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James G. (James George) Needham.

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like fore feet, and with them scrapes the sand
aside, making a hole. Then it thrusts its tusks into
the bottom of the hole and lifts the earth forward and
upward. Then, moving forward into the opening
thus begun and repeating these operations, it quickly
descends from view.

Squeezing thro the bottom is the method of progress
most available to soft-bodied animals. Those lacking
hard parts such as shovels and tusks with which to dig
make progress by pushing a slender front into a narrow
opening, and then distending and, by blood pressure
enlarging the passageway. The horsefly larva shown in
figure 137 on page 230 (discussed on page 227) is a good
example. The body is somewhat spindle-shaped, taper-
ing both ways, and adapted for traveling forward or
backward. It is exceedingly changeable in proportions
being adjustable in length, breadth and thickness.
Indeed, the whole interior is a moving mass of soft
organs, any one of which may be seen thro the trans-
parent skin, slipping backward or forward inside for a
distance of several segments. The body wall is lined
with strong muscles inside, and outside it bears rings
of stout tubercles, which may be drawn in for passing,
or set out rigidly to hold against the walls of the burrow.
The extraordinary adjustability of both exterior and
interior is the key to its efficiency. When such a larva
wishes to push forward in the soil, it distends and sets
its tubercles in the rear* to hold against the walls, and
drives the pointed head forward full length into the mud ;
then it compresses the rear portion, forcing the blood



*Certain cranefly larvae (such as Pedicia albivitta and Eriocera spinosa) that
live in beds of gravel have one segment near the end of the body expansible to
almost balloon-like proportions, forming a veritable pushing-ring in the rear.



Shelter Building 257



forward to distend the body there, thus widening the
burrow. And if anyone would see how such a larva gets
through a narrow space when the walls cannot be
pushed farther apart, let him wet his hand and close the
larva in its palm ; the larva will quickly slip out between
the fingers of the tightly closed hand; and when half
way out it will present a strikingly dumb-bell-shaped
outline. Here, again, we see the advantage of its
almost fluid interior.

This adjustability of body, is of course, not peculiar to
soft bodied insect larvae; it is seen in leeches and slugs
and many worms.

The mussel's mode of burrowing is not essentially
different from that above described. The slender
hollow foot is pushed forward into the sand, and then
distended by blood forced into it from the rear. When
sufficiently distended to hold securely by pressure
against the sand, a strong pull drags the heavy shell
forward.

Ill

Shelter building Some animals produce adhesive
secretions that harden on contact with the water.
Thus, these are able to bind loose objects together into
shelters more suitable for their residence than any that
nature furnishes ready made. The habit of shelter
building has sprung up in many groups; in such
protozoans as Difflugia (see fig. 69 c on p. 39) ; in such
worms as Dero (see fig. 82 on p. 174) ; in such rotifers
as Melicerta (see fig. 86 on p. 178) ; in such caterpillars
as Hydrocampa (see fig. 127 on p. 219); in nearly all
midges, as Chironomus (see figure 134 on p. 226) and
Tanytarsus (see fig. 223 on p. 373) ; and especially in
the caddis- worms, all of which construct shelters of some
sort and most of which build portable cases. The
extraordinary prevalence in all fresh waters of such forms



258 Adjustment to Conditions of Aquatic Life

as the larvae of midges and caddis-flies would indicate
that the habit has been biologically profitable.

According to Betten the habit probably began with
the gathering and fastening together of fragments for a
fixed shelter, and the portable, artifically constructed,
silk lined tubes of the higher caddis-worms are a more
recent evolution.

IV

Withstanding the wash of moving waters Where
waters rush swiftly, mud and sand and all loose shelters




FIG. 158. Stone from a brook bed, bearing tubes of midge
larvae and portable cases of two species of caddis- worms.
The more numerous spindle-shaped cases are those of
the micro-caddisworms of the genus Hydroptila. For
more distinct midge tubes see figs. 134 and 223.

are swept away. Only hard bare surfaces remain, and
the creature that finds there a place of residence must
build its own shelter, or must possess more than ordi-
nary advantages for maintaining its place. The gifts of
the gods to those that live in such places are chiefly
these three:

I. Ability to construct flood-proof shelters. Such
are the fixed cases of the caddis-worms and midge larvae
(fig. 158) to which we shall give further consideration in
the next chapter.



