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James G. (James George) Needham.

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disc-like foot to some solid support or to the surface
film, from which they often hang suspended. But at
times of abundance (and under conditions that are not
at present well understood) they become detached and
drift about in the water. A hydra of a brick-red color
swarms about the outlet of Little Clear Pond at Saranac
Inn, N. Y., in early summer, and drifts down the out-
flowing stream, often in such abundance that the water
is tinged with red. The young trout in hatching ponds
through which this stream flows, neglect their regular
ration of ground liver, and feed exclusively upon the
hydras, so long as the abundance continues. The
hydras play fast-and-loose in the stream, attaching
themselves when they meet with some solid support,
and then loosening and drifting again.

Clear, sunlit pools are the favorite haunts of hydras,
and the early summer appears to be the time of their
maximum abundance. They attach themselves mainly
to submerged stems and leaves, and to the underside of
floating duckmeat. They feed upon lesser animals
which abound in the plancton, and, multiplying rapidly
by a simple vegetative process of budding with subse-



1 64



Aquatic Organisms



quent detachment, they become numerous when
plancton abounds. Kofoid ('08) found a maximum
number of 5335 hydras per cubic meter of water in
Quiver Lake during a vernal plancton pulse in 1897.

Fresh-water sponges grow abundantly in the margins
of lakes and pools and in clear, slow-flowing streams.
They are always sessile upon some solid support. In
sunlight they are green, in the shade they grow pale.
The species that branch out in slender finger-like pro-
cesses are most suggestive of plants in both form and

color, but even the slen-
derest sponge is more
massive than any plant
body; and when one
looks closely at the
surface he sees it rough-
ened all over with the
points of innumerable
spicules, and sees open
osteoles at the tips. By

FIG. 74. Three simple metazoans of these sienS Sponges of

isolated structural types. 1 * r

A. a scruff back, ChoUmctus; B. Hydra, bearing a Whatever torm Or




ud; C. a tardigrade. Macrobiotus.

The commonest sponges are low encrusting species
that grow outspread over the surfaces of logs and
timbers. When, in early summer, one overturns a
floating log that has been long undisturbed he may find
it dotted with young sponges, growing as little yellow,
circular, fleshy discs, bristling with spicules, and each
with a large central osteole. Later they grow irregular
in outline, and thicker in mass. Toward the end of
their growing season they develop statoblasts or
gemmules (winter-buds) next to the substratum (see
fig. 164 on p. 264), and then they die and disintegrate.
So our fresh-water sponges are creatures of summer,
like daffodils.



Fresh-Water Sponges



165



All sponges are aquatic, and most of them are marine.
Only the fresh-water forms produce statcblasts, and
live as annuals.

In figure 74 we show two other simple metazoans
(unrelated to Hydra and of higher structural rank




FIG. 75. A semi-columnar sponge from the Pulton Chain of Lakes near Old
Forge, N. Y. Half natural size. Photo, kindly loaned by Dr. E. P. Felt
of the N. Y. State Museum.

than the sponges) that during the history of syste-
matic zoology, have been much bandied about among
the groups, seeking proper taxonomic associates.
Chcetonotus often appears on the side of an aquarium jar
gliding slowly over the surface of the glass as a minute
oblong white speck. It is an inhabitant of water con-
taining plant infusions, and an associate of Paramecium
which to the naked eye it somewhat resembles.



1 66



Aquatic Organisms



Macrobiotus may be met in the same way and place,
but less commonly. It may also be taken in plancton ;
but its favorite habitat appears to be tangles of water-
plants, over whose stems it crawls clumsily with the aid

_ of its four pairs of stub-

^L by strong-clawed feet.

i It also inhabits the

I most temporary pools,

Sj even rainspouts and

^| Kb \Jr Vx stove urns, and is able

to withstand dessi-

^tfr^ cation.

Chaetonotus is probably
most nearly related to
the Rotifers; Macro-
biotus, to the mites.

