_MARINE WORMS_
Some groups of animals are so well defined that the individual species
contained in them can be assigned their proper place without any
difficulty, the main characteristics by which the group is distinguished
running with more or less precision throughout the series; but,
unfortunately this is not the case with the ‘worms,’ which constitute
the sub-kingdom _Vermes_. Here we have a most heterogeneous assemblage
of animals, collectively exhibiting exceedingly wide variations in both
form and structure.
We have already referred to the sea cucumber as wormlike in form, and
this creature is only one of a large number of wormlike animals that are
not worms; and it is also a fact that a considerable number of the worms
are not wormlike. It appears as if the sub-kingdom Vermes were a kind of
receptacle into which we may throw almost any invertebrate animal that
does not readily fall in line with the general characteristics of the
other important groups; for in it we have such a varied assemblage of
creatures that, speaking of them collectively as worms, it becomes most
difficult, if not absolutely impossible, to say exactly what a worm is;
and it is a question whether the sub-kingdom ought not to be divided
into at least two or three groups of the same standing.
This being the case we can hardly give a satisfactory summary of the
characteristics of the group, and therefore it must be understood that
in our attempt to do so we unavoidably exclude some forms that belong to
it according to our present system of classification. This being
remembered, we will define worms as soft-bodied and elongated animals,
exhibiting a bilateral symmetry (that is, having appendages and organs
arranged symmetrically on each side of a plane extending from the dorsal
to the ventral surface through the centre of the body), and with the
body usually divided into a succession of segments, each of which
resembles the one preceding and following it. Though many of the worms
are generally looked upon as uninteresting creatures, of such an
unattractive appearance and with such depraved habits that they are
beneath respect, yet a study of the sub-kingdom will prove that not only
does it include a number of wonderful forms with the most marvellous
life histories, but that some of them are very beautiful objects; and
this last remark refers more particularly to many of the marine worms,
which come directly within the scope of our work.
Before passing on to the special study and classification of the marine
species, however, we must say a few words concerning the worms in
general, reminding the reader that all our statements regarding the
anatomy of the creatures may be readily verified by simple dissections
of one or two typical species, such as the common earthworm, the
fisherman's lugworm, the sea mouse, or the common horse-leech of our
fresh-water ponds. With this object in view, the animal may be killed by
immersion in spirit, then pinned out in the dissecting tray under water,
and the body-wall opened by means of a pair of sharp-pointed scissors.
The digestive tube of a worm runs completely through the length of the
body, and though there is no distinct head, there is always a mouth, and
this is often provided with horny jaws, and sometimes also with horny
teeth, with which the animal is enabled to inflict wounds on its prey.
Like the preceding sub-kingdom--the _Echinodermata_--worms possess a
system of water tubes; this system, however, is not in any way connected
with the function of locomotion, but is, in many cases at least, if not
in all, intimately associated with the process of respiration. It
consists of a series of tubes, arranged in pairs in the successive
segments, communicating with the body-cavity internally, and opening at
the exterior by means of pores in the cuticle. In some there is a highly
organised system of bloodvessels, containing blood that is usually
either colourless, red, or green, but the colour of the blood is never
due to the presence of corpuscles, as is the case with higher animals,
the tint being due to the plasma or fluid portion of the blood; and
though worms cannot be said to possess a true heart, yet they often have
one or more contractile bloodvessels which serve the purpose of
propelling the blood.
Most worms possess a nervous system, and, where this is present, it
consists of a chain of ganglia, placed along the ventral side of the
body, beneath the digestive tube, all united by means of a nerve cord,
and distributing nerves in pairs to various parts of the body; and it
may be well to note here one very important point of distinction between
the general arrangement of the central portion of the nervous system in
the worms and higher invertebrates, as compared with that of the
corresponding structure in the vertebrates:--In the former the main axis
of the system, consisting, as we have seen, of a chain of ganglia
connected by a nerve cord, is invariably placed along the _ventral_
portion of the body-cavity--the surface on which the animal crawls;
while in the vertebrates the axis of the nervous system lies along the
upper or dorsal part of the body; and, instead of lying in the general
body-cavity, in company with the organs of digestion and circulation, is
enclosed in the bony canal formed by the vertebral column. It will be
seen from this that when it is desired to examine the nervous system of
the invertebrate animal, the body-wall should be opened along the middle
of the ventral surface, while, in the vertebrate, the central axis
should be exposed from above.
