first formed, of a single Hydra containing an oblong oil bubble (Fig.
107). The whole organisation of such a Hydra is limited to a simple
digestive cavity; it has, in fact, but one organ, and one function,
and consists of an alimentary sac resembling the proboscis of a Medusa
(Fig. 107); the oil bubble is separated from it by a transverse
partition, and has no connection with the cavity. Presently, between
the oil bubble and the cavity arise a number of buds of various
character (Fig. 108), which we will describe one by one, beginning
with those nearest the oil bubble, since these upper members of the
little swimming community bear a very important part in its history.
The infant community (Fig. 108) passes rapidly into the stage
represented in Fig. 109, and then through all the stages intermediate
between this and the adult, shown in its natural size in Fig. 115. The
upper buds enlarge gradually, and soon take upon themselves a perfect
Medusa structure (Fig. 110), with the exception of the proboscis, the
absence of which is easily understood, when we find that these Medusæ,
serve the purpose of locomotion only, having no share in the function
of feeding the community, so that a digestive apparatus would be quite
superfluous for them. In every other respect they are perfect Medusæ,
attached to the Hydra as the Medusa buds always are when first formed,
having the (four) chymiferous tubes, characteristic of all Hydroid
Medusæ, radiating from the centre to the periphery; two of these tubes
are very winding, as may be seen in Fig. 110, while the other pair are
straight. The Medusæ themselves are heart-shaped in form, depressed at
the centre of the upper surface, and bulging on either side into
wing-like expansions, where they join the stem. These expansions
interlock with one another, crossing nearly at right angles. The
Medusæ-like buds are the swimming bells; by their contractions,
alternately taking in and throwing out the water, they impel the whole
community forward, so that it seems rather to move like one animal,
than like a combination of individuals.
[Illustration: Fig. 107. Young Nanomia; magnified.]
[Illustration: Fig, 108. Young Nanomia with rudimentary Medusæ.]
[Illustration: Fig. 109. Young Nanomia, older than Fig. 108.]
[Illustration: Fig. 110. Heart-shaped swimming bell of Nanomia;
magnified.]
Besides these locomotive members, the community contains three kinds
of Hydræ arising as buds from the primitive Hydra below the swimming
bells, the latter remaining always nearest the oil bubble at the top,
while the first Hydra, the founder of the community, in proportion as
the new individuals are added, is gradually pushed downward, and
remains always at the end of the string, the stem of which is formed
by the elongated neck of the primitive Hydra. All the three sets of
Hydræ have certain features in common, while they have other
distinguishing characteristics marking them as distinct individuals.
They are all accompanied by triangular shields (Fig. 111), arising
with them at the same point on the parent stem, and all are furnished
with tentacles hanging down from the summit of the Hydra at the side
opposite the shield. These facts are important to remember, since we
shall presently perceive, upon analyzing their parts, that these Hydræ
have a close homology to the Hybocodon. The tentacles differ in
structure as well as in number for each kind of Hydra. Having shown in
what characters they agree, let us now take each set individually, and
see what differences they present.
[Illustration: Fig. 111. Cluster of Medusæ with tentacles having
pendent knobs.]
In the first set which we will examine the Hydra is open-mouthed. Like
the original Hydra, it is only a digestive tube, similar in all
respects to the proboscis of a Medusa-disk. Its only function is that
of feeding, and it shows a laudable fidelity to its calling, being
very constantly and earnestly engaged in the work. Let us add,
however, that in this instance the occupation is not a wholly selfish
one, since the cavity of every Hydra communicates with that of the
stem, and the food taken in at these over-gaping mouths, is at once
circulated through all parts of the community, with the exception of
the oil bubble, from which it is excluded by the transverse partition
dividing it from all the lower members of the stock. The shields share
in this general nourishment of the compound body by means of
chymiferous tubes extending toward the outer surface, and opening into
the cavity of the stem. The mouth of this Hydra is very flexible (Fig.