Bryozoans The
Bryozoans or "moss
animals" (called also
Polyzoans) are colonial
forms that are very
common in fresh water.
They grow always in
sessile colonies, which
have a more or less
plant-like mode of
branching. Their fixity
in place, their spreading
branches and the
brownish color of the
test they secrete give
the commoner forms

an aspect enough like minute brown creeping water
mosses to have suggested the name. The individ-
ual animals (zooids) of a colony are minute, requir-
ing a pocket lens for their examination, but the colo-




FIG. 76. Bryozoan colonies, slightly en-
larged; a dense colony of Plumatella on
a grass-stem; a beginning colony on a
leaf (above) ; and a loosely grown colony
of FredericeUa.



Bryozoans



167



nies are often large and conspicuous. Two of the
commoner genera are shown in figure 76, natural
size. These may be found in every brook or pond,
growing in flat spreading colonies on leaves or pieces of
bark or stones. Often a flat board that has long been
floating on the water, if overturned , will show a com-
plete and beautiful tracery of entire colonies outspread
upon the surface. New zooids are produced by bud-
ding. The buds remain permanently attached, each
at the tip of a branch. With growth in length and the
formation of a tough brown-
ish cuticle over every por-
tion except the ends, the
skeleton of the colony devel-
ops. This skeleton is what
we see when we lift the leaf
from the water and look at
the colony brown, branch-
ing tubes, with a hole in the
end of each branch. Noth-
ing that looks like an ani-
mal is visible, for the zooids
which are very sensitive and
very delicate have all with-
drawn into shelter. They
suddenly disappear on the slightest disturbance of the
water, and only slowly extend again.

If we put a leaf or stone bearing a small colony into a
glass of water and let it stand quietly for a time the
zooids will slowly extend themselves, each unfolding a
beautiful crown of tentacles. There are few more
beautiful sights to be witnessed through a lens than the
blossoming out of these delicate transparent, flower-
like, crowns of tentacles from the tips of the apparently
lifeless branches of a populous colony. They unfold
from each bud, like a whorl of slender petals and slowly




?IG. 77. Three zooids of the bryo-
zoan, Plumatella, magnified.

I, expanded; m, retracted; n, partly re-
tracted; i, anus; j, intestine; k, de-
veloping statoblast.



168



Aquatic Organisms



extend their tips outward in graceful curves. Then one
sees a mouth in the midst of the tentacles, and water-
currents set up by the lashing of the cilia which cover




FIG. 78. A colony of Pectinatella, one-half natural size. Note the
distribution of buds in close groups over the surface. The large
hole marks the location of the stick around which the colony grew.

them. A close examination with the microscope will
reveal in each zooid the usual system of animal organs.
The alimentary canal is U-shaped its two openings
being near together at the exposed end of the body.



Bryozoans 169



Several Bryzoans secrete a gelatinous covering
instead of a solid tube, and the colonies become in-
vested in a soft transparent matrix. Pectinatella
(fig. 78) is one of these. It grows in large, more or
less spherical colonies, often resembling a muskmelon
in size, shape and superficial appearance. It is a not
uncommon inhabitant of bayous and ditches and slow-
flowing streams. It grows in most perfect spherical
form when attached to a rather small twig. The
clustered zooids form grayish rosettes upon the surface
of the huge translucent sphere. Late in the season
when statoblasts appear the surface becomes thickly
besprinkled with brown. Still later, after the zooids
have died, and the statoblasts have been scattered the
supporting gelatin persists, blocks and segments of
it, derived from disintegrating colonies, now green from
an overgrowth of algae, are scattered about the shores.

There are but a few genera of fresh-water bryozoans
some six or seven and Plumatella is much the com-
monest one. Plumatella and allied forms grow in water
pipes. They gather in enormous masses upon the sluice-
ways and weirs of water reservoirs. They sometimes
cover every solid support with massive colonies of inter-
laced and heaped-up branches. Thus they form an
incrusting layer thick enough to be removed from flat
surfaces with shovels. Its removal is demanded because
the bryozoans threaten the potability of the water sup-
ply. They do no harm while living and active, but
when with unfavorable conditions they begin to die,
their decomposing remains may befoul the water of an
entire reservoir.