Many of the vermes are parasitic, either attaching themselves to the
exterior of other animals, and deriving nourishment by sucking their
blood, or they are internal parasites, living in the digestive canal of
their hosts and partaking of the digested food with which they are
almost perpetually surrounded, or burrowing into the tissues and
imbibing the nutritive fluids which they contain; and it is interesting
to study even these degraded members of the group, if only to observe
how their physical organisation degenerates in accordance with their
depraved mode of living. In them we find no digestive system with the
exception of the simplest sac from which the fluids they swallow may be
absorbed, for their food is taken in a condition ready for direct
assimilation; and the food so obtained being readily absorbed into all
parts of their soft bodies, and being sufficiently charged with oxygen
gas by the respiration of their hosts, they require no special organs
for circulation or respiration, nor, indeed, do we find any. Further, we
find that the nervous system is often undeveloped; for since the
parasites, and especially the internal ones, are so plentifully
surrounded with all the necessaries of existence, their bodies are so
simple in construction that no complex nervous system is required to
promote or control either locomotion or internal functions. Even the
general body-cavity often disappears in these degraded creatures, the
internal organisation being of such a low type that there is no
necessity for it; and all the abundant nourishment absorbed over and
above that required for the sustenance of their simple bodies is
utilised in the reproduction of the species; consequently we find, as a
rule, the reproductive organs well represented, and the species
concerned very prolific.
It is an interesting fact, too, that these parasites, in their earliest
stage, possess organs which are present in the higher worms, but which
degenerate as they approach the adult form, thus indicating that they
have descended from more respectable members of the animal world, and
that the low physical development which they ultimately attain is the
natural result of their base mode of living.
The young marine naturalist, working on our coasts, will not be brought
into intimate contact with parasitic worms to any large extent, yet we
have said this little on parasitism to show that these degenerate
creatures are not really devoid of interest, and that they will repay
study whenever they are found. They will be more frequently met with
during the examination of the animals--usually higher types--that become
their hosts, and thus they hardly come within the scope of this work.
[Illustration: FIG. 116.--A TURBELLARIAN, MAGNIFIED
_a_, mouth; _b_, cavity of mouth; _c_, gullet; _d_, stomach; _e_,
branches of stomach; _f_, nerve ganglion; _g_ to _m_, reproductive
organs.]
The simplest of the worms are those forming the class _Turbellaria_, so
designated on account of the commotion they produce in the water
surrounding them by means of the vibratile cilia that fringe their
bodies--a characteristic that is also expressed by their popular name of
Whirl Worms. They are usually small creatures, with soft, flattened,
unsegmented bodies, though some of the larger species are really
wormlike in form, and are more or less distinctly divided into a chain
of segments. Many of them are marine, and may be seen gliding over
stones left uncovered by the receding tide with a smooth slug-like
motion, and when disturbed in a rock pool, occasionally swimming with a
similar smooth motion by the aid of their cilia. They avoid bright
light, and are consequently generally found on the under surfaces of
stones, especially in rather muddy situations, and where the stones are
covered with a slimy deposit of low forms of life. In these
turbellarians the mouth is situated on the under surface, thus enabling
the animal to obtain its nourishment from the slimy surface over which
it moves, and it is also provided with an extensile proboscis that aids
it in the collection of its food. The digestive tube is generally very
complex in form, extending its branches into every part of the soft
body; and, there being no special organs of respiration, the animal
derives all the oxygen required by direct absorption from the water
through the soft integument.
When searching for turbellarians on the sea shore one must be prepared
to meet with interesting examples of protective colouring that will
render a close examination of rocks and stones absolutely necessary.
Some of these worms are of a dull greyish or brownish colour, so closely
resembling that of the surface over which they glide that they are not
easily distinguished; and the thin bodies of others are so transparent
that the colour of the stone beneath is visible through them, thus
preventing them from being clearly observed.