111), expanding and contracting at the will of the animal, and
sometimes acting as a sucker, fastening itself, leech-like, on the
object from which it seeks to draw its sustenance. (See Fig. 111.) The
tentacles attached to this set of Hydræ are exceedingly long and
delicate. They arise in a cluster at the upper and inner edge of the
Hydra, just at its point of juncture with the stem, and being
extremely flexible and contractile, their long tendril-like sprays are
thrown out in an endless variety of attitudes. (See Fig. 115.) Along
the whole length of this kind of tentacle are attached little pendent
knobs at even distances; Fig. 112 represents such a knob greatly
magnified, and absolutely paved with lasso-cells, the inner and
smaller ones being surrounded by a row of larger ones.
[Illustration: Fig. 112. Magnified pendent knob.]
[Illustration: Fig. 113. Medusa with corkscrew shaped tentacles.]
The second set of Hydræ (Fig. 113), are also open-mouthed,
corresponding with those described above, in everything except the
tentacles, which are both shorter and thicker, and are coiled in a
corkscrew-like spiral. These are thickly studded for their whole
length with lasso-cells. (See Fig. 113.)
In the third and last set of Hydræ (Fig. 114), the mouth is closed;
they have, therefore, no share in feeding the community, but receive
their nourishment from the cavity of the stem into which they open.
They differ also from the others in having a single tentacle instead
of a cluster, and on this tentacle the lasso-cells are scattered at
uneven distances (Fig. 114). The special function of these closed
Hydræ is yet to be explained; they have oil bubbles at their upper end
(see Fig. 111, the top Hydra), and though we have never seen them drop
off, it seems natural to suppose that they do separate from the parent
stock, and found new communities similar to those from which they
arise.
[Illustration: Fig. 114. Medusa with a simple thread-like
tentacle.]
The intricate story of this singular compound existence does not end
here. There is still another set of individuals whose share in
maintaining the life of the community is by no means the least
important. Little bunches of buds, of a different character from any
described above, may be seen at certain distances along the lower part
of the stem. These are the reproductive individuals. They are clusters
of imperfect sexual Medusæ, resembling the rudimentary Medusæ of
Tubularia (Fig. 99), which are never freed from the parent stem, but
discharge their contents at the breeding season. Like many other
compound Hydroids, the sexes are never combined, in one of these
communities; they are always either male or female, and as those with
female buds have not yet been observed, we can only judge by inference
of their probable character. Front what is already known, however, of
Hydroid communities of a like description, we suppose that the process
of reproduction must be the same in these, and that the female stocks
of Nanomia give birth to small Jelly-fishes, the eggs of which become
oil bubbles, similar to that with which our little community began.
(Fig. 108.)
[Illustration: Fig. 115. Adult Nanomia, natural size, at rest.]
By the time all these individuals have been added along the length of
the stem, the stem itself has grown to be about three inches long
(Fig. 115), though the tentacles hanging from the various members of
the community give to the whole an appearance of much greater length.
The motion of this little string of living beings is most graceful.
The oil bubble (Fig. 116) at the upper end is their float; the
swimming bells immediately below it (Fig. 110), by the convulsive
contractions of which they move along, are their oars. The water is
not taken in and expelled again by all the bells at once, but first
from all the bells on one side, beginning at the lower one, and then
from all those on the opposite side, beginning also at the lower one;
this alternate action gives to their movements a swinging, swaying
character, expressive of the utmost freedom and grace. Whether such a
little community darts with a lightning-like speed through the water,
or floats quietly up and down, for its movements are both rapid and
gentle, it always sways in this way from side to side. Its beauty is
increased by the spots of bright red scattered along the length of the
stock at the base and tips of the Hydræ, as well as upon the
tentacles. The movements and attitudes of the tentacles are most
various. Sometimes they shoot them out in straight lines on either
side, and then the aspect of the whole thing reminds one of a tiny
chandelier in which the coral drops make the pendants, or they may be
caught up in a succession of loops or floating in long streamers;
indeed, there is no end to the fantastic forms they assume, ever
astonishing you by some new combination of curves. The prevailing hue
of the whole community is rosy, with the exception of the oil bubble
or float, which looks a bright garnet color when seen in certain
lights.