Withstanding the Wash of Moving Waters 259




2. Special organs for hanging on to water-swept
surfaces. Such organs are the huge grappling claws of
the nymphs of the larger stoneflies (see fig. 1 1 1 on p.
204) and of the riffle beetles: also powerful adhesive
suckers, such as those of the larvae of the net-winged

midges.

3 . Form of body that
diminishes resistance to
flow of the water. This
we have already seen is
stream-line form. In
our discussion of swim-
ming we pointed out
that the form of body
that offers least resist-
ance to the progress of
the body through the
water will also offer least
resistance to the flow of
water past the body. So
we find the animals that
stand still in running
water are of stream-line
form ; darters and other
fishes of the rapids ; may-
flies, such as Siphlon-
urus and Chirotenetes;
even such odd forms as the larvae of Simulium, which
hangs by a single sucker suspended head downwards in
the stream. Indeed, the case of Simulium is especially
significant, for with the reversal of the position of the
body the greater widening of the body is shifted from
the anterior to the posterior end, and stream-line form
is preserved. Such forms as these live in the open,
remain for the most part quietly in one position and
wait for the current to bring their food to them.



FIG. 159. The larva of the net-
winged midge, Blepharocera, dorsal
and ventral views.



260 Adjustment to Conditions of Aquatic Life



FIG. 1 60. Limpet-shaped
animals. At right the larva
of the Parnid beetle, Pse-
phenus, known as the
"water-penny." At left,
the snail, Ancylus.



There are other more numerous forms living in rapid

water that cling closer to the solid surfaces, move about

upon and forage freely on these surfaces, and the

adaptations of these are related to the surfaces as much

^^^^ ^^^ as to the open stream. These

^B ^ have to meet and withstand the

mmiP* : Si pi II wa "ter also, but only on one side;

H and the form is half of that of

|^ it, S our diagram (fig. 1 53) . It is that

^1 figure divided in the median

vertical plane, with the flat side
then applied to the supporting
surface, and flattened out a bit at
the edges. This is not fish form,
but it is the form of a limpet.
This is the form taken on by a
majority of the animals living in rapid waters. When
the legs are larger they fall outside of the figure, as in
the mayfly shown on page 369, and
are flattened and laid down close
against the surface so as to present only
their thin edges to the water. When
the legs are small, as in the water-
penny, (fig. 1 60) they are covered in
underneath. Sometimes there are no
legs, as in the flatworms, and in the
snail, Ancylus.

Here, surely, we have the impress
of environment. Many living beings
of different structural types are mould-
ed to a common form to meet a com-
mon need; and even the non-living
shelters built by other animals are
fashioned to the same form. The case
of the micro-caddisworm, Ithytrichia FlG - J 61 - T. he larva

., , , \ 11- , i 1 of the caddis- worm,

confusa (ng. 1 6 1 ) is also limpet-shaped ; Ithytrichi




Adjustment of the Life Cycle



261



so also is the pupal shelter of the caterpillar of Elophila
fulicalis; hardly less so is the portable case of the larva
of the caddis-fly, Leptocerus ancylus or of Molanna
angustata.

ADJUSTMENT OF THE LIFE CYCLE

Life runs on serenely in the depths of the seas where,
as we have noted in Chapter II, there is no change of
season ; but in shoal and impermanent waters it meets
with great vicissitudes. Winter's freezing and summer's
drouth, exhaustion of food and exclusion of light and of
air, impose hard conditions here. Yet in these shoals
is found perhaps the world's greatest density of popula-




FIG. 162. The flattened and limpet-shaped cases of Ithytrichia
confusa, as they appear attached to the surface of a sub-
merged stone.



262 Adjustment to Conditions of Aquatic Life

tion. Here competition for food and standing room is
most severe. And here are made some of the most
remarkable shifts for maintaining "a place in the sun."

Encystment The shifts which we are here to consider
are those made in avoidance of the struggle shifts
which have to do with the tiding over of unfavorable
seasons by withdrawal from activity. This means
encystment or encasement of some sort or in some
degree. The living substance secretes about itself
some sort of a protective layer, and, enclosed within it,
ceases from all its ordinary functions.