Overturned stones should be examined for their flattened bodies gliding
along rapidly in close contact with the surface. They may be removed
without injury by placing a wet frond of a sea weed close to the stone,
in front of one end of the body, and then urging them to glide on to it
by gently touching the opposite end. Sometimes, however, the
turbellarians remain perfectly still when exposed to the light, in which
case they are even more difficult to detect, but a little practice will
soon enable one to distinguish them with readiness.
Allied to the turbellarians are the Spoon Worms or Squirt Worms, some
species of which inhabit deep water round our shores, where they burrow
into the sand or mud of the bed of the sea. These form the class
_Gephyrea_, and consist of creatures with sac-like or cylindrical and
elongated bodies, and a protrusible proboscis, which is often of great
length. Their bodies are not distinctly segmented, nor do they bear any
appendages. The skin is tough and horny, and the body-wall, which is
very thick and muscular, is often contracted when the animal is
disturbed, thus causing a jet of water to be forcibly ejected.
All the most interesting of the marine worms belong to the _Annelida_ or
_Chætopoda_, popularly known as the Bristle-footed worms, because their
locomotion is aided more or less by the presence of stiff bristles that
project beyond the surface of the skin. These are all highly organised
worms, mostly with very elongated bodies that are distinctly segmented
exteriorly by a number of transverse grooves, while the interior is
correspondingly divided into a number of compartments by means of a
series of _septa_.
In addition to the bristles already mentioned, there are often numerous
appendages, but these must be distinguished from the more perfect
appendages of the arthropods, to be hereafter described; for while the
latter are distinctly jointed to the body, and are themselves made up of
parts that are jointed together, the former are mere outgrowths of the
body-wall. The digestive and circulatory systems are well developed, as
is also the system of water tubes that connect the body-cavity with the
exterior, while the body-cavity itself is full of fluid.
This group of worms is subdivided into two divisions, the many bristled
(_Polychæta_) and the sparsely bristled (_Oligochæta_) worms. The latter
contain the common earthworms and some less known species, while the
former include a number of interesting and even beautiful worms, all of
which are marine, and many of them among the commonest objects of the
sea shore.
These Polychætes exhibit a great variety of habit as well as of
appearance. Some live in crevices of the rocks or under stones and
weeds, or make burrows in the sand or mud of the bed of the sea, and
roam about freely at times in search of food. They are continually
coming within the ken of the sea-side collector, being revealed by
almost every overturned stone near the low-water mark, and are often
seen crawling over the wet rocks just left uncovered by the receding
tide; while their burrows are often so numerous that hundreds may be
counted in every few square feet. But many are sedentary species, and
these are not so generally known to young sea-side naturalists, who
frequently observe, and even preserve, the interesting homes they
construct, while less attention is given to the architects that build
them.
It is very interesting to observe some of the general differences
between the roving and the sedentary species--differences which
illustrate the principle of adaptation of structure to habit. The roving
species are provided with a lobe that overhangs the mouth, bearing
feelers and eyes, and are thus enabled to seek out any desired path and
to search for their food. They are provided with bristles and other
appendages by means of which they can travel freely over the surfaces of
solid objects, and are able to swim well either by undulations of the
body, or by fringed appendages, or both. The carnivorous species, too,
are provided with strong, horny jaws, and sharp, curved teeth, by means
of which they can capture and hold their prey. The sedentary species, on
the other hand, unable to move about in search of food, are supplied
with a number of appendages by means of which they can set up water
currents towards their mouths, and which also serve the purpose of
special breathing organs, and, having no means of pursuing and devouring
animals of any size, they do not possess the horny jaws and curved teeth
so common in the rovers. Their eyes, too, are less perfectly developed,
and the tactile proboscis of their free-moving relatives is absent.
[Illustration: FIG. 117.--_Arenicola piscatorum_]
Of the roving worms, perhaps, the Lugworm or Sandworm (_Arenicola
piscatorum_) is the best known. Its burrows may be seen on almost every
low sandy or muddy shore, and, being so highly valued as a bait, its
general appearance is well known to all professional and amateur sea
fishers. It reaches a length of eight inches or more, and varies in
colour according to the sand or mud in which it lives. The segments of
this worm are very different in structure in different parts of the
body. Those in the front of the body have a few tufts of bristles
arranged in pairs, while the middle portion of the body has large
brush-like tufts of filamentous gills placed rather close together; and
the hindmost part has no bristles or appendages of any kind, and is so
well filled with the sand or mud that it is quite hard and firm to the
touch. As is the case with our common earthworms, the sand or mud is
swallowed in enormous quantities, and this is not only the means by
which the lugworm derives its food, but also assists it considerably in
making its burrows; the extent to which this creature carries on its
work of excavation may be estimated by the thousands of little
contorted, worm-like heaps of sand that lie on the surface at every
period of low water. These little heaps are known as ‘castings,’ and
consist of the sand that passed through the worm’s body as the burrowing
proceeded.
