusively belong to the abysmal Gephyrea or are
more frequently found in them than in the shallow-water forms. Nor are
there any genera, at present brought to light, that are confined to
those regions of the sea.
The group of the Annelida is not very well represented in the deep-sea
fauna. The genera _Serpula_ and _Terebella_ have been found very widely
distributed over the earth, at all depths from the shore to the abyss,
but there do not seem to be many genera that are confined to deep water.
In some cases, where there is a scarcity of lime in the water, the thin
protecting tubes of the sedentary forms are strengthened by the adhesion
of foreign particles, such as sponge spicules and arenaceous
foraminifera, but in others, the tubes are formed of successive layers
of a transparent quill-like substance (_Nothria Willemoesii_) which is
frequently armed with spiny projections.
Most of the errant Polychætes found at great depths are said to be most
brilliantly coloured, and some of these, such as _Eunice amphiheliæ_,
have the power of emitting a bright phosphorescent light; but there seem
to be no very definite and constant characters separating these forms
from the Polychætes of shallow waters.
As is the case with many other orders of animals, the species of
Annelida living in deep water are either blind or possess eyes of a
remarkably large size. _Genityllis oculata_ may be taken as an example
of a deep-sea annelid with large eyes. This annelid, belonging to the
family Phyllodocidæ, was found at a depth of 500 fathoms in the Celebes
sea. It possesses two enormous eyes which cover almost the whole of the
head, and there can be no doubt, from the investigations of Dr. Gunn on
their minute anatomy, that they are perfectly functional.
Before leaving the Annelida a brief notice must be made of the very
extraordinary form _Syllis ramosa_, found parasitic on a hexactinellid
sponge at depths of about 100 fathoms. It is chiefly remarkable for the
very complicated manner it has of producing buds which do not
immediately become detached from the parent, but form a compound network
which ramifies through the interstices of the sponge like the colony of
a Hydromedusan.
Passing now to the sub-kingdom Mollusca, we shall find that all the
classes are represented in the abysmal fauna.
The Lamellibranchiata, or bivalves, occur in almost all depths of the
ocean, _Callocardia pacifica_ and _Callocardia atlantica_ having been
found at the enormous depth of 2,900 fathoms. Some species, such as
_Venus mesodesma_, have a very wide bathymetrical distribution, but
others are only known to occur in deep water.
Concerning the characters of the deep-sea Lamellibranchiates, Mr. Smith,
in his report on the Lamellibranchia of the ‘Challenger’ expedition,
says ‘very deep-water benthal species certainly have a tendency to be
without colour, and of thin structure, no doubt resulting from the
absence of light the difficulty of secreting lime, the scarcity of food
and other unfavourable conditions of existence.’ But notwithstanding
this, the same author continues: ‘The species are apparently few in
number in comparison with those of shallow water; and new and peculiar
generic forms which we naturally expected would have been discovered are
of even still rarer occurrence.’
As in the group of the Lamellibranchia, so in the Gasteropoda, no very
remarkable new genera or species have been found in deep sea. Some
shallow-water genera, such as _Fusus_ for example, have representative
species in the abyss; but, with the exception of a want of brilliant
coloration and marking and a thinness of the shell, the deep-sea forms
do not exhibit any characteristic features. One of the most brightly
coloured shells found at great depths is that of _Scalaria mirifica_,
which is tinted rose and white, but this seems to be quite an
exceptional character among the deep-sea Gasteropods. Several new genera
were found in the deep water, but their general characters do not call
for any special remark.
Among the Cephalopoda there seems to be little doubt that the genera
_Cirroteuthis_, _Bathyteuthis_, and _Mastigoteuthis_ are entirely
abysmal, and the same applies probably to one or two species of octopus;
but as Hoyle remarks, ‘apart from the single fact that _Bathyteuthis_
and _Mastigoteuthis_ both have slender filiform tentacles with minute
suckers, no structural features have been discovered which will serve to
diagnose a deep-sea form from a shallow-water one.’
The exact habitat of the interesting genus _Spirula_ is unfortunately
still unknown. In some parts of the tropics the shores are covered with
spirula shells, and yet the animals that secrete them are still to be
reckoned amongst the greatest rarities of our museums. The numerous
dredgings of the ‘Challenger’ only brought to light one specimen of this
animal, and that from a depth of 360 fathoms, and the ‘Blake’ caught one
at a depth of 950 fathoms, so that there can be little doubt that
_Spirula_ lives in deep water.
[Illustration: FIG. 14.—_Bathyteuthis abyssicola._ (After Hoyle.)]
It seems to be very probable that some day, when the right place and
depth are discovered, _Spirula_ may be discovered in great abundance,
but we have at present no means of judging whether this will be in very
deep water or not.
Almost precisely similar remarks apply to the distribution of the pearly
Nautilus. The shells of this Cephalopod are sometimes found in great
number on the shores of some of the islands of the Southern Pacific
Ocean and the Malay Archipelago, but the living animal is but rarely
captured. It has been asserted by some travellers that the pearly
Nautilus floats on the surface of the ocean and possesses the power of
suddenly diving to great depths on being disturbed; but it must be
remembered that Rumphius originally caught his Amboyna specimens of
Nautilus in traps set at a depth of 200 fathoms baited with sea-urchins,
and that the ‘Challenger’ captured a single living specimen off Matuku
island in 300 fathoms.
The probability, then, is that both _Nautilus_ and _Spirula_ should be
included in the deep-sea fauna, but we are still in want of a great deal
more information concerning their life and habits before this point can
be definitely determined.