This is the most familiar to us in the reproductive
bodies of plants and animals; in the zygospores of
Spirogyra and desmids and other conjugates; in the
fruiting bodies of the stoneworts; in the seeds of the
higher plants; and in the over-wintering eggs of many
animals. Most remarkable perhaps is the brief seasonal
activity of forms that inhabit temporary pools. Such
Branchipods as Chirocephalus (see fig. 90 on p. 184)
Estheria and Apus, appear in early spring in pools
formed from melting snow. They run a brief course of
a few weeks of activity, lay their eggs and disappear to
be seen no more until the snows melt again. Their
eggs being resistant to both drying and freezing, are
able to await the return of favorable conditions for
growth. The eggs of Estheria have been placed in
water and hatched after being kept dry for nine years.

But it is not alone reproductive bodies that thus tide
over unfavorable periods. The flatworm, Planaria
velata, divides itself into pieces which encyst in a layer
of slime and thus await the return of conditions favor-
able for growth. The copepod, Cyclops bicuspidatus,
according to Birge and Juday (09) spends the summer
in a sort of cocoon composed of mud and other bottom
materials rather firmly cemented together about its



Encystment 263

body. It forms this cocoon about the latter end of
May. It reposes quietly upon the bottom during the
entire summer thro a longer period, indeed, than
that of absence of oxygen from the water. Hatch-
ing and resumption of activity begin in September and
continue into October. Marsh (09) suggests that
with us this species "may be considered preeminently a




FIG. 163. Hibernacula of the common bladderwort.

winter form." It is active in summer only in cold
mountain lakes.

The over- wintering buds (hibernacula) of some aquat-
ic seed plants are among the simplest of these devices.
Those of the common bladderwort are shown in figure
1 63 . At the approach of cold weather the bladderwort
ceases to unfold new leaves, but develops at the tip of
each branch a dense bud composed of close-laid incom-
pletely developed leaves. This is the hibernaculum.
It is really an abbreviated and undeveloped branch.



264 Adjustment to Conditions of Aquatic Life

Unlike other parts of the plant, its specific gravity is
greater than that of water. It is enveloped only by a
thin gelatinous covering. With its development the
functional activity of the old plant ceases; the leaves
lose chlorophyl; their bladders fall away; the tissues




FIG. 164. The remains of a fresh- water sponge that has
grown upon a spray of water-weed. The numerous
rounded seed-like bodies embedded in the disintegrating
tissue are statoblasts. See text.

disintegrate; and finally the hibernacula fall to the
bottom to pass the winter at rest. When the water
begins to be warmer in spring, the buds resume growth,
the axis lengthens, the leaves expand, air spaces
develop and gases fill them, buoying the young shoots
up into better light, and the activities of another season
are begun.



Winter Eggs 265



Statoblasts Perhaps the most specialized of over-
wintering bodies are those of the Bryozoans and fresh-
water sponges, known as statoblasts. These are little
masses of living cells invested with a tough and hard
and highly resistent outer coat. They are formed
within the flesh of the parent animal (as indicated for
Bryozoan in fig. 77 on p. 167), and are liberated at its
dissolution (as indicated for a sponge in the accompany-
ing figure) . They alone survive the winter. As noted
earlier in this chapter, their chitinous coats are often
expanded with air cavities to form efficient floats:
sometimes in Bryozoan statoblasts there is added to
this a series of hooks for securing distribution by ani-
mals (see fig. 150 on p. 247). Often in autumn at the
Cornell Biological Field Station collecting nets become
clogged with these hooked statoblasts.

In the fresh-water sponges the walls of the statoblast
are stiffened with delicate and beautiful siliceous
spicules, and there is at one side a pore through which
the living cells find exit at the proper season. Since
marine sponges lack statoblasts, and some fresh-water
species do not have them, it is probable that they are
an adaptation of the life cycle to conditions imposed
by shoal and impermanent waters.

Winter Eggs Another seasonal modification of the
life cycle is seen in the Rotifers and water-fleas. Here
there are produced two kinds of eggs; summer eggs
that develop quickly and winter eggs that hibernate.
The summer eggs for a long period produce females
only. They develop without fertilization. In both
these groups males are of very infrequent occurrence.
They appear at the end of the season. The last of the
line of parthenogenetic females produce eggs from which
hatch both males and females and the last crop of eggs
is fertilized. These are the over-wintering eggs.



266 Adjustment to Conditions of Aquatic Life

The accompanying figures illustrate both kinds of
eggs in the water-flea, Ceriodaphnia, an inhabitant of
bottomland ponds. Figure 165 shows a female with
the summer eggs in the brood chamber on her back.
These thin-shelled eggs are greenish in color. They
hatch where they are and the young Ceriodaphnias live




FIG. 165. Ceriodaphnia, with summer eggs.

within the brood-chamber until they have absorbed all
the yolk stored within the egg and have become very
active. Then they escape between the valves of the
shell at the rear.