The Ragworm is another species that is highly valued as bait. It burrows
into the odorous mud that is so commonly deposited in harbours and the
mouths of sluggish rivers. In this species the segments are similar
throughout the length of the body, and the numerous flattened appendages
give it the ragged appearance that has suggested its popular name. Quite
a number of marine worms closely allied to the common ragworm, and
resembling it in general form, are to be found on our shores. Many of
these may be seen by turning over stones that are left exposed at low
tide, while others hide themselves in snug little crevices of the rock,
or in the empty shells of the acorn barnacle and various molluscs; and
some species, including one of a bright-green colour, creep freely over
the wet rocks in search of food or home, often exposing themselves to
the rays of a fierce summer sun.
[Illustration: FIG. 118.--THE SEA MOUSE]
The Sea Mouse (_Aphrodita aculeata_) is certainly one of the most
interesting of the roving marine worms, and, though seldom seen above
low-water line, may often be obtained by the sea-side collector with the
aid of friendly fishermen, who sometimes find it plentifully among the
contents of their trawl nets. Failing such aid, it may be looked for
among the encrusted stones that are exposed only at the lowest spring
tides, especially in places where a certain amount of mud has been
deposited under the shelter of outlying rocks; and the chances of
success are much greater if the search is made immediately after a
storm, for at such times much of the life that exists in deep water will
have been driven shoreward by the force of the waves.
At first sight the sea mouse would hardly be associated with the worms;
for, instead of having the elongated and cylindrical form that is
usually regarded as characteristic of these creatures, it is broad and
slug-like in shape, the under surface, on which it crawls, being flat,
while the upper side is convex. The segmentation of the body, too, is
not readily seen in the upper surface on account of the thick felt-like
covering of hairs, but is at once apparent when the creature has been
turned over to expose the ventral side.
When seen for the first time in its natural haunt one naturally wonders
what the moving mass may be. Crawling sluggishly over incrusted stones,
or remaining perfectly still in a muddy puddle that has been exposed by
overturning a stone, it looks like a little mound of mud itself, about
four or five inches long, and its general colour and surface so closely
resembles that of its surroundings that an inexperienced collector may
never even suspect that the mass is a living animal form. But take the
creature and wash it in the nearest rock pool, and it will be recognised
as a broad segmental worm, thickly covered with fine hairs above, and
its sides adorned by bristles that display a most beautiful iridescence.
It is not easy to see the value of this gorgeous colouring to the
animal, and it is doubtful whether, on account of the muddy nature of
the creature’s home, such colouring is often displayed to the view of
other inhabitants of the sea; but it is well known, on the other hand,
that sea mice are readily devoured by fishes, even though they possess
an armature of stiff and sharp spines, and that they must therefore be
often preserved from destruction by the close resemblance of the general
colour to that of their surroundings.
The gills of the sea mouse are not prominent appendages, as with most
marine worms, but are soft fleshy structures situated beneath the
overlapping scales that lie hidden below the thick hair of the upper
surface.
As it is most probable that the reader may desire to preserve a sea
mouse at some time or other, a few words concerning the best methods of
doing this may be of value. If it is to be preserved in fluid, it should
be thoroughly washed to remove all the mud that normally covers its
body, and then placed in spirit or formaldehyde, both of which fluids
have no destructive effects on the iridescent colouring of the bristles.
If, however, it is desired to keep the specimen in a dry state, it
should first be put into strong spirit containing a few grains of
corrosive sublimate, for a few days. It should then be put under
considerable pressure between several thicknesses of absorbent paper to
expel the fluid it contains, as well as all the softer internal
structures. By this means it will have been squeezed quite flat, so that
it presents anything but a natural appearance; but the skin may be blown
out to the normal shape by means of a glass tube inserted into the
mouth, and then set aside to dry. As the water it originally contained
has been extracted by the strong spirit, the drying takes place very
quickly; and the small amount of corrosive sublimate that has penetrated
into its substance will be sufficient to protect it from the invasion of
those pests that commonly attack our museum specimens.