Cristatella is a flat, rather leech-shaped form that is
often found on the under side of lily pads. It is re-
markable for the fact that the entire colony is capable
of a slow creeping locomotion. The zooids act together
as one organism.



170



Aquatic Organisms



The free-living flatworms abound in most shoal fresh
waters. Some live in shallow pools; others in lakes
and rivers, others in spring-fed brooks. They gather
on the under sides of stones, sticks and trash, and con-
ceal themselves amid vegetation, usually shunning
the light. They are often collected unnoticed, and
crawl at night from cover and lie outspread upon the




FIG. 79. Flatworms.

A, diagram of a planarian, showing food cavity; M, mouth at end of cylindric pharynx, directed
downward underneath the body; B, Dendroccelum; C, a chain of five individuals of Stenps-
tomum formed by automatic division of the body, (after Keller). Note the anterior position
of the mouth and the unbranched condition of the alimentary canal in this Rhabdoccele type.

sides of our aquaria. We may usually find the larger
species by lifting stones from a stream bed or a lake
shore, and searching the under side of them.

Flatworms are covered with vibratile cilia and travel
from place to place with a slow gliding motion. They
range in length from less than a millimeter to several
centimeters. The smaller among them are easily mis-



Flatworms 171



taken for large ciliate protozoans, if viewed only with
the unaided eye; but under the microscope the alimen-
tary canal and other internal organs are at once
apparent. They are multicellular and have little like-
ness to any infusoria, save in the ciliated exterior.
Most members of the group are flattened, as the com-
mon name suggests, but a few are cylindric, or even
filiform. A few are inclined to depart from shelter and
to swim in the open water, especially at time of abund-
ance. Kofoid ('08) found them in the channel waters
of the Illinois River in average numbers above 100 per
cubic meter, with a maximum record of 19250 per cubic
meter.

The large flatworms resemble leeches somewhat in
form of body, but they have more of a head outlined
at the anterior end. They lack the segmentation of
body and the attachment discs of leeches, and their
mode of locomotion is so very different they are readily
distinguished. They do not travel by loopings of the
body as do leeches, but they glide along steadily, pro-
pelled by invisible cilia.

The most familiar flatworms are the planarians:
soft and innocuous-looking little carnivores, having
the mouth opening near the midventral surface of the
body, and the food-cavity spreading through the body
in three complexly ramifying branches. They are
often brightly colored, mottled white, or brick red, or
plumbeous, and they have a way of changing color with
every full meal; for the branched alimentary canal
fills, and the color of the food glows through the skin
in the more transparent species. The eggs of planarians
are often found in abundance on stones in streams in
late summer. They are inclosed in little brownish
capsules, of the size and appearance of mustard seeds,
and each capsule is raised on a short stalk from the
surface of the stone. Increase is also by automatic



172



Aquatic Organisms



transverse division of the body, the division plane lying
close behind the mouth. When a new head has been
shaped on the tail-piece, and a new tail on the head-
piece, and two capable organisms have been formed,
then they separate. In some of the simple (Rhab-
doccele) flatworms the body divides into more than two
parts simultaneously and thus chains of new individuals
arise (fig. 79 c).

Thread-worms or Nematodes, abound in all fresh
waters, where they inhabit the ooze of the bottom, or
thick masses of vegetation. They are minute, color-
less, unsegmented, smoothly-contoured cylindric worms
rarely more than a few millimeters long. The tail end
is usually sharply pointed. The mouth is terminal at
the front end of the body, and is surrounded by a few
short microscopic appendages. Within the mouth
cavity there are often little tooth-like appendages.
The alimentary canal is straight and cylindric and
unappendaged, and the food is semifluid organic sub-
stances.



ov



ov




FIG. 80. Diagram of a Nematode worm.