Winter eggs in this species are produced singly.
Figure 166 shows one in the brood chamber of another
female. It is inclosed in a chitinized protective cover-



Winter Eggs 267



ing, which, because of its saddle-shaped outline, is called
an ephippium. This egg is liberated unhatched by the
molting of the female, as shown in figure 167. It
remains in its ephippium over winter, protected from
freezing, from drouth and from mechanical injury,




FIG. 166. Ceriodaphnia bearing an ephippium containing the
single winter egg.

and buoyed up just enough to prevent deep sub-
mergence in the mud of the bottom. With the return
of warmer weather it may hatch and start a new
line of parthenogenetic female Ceriodaphnias.

Thus, it is that many organisms are removed from our
waters during a considerable part of the winter season.



268 Adjustment to Conditions of Aquatic Life

The water-fleas and many of our rotifers are hibernating
as winter eggs. The bryozoans and sponges are hiber-
nating as statoblasts. Doubtless many of the simpler
organisms whose ways are still unknown to us have their




FIG. 167. Ceriodaphnia, molted skin and liberated ephippium
of the same individual shown in the preceding figure. This
photograph was taken only a few minutes after the other.
The female after molting immediately swam away.

own times and seasons and modes of passing a period of
rest. It is doubtless due, also, to the ease and safety
with which they may be transported when in such
condition that they all have a wide distribution over the
face of the earth. In range, they are cosmopolitan.



Readaptation to Life in the Water 269

Readaptations to life in the water The more primitive
groups of aquatic organisms have, doubtless, always
been aquatic; but the aquatic members of several of
the higher groups give evidence of terrestrial ancestry.
Among the reasons for believing them to have devel-
oped from forms that once lived on land is the possession
of characters that could have developed only under
terrestrial conditions, such as the stomates for intake of
air in the aquatic vascular plants, the lungs of aquatic
mammals, and the tracheae and spiracles of aquatic
insects. Furthermore, they are but a few members
(relatively speaking) of large groups that remain
predominantly terrestrial in habits, and there are among
them many diverse forms, fitted for aquatic life in very
different ways, and showing many signs of independent
adaptation.

I

The vascular plants are restricted in their distribution
to shores and to shoal waters. They are fitted for
growth in fixed position and they possess a high degree
of internal organization with a development of vessels
and supporting structures that cannot withstand the
beating of heavy waves. As compared with the land
plants of the same groups, these are their chief structural
characteristics :

1. In root: reduced development. With submer-
gence there is less need of roots for food-gathering, since
absorption may take place over the entire surface.
Roots of aquatic plants serve mainly as anchors ; in a
few floating plants as balancers; sometime they are
entirely absent.

2. In stems: many characteristics, chief of which
are the following:

a. Reduction of water-carrying tubes, for the ob-
vious reason that water is everywhere available



270 Adjustment to Conditions of Aquatic Life

b. Reduction of wood vessels and of wood fibers
and other mechanical tissues. In the denser
medium of the water these are not needed, as
they are in the air. to support the body. Pliancy,
not rigidity, is required in the water.

c. Enlargement of air spaces. This is prevalent
and most striking. One may grasp a handful
of any aquatic stems beneath the water and
squeeze a cloud of bubbles out of them.

d. Concentration of vessels near the center of
the stem where they are least liable to injury
by bending.

e. A general tendency toward slenderness and
pliancy in manner of growth, brought about
usually by elongation of the internodes.

3. In leaves: many adaptive characters; among
them these:

a. Thinness of epidermis, with absence of cuticle
and of ordinary epidermal hairs. This favors
absorption through the general surfaces.

b. Reduction of stomates, which can no longer
serve for intake of air.

c. Development of chlorophyl in the epidermis,
which, losing the characters which fit it for
control of evaporation, takes on an assimilatory
function.

d. Isolateral development, i. e., lack of differ-
entiation between the two surfaces.

e. Absence of petioles.

/. Alteration of leaf form with two general ten-
dencies manifest: Those growing in the most
stagnant waters become much dissected (blad-
derworts, milfoils, hornworts, crowfoots, etc.).