Passing now to the sedentary or fixed worms, we meet with some that are
very interesting and beautiful creatures, even when considered apart
from the wonderful homes they construct. The several species of the
genus _Terebella_ form a soft and flexible tube by binding together
particles of sand, shells, or mud with a sticky substance that exudes
from their own bodies. These tubes are to be found in abundance between
the tide-marks on almost every low, sandy shore, the nature of the tubes
varying, of course, with the character of the materials at the disposal
of the builder.
In some cases the tubes are exposed throughout the greater part of their
length, but very frequently they are more or less buried in the sand or
other material of the beach, so that one has to dig to a moderate depth
in order to extricate them. In either case, however, the tube of
Terebella may be known by the free tufts of sandy threads that form a
deep fringe around its mouth.
These worms almost invariably select a sheltered situation for their
abode, and should be searched for at the foot of rocks, or under stones,
and it is no easy matter to move the buried tube with its occupant
intact.
When turning over the stones of a sandy or muddy beach one frequently
discovers the slender, thread-like tentacles of the Terebella, together
with the sandy filaments that surround the mouth of the tube, the
remainder of the tube and its occupant being beneath the surface, and
the ground is often so hard and stony that a strong tool is necessary
to dig it out; but the work entailed will be amply repaid if a perfect
specimen be obtained and placed for observation in the aquarium.
[Illustration: FIG. 119.--TUBE-BUILDING WORMS: _Terebella_ (LEFT),
_Serpula_ (MIDDLE), _Sabella_ (RIGHT)]
The reader may possibly be acquainted with the tubes or cases that are
constructed by the larvæ of caddis flies in fresh-water ponds and
streams, and perhaps has noticed the ease with which these creatures may
be made to construct new homes after having been turned out of doors.
Similar experiments may be performed with Terebella; for when the worm
has been extricated from its tube without injury--a work that requires
great care on account of the soft and slender nature of the creature’s
body--and placed in the aquarium with a bed of suitable material, it
will build itself a new dwelling. As with the caddis larvæ, the
different species may be known by the materials they select to construct
their tubes, but in captivity they may be compelled to employ other than
their favourite substance for this purpose. It is unfortunate, however,
that Terebella is a nocturnal builder, and thus its movements are not so
easily observed.
When removed from its tube its first movements suggest a resentment at
the untimely ejection. This being over, it seeks a sheltered situation
beneath the edge of a stone, and, at nightfall, commences the slow
process of the construction of a fresh home. The particles of material
at hand are seized by the tentacles, placed in position round the body,
where they are held together by the sticky secretion already mentioned.
[Illustration: FIG. 120.--_Terebella_ REMOVED FROM ITS TUBE]
The tentacles are employed in two distinct ways:--They may be flattened
into slender ribbon-like structures, which, by being folded
longitudinally at any point, may be made to grasp a particle of sand;
and, in addition to this, the tip of the tentacle may be converted into
a minute cup-shaped sucker by the withdrawal of the fluid it contains
into the body.
Some species of Terebella build their tubes of ordinary sand, while
others select fragments of shells. Some employ mud only, and
occasionally we meet with tubes constructed of the silky secretion of
the body with hardly any foreign matter.
We sometimes see edges of rocks, on low, sandy shores, covered with what
appears to be large masses of consolidated sand, full of holes a little
more than an eighth of an inch in diameter; and these masses are often
so extensive and so firm that they seem to form the greater part of the
rock itself. Such masses are particularly abundant on the south coasts
of Devon and Cornwall, but are more or less plentiful on most sandy
shores of Great Britain. They consist of the tubes of a species of the
marine worm _Sabella_, which have been built up much in the same manner
as those of Terebella, but usually exist in such numbers in the same
spot that, together with the sand that has been washed between them,
they form the dense masses just described.