[Illustration: Fig. 116. Oil float of Nanomia; greatly magnified.]
Let us now compare one of the Hydræ hanging from the stem (Fig. 113)
with the Hybocodon (Fig. 102). The reader will remember the
unsymmetrical bell of this singular Medusa, one half of its disk more
largely developed than the other, with the proboscis hanging from the
centre, and the cluster of tentacles from one side. Let us now split
the bell so as to divide it in two halves with the proboscis hanging
between them; next enlarge the side where there are no tentacles, and
give it a triangular outline; then contract the opposite side so as to
draw up the cluster of tentacles to meet the base of the proboscis,
and what have we? The proboscis now corresponds to the Hydra of our
Nanomia, with the cluster of tentacles attached to its upper edge
(Fig. 113), while the enlarged half of the bell represents the shield.
If this homology be correct it shows that the Nanomia is not, as some
naturalists have supposed all the Siphonophores to be, a single
animal, its different parts being a mere collection of organs endowed
with special functions, as feeding, locomotion, reproduction, &c., but
that it is indeed a community of distinct individuals corresponding
exactly to the polymorphous Hydroids, whose stocks are attached, such
as Hydractinia, and differing from them only in being free and
floating.
The homologies of the Siphonophoræ or floating Hydroids, with many of
the fixed Hydroids, is perhaps more striking when we compare the
earlier stages of their growth. Suppose, for instance, that the
planula of our Melicertum (See Fig. 81) should undergo its development
without becoming attached to the ground,--what should we then have? A
floating community (Fig. 83), including on the same stock like the
Nanomia, both sterile and fertile Hydræ, from the latter of which
Medusæ bells are developed. The little Hydractinia community (Fig.
100), in which we have no less than four distinct kinds of
individuals, each performing a definite distinct function, affords a
still better comparison.
_Physalia_. (_Physalia Arethusa_ TIL.)
[Illustration: Fig. 117. Physalia; _a_ _b_ air sac with crest _c_,
_m_ bunches of individuals, _n_ central tentacles, _t_ _t_
expanded tentacles. (_Agassiz_.)]
Among the most beautiful of the Siphonophores, is the well-known
Physalia or Portuguese man-of-war, represented in Fig. 117. The float
above is a sort of crested sac or bladder, while the long streamers
below consist of a number of individuals corresponding in their nature
and functions to those composing a Hydroid community. Among them are
the fertile and sterile Hydræ (Fig. 118), the feeders and Medusæ bells
(Fig. 119). The Physalia properly belongs to tropical waters, but
sometimes floats northward, in the warm current of the Gulf Stream,
and is stranded on Cape Cod. When found so far from their home,
however, they have usually lost much of their vividness of color; to
judge of their beauty one should see them in the Gulf of Mexico,
sailing along with their brilliant float fully expanded, their crest
raised, and their long tentacles trailing after them.
[Illustration: Fig. 118. Bunch of Hydræ; _a_ base of attachment,
_b_ _b_ _b_ single Hydræ, _c_ _c_ tentacles. (_Agassiz_.)]
[Illustration: Fig. 119. Bunch of Hydræ; cluster of Medusæ; _b_
_b_ Hydræ with tentacles, _c_ _d_ bunches of Medusæ (_Agassiz_.)]
_Velella_. (_Velella mutica_ BOSC.)
Another very beautiful floating Hydroid, occasionally caught in our
waters, though its home is also far to the south, is the Velella (Fig.
120). It is bright blue in color, and in form not unlike a little flat
boat with an upright sail. Its Medusa (Fig. 121) resembles so much
that of some of our Tubularians, that it has actually been removed on
this account from the old group of Siphonophoræ, and placed next the
Tubularians; another evidence of the close affinity between the former
and the Hydroids.
[Illustration: Fig. 120. Velella; _m_ so-called mouth, _a_
tentacles. (_Agassiz_.)]
[Illustration: Fig. 121. Free Medusa of Velella; _a_ proboscis,
_b_ chymiferous tube, _c_ circular tube. (_Agassiz_.)]
MODE OF CATCHING JELLY-FISHES.