Those growing in the more open and turbu-
lent waters become long, ribbonlike, and very
flexible (eelgrass, etc.).



Readaptations to Life in the Water 271

In general, the following characteristics:

a. The production of abundance of mucilage,
which, forming a coating over the surface, may
be of use to the plants in various ways :

1. For flotation, when the mucilage is of low
specific gravity.

2. For defense against animals to which the
mucilage is inedible or repugnant.

3. For lubrication: a very important need;
for, when crossed plant stems are tossed by
waves, the mucilage reduces their mutual
friction and prevents breaking.

4. For preventing evaporation on chance ex-
posure to the air.

5. For regulating osmotic pressure, and aiding
in the physical processes of metabolism.

b . Development of vegetative reproductive bodies :

1. Hibernacula, such as those of the bladder-
wort (fig. 162).

2. Tubers such as those of the sago pondweed
(see fig. 228), the arrow-head, etc.

3. Burs, such as terminate the leafy shoots of
the ruffled pondweed (see fig. 63).

4. Offsets and runners, such as are common
among land plants.

5. Detachable branches and stem segments,
that freely produce adventitious roots and
establish new plants.

c. Diminished seed production. This is correlated

with the preceding. Some aquatics such as
duckweeds and hornworts are rarely known to
produce seeds; others ripen seeds, but rarely
develop plants from them. Their increase is
by means of the vegetative propagative struc-
tures above mentioned, and they hold their
place in the world by continuous occupation
of it.



272 Adjustment to Conditions of Aquatic Life

II

The mammals that live in the water are two small
orders of whales, Cetacea and Sirenia, and a few
scattering representatives of half a dozen other orders.
Tho few in number they represent almost the entire
range of mammalian structure. They vary in their
degree of fitness for water life from the shore-haunting
water-vole, that has not even webbing between its toes,
to the ocean going whales, of distinctly fish-like form,
that are entirely seaworthy. It is a fine series of
adaptations they present.

For all land-animals, returned to the water to live,
there are two principal problems, (i) the problem of
getting air and (2) the problem of locomotion in the
denser medium. Warm-blooded animals have also
the problem of maintaining the heat of the body in
contact with the water. To begin with the point last
named, aquatic mammals have solved the problem of
heat insulation by developing a copious layer of fat and
oils underneath the skin. This development culminates
in the extraordinary accumulation of blubber in arctic
whales.

No aquatic mammals have developed gills. They all
breathe by means of lungs as did their terrestrial ances-
tors. All must come to the surface for air. Their
respiratory adaptations are slight, consisting in the
shifting of the nostrils to a more dorsal position and
providing them with closable flaps or valves, to prevent
ingress of the water during submergence.

It is with reference to aquatic locomotion that
mammals show the most striking adaptations. About
in proportion to their fitness for life in the water they
approximate to the fish-like contour of body that we
have already discussed (page 249) as stream-like form.
Solidity and compactness of the anterior portion of the



Aquatic Adaptations of Insects 273

body are brought about by consolidation of the neck
vertebrae and shortening of the cranium. Smoothness
of contour, (and therefore diminished resistance to
passage through the water) is promoted by (i) the loss
of hair; (2) the loss of the external ears; (3) the
shortening and deflection of the basal joints of the legs;
(4) elongation of the rear portion of the body. Caudal
propulsion is attained in the whales by the huge
dorsally flattened tail; in the seals (whose ancestors
were perhaps tailless) by the backwardly directed hind
legs.

Compared with these marine mammals those of our
fresh waters show very moderate departures from
terrestrial form. The beaver has broadly webbed hind
feet for swimming. The muskrat has a laterally
flattened tail. The mink, the otter and the fisher, with
their elongate bodies and paddle-like legs, are best
fitted for life in the water, and spend much time in it.
But all fresh-water mammals make nests and rear their
young on land.

Ill

The insects that live in the water have adaptations for
swimming that parallel those of mammals, just noted;
but some other adaptations grow out of the different
nature of their respiratory system, and, more grow out
of the difference in their life cycle. The free-living
larval stage of insects offers opportunity for independ-
ent adaptation in that stage. Adult insects of but two
orders, Coleoptera and Hemiptera, are commonly
found in the water. These, as compared with their
terrestrial relatives, exhibit many of the same adapta-


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Online LibraryJames G. (James George) NeedhamThe life of inland waters; an elementary text book of fresh-water biology for students → online text (page 15 of 26)