A cluster of some dozens of these tubes may be detached with the aid of
a hammer and chisel; or, in some instances, where the mass of tubes is
not held so firmly together, by the mere pressure of the hand; and it
will then be observed that each tube consists of a flexible membrane, of
a somewhat leathery nature, formed by a sticky secretion from the body
of the worm, with its outer surface covered with grains of sand. The
tubes may be easily opened, and the occupants extracted for examination,
when it will be observed that the front or upper portion of the worm is
short and thick, while the hindmost portion is much thinner, and is
doubled forwards in the tube. The body is also provided with numerous
bristles, by means of which the worm is enabled to grasp the membranous
lining of the tube, and thus secure a firm hold within its home.
A cluster of these tubes should be placed in a rock pool, or in the
marine aquarium, when the worms may be seen to protrude gradually, and
expose a large number of feathered tentacles, which, by their incessant
motion, keep up the constant circulation of the water for the purpose of
respiration as well as to bring food particles towards the mouths of the
worms.
It is possible to keep these worms alive for some time in the aquarium,
but special care is required for the reason that it is a very difficult
matter to secure a cluster of tubes without injury to a certain number
which are sure to be broken or otherwise damaged; and these, dying and
decomposing within their homes, speedily pollute the water. Hence it is
necessary to keep a sharp watch for dead specimens, which should, of
course, be removed at once. The presence of a putrefying worm may often
be detected by the appearance of a whitish fungoid growth round the
mouth of what appears to be an empty tube; and if, through neglect, the
water of the aquarium has been allowed to become contaminated by the
products of decomposition, it will often happen that some of the living
worms will come entirely out from their tubes, as if to seek a more
sanitary situation. Thus, the exit of worms from their homes may always
be looked upon as pointing to a suspicious condition of the water which,
if not corrected immediately, may lead to the death of all.
The species we have briefly described is by far the commonest of the
genus Sabella, but there are several others to be found on our shores.
Some are of a solitary nature, and construct a sandy tube so much like
that of a certain species of Terebella that they may be mistaken for
that genus. Another solitary species builds a hard stony tube of
carbonate of lime that has been extracted from the sea water; and
although it is hardly possible to take the live worm from this
calcareous tube without injury, the animal may be obtained intact for
examination or preservation by dissolving away the tube in dilute
hydrochloric acid.
[Illustration: FIG. 121.--A TUBE OF _Serpula_ ATTACHED TO A SHELL]
While engaged in collecting specimens on the sea shore we are
continually meeting with stones and shells that are more or less covered
with white, limy tubes twisted into all manner of serpentine forms.
These are the tubes of other marine worms known as the _Serpulæ_, which,
like the species previously mentioned, are interesting objects for the
aquarium.
The tubes themselves are worthy of study and preservation, more
especially as they vary in form, and may, to some extent, provide a
means by which the different species may be identified. They are
composed of fine layers of calcareous matter secreted by the body of the
worm within, and lined by a thin leathery membrane which may be easily
exposed by dissolving away the mineral matter as just described. Some
are triangular in section, and often distinctly keeled, while others are
cylindrical, and flattened more or less on the lower side. The
triangular tubes are attached to stones or shells throughout their
length, but the cylindrical ones are often elevated above the surface in
the wider and newer part.
If a cluster of these tubes, freshly gathered from between the
tide-marks, be placed in the aquarium, the worms will soon protrude the
foremost portion of their bodies, exposing beautiful fan-like gills,
often brilliantly coloured in shades of scarlet, blue, or purple, which
are kept in motion in such a manner as to convey water, and consequently
also food, towards the mouth. The gills are of course, richly supplied
with blood, for their main function is to aërate that liquid by exposing
it to the water in order to absorb oxygen gas. The body of the worm is
provided also with little cilia, which, by their constant vibratory
motion, keep up a circulation of water through the tube; and this not
only keeps the tubular home free from excrement and other sedimentary
matter, but also probably assists in the function of respiration by
bringing fresh supplies of water in contact with the animal’s soft and
absorbent skin.
[Illustration: FIG. 122.--_Serpula_ REMOVED FROM ITS TUBE]
When the worms are disturbed they immediately withdraw themselves within
the tubes, this being done by the aid of the numerous minute hooklets on
the surface of the body that enable the worms to cling firmly to the
membranous linings of their homes; and it will then be observed that the
mouth of each tube is closed by a lid (_operculum_), which hangs as by a
hinge when not in use. These operculi vary much in character, and supply
another aid in the identification of the various species. They differ
much in shape, and may be either membranous, horny, or calcareous.