Not the least attractive feature in the study of these animals, is the
mode of catching them. We will suppose it to be a warm, still morning
at Nahant, in the last week of August, with a breath of autumn in the
haze that softens the outlines of the opposite shore, and makes the
horizon line a little dim. It is about eleven o'clock, for few of the
Jelly-fishes are early risers; they like the warm sun, and at an
earlier hour they are not to be found very near the surface. The sea
is white and glassy, with a slight swell but no ripple, and seems
almost motionless as we put off in a dory from the beach near
Saunders's Ledge. We are provided with two buckets, one for the larger
Jelly-fishes, the Zygodactyla, Aurelia, &c., the other for the smaller
fry, such as the various kinds of Ctenophoræ, the Tima, Melicertum,
&c. Beside these, we have two nets and glass bowls, in which to take
up the more fragile creatures that cannot bear rough handling. A bump
or two on the stones before we are fairly launched, a shove of the oar
to keep the boat well out from the rocks along which we skirt for a
moment, and now we are off. We pull around the point to our left and
turn toward the Ledge, filling our buckets as we go. Now we are
crossing the shallows that make the channel between the inner and
outer rocks of Saunders's Ledge. Look down,--how clear the water is
and how lovely the sea-weeds, above which we are floating, dark brown
and purple fronds of the Ulvæ, and the long blades of the Laminaria
with mossy green tufts between. As we issue from this narrow passage
we must be on the watch, for the tide is rising, and may come laden
with treasures, as it sweeps through it. A sudden cry from the oarsman
at the bow, not of rocks or breakers ahead, but of "A new Jelly-fish
astern!" The quick eye of the naturalist of the party pronounces it
unknown to zoölogists, un-described by any scientific pen. Now what
excitement! "Out with the net!--we have passed him! he has gone down!
no, there he is again! back us a bit." Here he is floating close by
us; now he is within the circle of the net, but he is too delicate to
be caught safely in that way, so, while one of us moves the net gently
about, to keep him within the space enclosed by it, another slips the
glass bowl under him, lifts it quickly, and there is a general
exclamation of triumph and delight,--we have him. And now we look more
closely; yes, decidedly he is a novelty as well as a beauty. (See Fig.
122, _Ptychogena lactea_ A. Ag.) Those white mossy tufts for ovaries
are unlike anything we have found before (Fig. 123), and not
represented in any published figures of Jelly-fishes. We float about
here for a while, hoping to find more of the same kind, but no others
make their appearance, and we keep on our way to East Point, where
there is a capital fishing ground for Medusæ of all sorts. Here two
currents meet, and the Jelly-fishes are stranded as it were along the
line of juncture, able to move neither one way nor the other. At this
spot the sea actually swarms with life; one cannot dip the net into
the water without bringing up Pleurobrachia, Bolina, Idyia,
Melicertum, &c., while the larger Zygodactyla and Aurelia float about
the boat in numbers. These large Jelly-fishes produce a singular
effect as one sees them at some depth beneath the water; the Aureliæ,
especially, with their large white disks, look like pale phantoms
wandering about far below the surface; but they constantly float
upward, and if not too far out of reach, one may bring them up by
stirring the water under them with the end of the oar.
[Illustration: Fig. 122. Ptychogena, natural size.]
[Illustration: Fig. 123. Ovary of Ptychogena; magnified.]
When we have passed an hour or so floating about just beyond East
Point, and have nearly filled our buckets with Jelly-fishes of all
sizes and descriptions, we turn and row homeward. The buckets look
very pretty as they stand in the bottom of the boat with the sunshine
lighting up their living contents. The Idyia glitters and sparkles
with ever-changing hues, the Pleurobrachiæ dart about, trailing their
long graceful tentacles after them, the golden Melicerta are kept in
constant motion by their quick, sudden contractions, and the delicate
transparent Tima floats among them all, not the less beautiful because
so colorless. There is an unfortunate Idyia, who, by some mistake, has
got into the wrong bucket with the larger Jelly-fish, where a
Zygodactyla has entangled it among his tentacles and is quietly
breakfasting upon it.