Little calcareous tubes, somewhat similar to those of the _Serpulæ_, but
always in the form of a spiral, may often be seen on stones and shells,
and the fronds of sea weeds, sometimes so closely packed together as to
almost entirely cover the surface. The average diameter of these spirals
is only about a sixteenth of an inch, and many are so small that a lens
is necessary to discern their shape. In general form they closely
resemble some of the small species of _Planorbis_ shells that are so
common in our ponds and streams, but these latter are the shells of
freely moving _molluscs_, and are generally of a brownish colour.
[Illustration: FIG. 123.--THE SEA MAT (_Flustra_)]
The minute worms that live within the tubes in question belong to the
genus _Spirorbis_, and are very similar to those of the _Serpulæ_, and
their pretty plumed gills may be seen with a lens when a cluster of the
tubes is placed in a shallow vessel of sea water. A sharp tap on the
table on which the vessel rests will cause the little creatures to
suddenly retire into their homes, the entrances to which may then be
seen to be closed by an operculum.
There is an interesting group of animals known collectively as the
_Bryozoa_ or _Polyzoa_, or, popularly, as the Moss Polyps, that are
often classed with the worms, though they are not, according to the
general idea, wormlike in appearance. They live in pretty colonies, many
of which are certainly familiar objects to all who ramble along the sea
shore. Some form pretty lacelike patches on the fronds of sea weeds,
while others are built up into flat, frond-like, branching objects that
are often mistaken for sea weeds by young collectors. Among the latter
is the Sea Mat (_Flustra_), that is so commonly washed up on the shore
in great abundance. An examination with a lens will show that, in both
instances, the mass consists of very many minute cells, with horny or
calcareous walls, the mouth of each cell being close by an operculum.
On placing the colony in sea water, however, we find that each little
cell is the home of a small animal, that protrudes from the cell,
exposing a mouth that is surrounded by a crown of tentacles. A
moderately high magnifying power will also show that the tentacles are
covered with minute vibratile cilia, by means of which currents of water
are set in motion towards the mouth to supply the animal with food.
Some, too, have a lip by means of which the mouth may be closed.
[Illustration: FIG. 124.--_Flustra_ IN ITS CELL, MAGNIFIED]
In addition to the colonies just briefly described, there are other moss
polyps that build up little, branching, tree-like clusters which closely
resemble some of the sea firs, and many of these are to be found in the
sheltered crevices of rocks, or attached to the under sides of stones
between the tide-marks.
While searching the surfaces of rocks and weeds at low tide, one’s
fingers will be constantly coming in contact with fixed, soft-bodied
animals that suddenly eject a fine stream of water as they are touched.
These are the Sea Squirts, sometimes spoken of as the Tunicate Worms.
They are semi-transparent creatures of oval or elongated form, and
usually of a pale yellow, brown, or pink colour; and derive their
popular name from the fact that they are covered externally by a
continuous tunic or wall of tough structure.
Although the tunicates resemble worms in many points of structure, it is
interesting to note that in their young or larval state the body
consists of two cavities, one of which contains the internal organs,
while in the other the central portion of the nervous system is
developed, in which respects they resemble the vertebrate or back-boned
animals--fishes, amphibians, reptiles, birds, and mammals. At this
stage, too, the creatures possess a tail that is supported by a rod of
gristle similar to that which gives place to the backbone in the
developing vertebrate. These features, though only transitory, are
regarded as a mark of relationship to the higher forms of animal life,
and thus the tunicates have been separated from the sub-kingdom Vermes
by some zoologists, and given an exalted place at the top of the
invertebrate scale, where they form a sub-kingdom of their own, and are
looked upon as a link connecting the invertebrates with the vertebrates.
[Illustration: FIG. 125.--SEA SQUIRT]
Before passing on to the next sub-kingdom, we should observe that the
interesting Rotifers or Wheel Animals also belong to the Vermes; but
although many of these minute creatures are to be found in sea water,
their principal home is the stagnant water of fresh-water ponds and
ditches, and thus we may be excused for neglecting them here.