During our row the tide has been rising, and as we near the channel of
Saunders's Ledge, it is running through more strongly than before, and
at the entrance of the shallows a pleasant surprise is prepared us; no
less than half a dozen of our new friends (the Ptychogena as he has
been baptized), come to look for their lost companion perhaps, await
us there, and are presently added to our spoils. We reach the shore
heavily laden with the fruits of our morning's excursion.
The most interesting part of the work for the naturalist is still to
come. On our return to the Laboratory, the contents of the buckets are
poured into separate glass bowls and jars; holding them up against the
light, we can see which are our best and rarest specimens; these we
dip out in glass cups and place by themselves. If any small specimens
are swimming about at the bottom of the jar, and refuse to come within
our reach, there is a very simple mode of catching them. Dip a glass
tube into the water, keeping the upper end closed with your finger,
and sink it till the lower end is just above the animal you want to
entrap; then lift your finger, and as the air rushes out the water
rushes in, bringing with it the little creature you are trying to
catch. When the specimens are well assorted, the microscope is taken
out, and the rest of the day is spent in studying the new
Jelly-fishes, recording the results, making notes, drawings, &c.
Still more attractive than the rows by day are the night expeditions
in search of Jelly-fishes. For this object we must choose a quiet
night, for they will not come to the surface if the water is troubled.
Nature has her culminating hours, and she brings us now and then a day
or night on which she seems to have lavished all her treasures. It was
on such a rare evening, at the close of the summer of 1862, that we
rowed over the same course by Saunders's Ledge and East Point
described above. The August moon was at her full, the sky was without
a cloud, and we floated on a silver sea; pale streamers of the aurora
quivered in the north, and notwithstanding the brilliancy of the moon,
they too cast their faint reflection in the ocean. We rowed quietly
along past the Ledge, past Castle Rock, the still surface of the water
unbroken, except by the dip of the oars and the ripple of the boat,
till we reached the line off East Point, where the Jelly-fishes are
always most abundant, if they are to be found at all. Now dip the net
into the water. What genie under the sea has wrought this wonderful
change? Our dirty, torn old net is suddenly turned to a web of gold,
and as we lift it from the water heavy rills of molten metal seem to
flow down its sides and collect in a glowing mass at the bottom. The
truth is, the Jelly-fishes, so sparkling and brilliant in the
sunshine, have a still lovelier light of their own at night; they give
out a greenish golden light as brilliant as that of the brightest
glow-worm, and on a calm summer night, at the spawning season, when
they come to the surface in swarms, if you do but dip your hand into
the water it breaks into sparkling drops beneath your touch. There are
no more beautiful phosphorescent animals in the sea than the Medusæ;
it would seem that the expression, "rills of molten metal" could
hardly apply to anything so impalpable as a Jelly-fish, but, although
so delicate in structure, their gelatinous disks give them a weight
and substance; and at night, when their transparency is not perceived,
and their whole mass is aglow with phosphorescent light, they truly
have an appearance of solidity which is most striking, when they are
lifted out of the water and flow down the sides of the net.
The various kinds present very different aspects; wherever the larger
Aureliæ and Zygodactylæ float to the surface, they bring with them a
dim spreading halo of light, the smaller Ctenophoræ become little
shining spheres, while a thousand lesser creatures add their tiny
lamps to the illumination of the ocean; for this so-called
phosphorescence of the sea is by no means due to the Jelly-fishes
alone, but is also produced by many other animals, differing in the
color as well as the intensity of their light, and it is a curious
fact that they seem to take possession of the field by turns. You may
row over the same course, which a few nights since glowed with a
greenish golden light wherever the surface of the water was disturbed,
and though equally brilliant, the phosphorescence has now a pure white
light. On such an evening, be quite sure that when you empty your
buckets on your return and examine their contents you will find that
the larger part of your treasures are small crustacea (little
shrimps). Of course there will be other phosphorescent creatures,
Jelly-fishes, &c., among them, but the predominant color is given by
these little crustacea. On another evening the light will have a
bluish tint, and then the phosphorescence is principally due to the
Dysmorphosa (Fig. 105).