IM, mouth; n, nerve ring; e, alimentary canal; ov,
o, ovaries; a, anus. (After Jagerskiold) .

We can hardly collect any group of pond-dwellers
without also collecting nematodes. They may occupy
any crevice. They slip in between the wing-pads of
insect nymphs, and into the sheaths of plant stems.
When we disturb the trash in the bottom of our collect-
ing dish, we see them swim forth, with violent swings
and reversals of the pliant body. They may easily be
picked up with a pipette.



Bristle-Bearing Worms



173



Oligochetes Associated with the nematodes in the
trash and ooze, there is a group of minute bristle-bear-
ing worms, the naiads (Family Naidae) , similar in slender-
ness and transparency of body, but very different on
close examination; for the body in Nais is segmented,
and each segment is armed with tufts of bristles of
variable length and form. There are many common
members of this family. Besides the graceful Nais
shown in our figure there is Chcetogaster, which creeps
on its dense bristle-clusters as on feet. There is
Stylaria with a long tongue-like
proboscis. There is Dero that lives
at the surface in a tube of some
floating plant stuffs, such as seeds
(fig. 82) or Lemna leaves, slipping
in and out or changing ends in
the tube with wonderful celerity;
and there are many others.

Dero bears usually two pairs of
short gill-lobes at the posterior end
of the body.

All these naiads reproduce habitually by automatic
division of the body, which when in process of develop-
ment, forms chains of incompletely formed individuals,
as in certain of the flat worms before described.

Another group of Oligochetes is represented by
Tubifex and its allies. These dwell in the bottom mud,
living in stationary tubes, which are in part burrows,
and in part chimneys extended above the surface.
The worms remain anchored in these and extend their
lithe bodies forth into the water. On disturbance they
vanish instantly, retreating into their tubes. They are
often red in color, and when thickly associated, as on
sludge in the bed of some polluted pool, they often
cover the bottom as with a carpet of a pale mottled
reddish color.




FIG. 81. Nais. (after
Leunis)



174



Aquatic Organisms




FIG. 82. Dero, in its case made of floating seeds.

Aquatic earthworms, more like the well-known
terrestrial species, burrow deeply into the mud of the
pond bottom.

Other worms occur in the water in great variety; we
have mentioned only a few of the commonest, and
those most frequently seen. There are many parasitic
worms that appear in the water for only a brief period
of their lives: hair-worms (Gordius, etc.), which are
freed from the bodies of insects and other animals in
which they have developed; these often appear in
watering troughs and were once widely believed to
have generated from horse-hairs fallen into the water.
There are larval stages (Cer carlo) of Cestodes and
others, found living in the water for only a brief interval

of passage from one
host animal to an-
other. There are
smaller groups also
like the Nemertine
worms, sparingly
represented in fresh-
water; for informa-
tion concerning
these the reader is
referred to the
larger textbooks of
zoology.




FIG. 83. Tubifex in the bottom mud.



Leeches 175



Leeches The leeches constitute a small group whose
members are nearly all found in fresh-water. They
occur under stones and logs, in water- weeds or bottom
mud, or attached to larger animals. The body is
always depressed, and narrowed toward the ends, more
abruptly toward the posterior end where a strong sucker
is developed. The front end is more tapering and neck-
like, and very pliant. There is no distinct head, but
at the front is a sort of cerebral nerve ring and there are
rudimentary eyes in pairs, and surrounding the mouth
is a more or less well-developed anterior sucker. The
great pliancy of the muscular body, the presence of the
two terminal suckers, and the absence of legs or other
appendages determine the leech 's mode of locomotion.
It ordinarily crawls about by a series of loopings like a
"measuring worm,'* using the suckers like legs for
attachment. The more elongate leeches swim readily
with gentle undulations of the ribbon-like body. The
shorter broader forms hold more constantly with the
rear sucker to some solid support, and when detached
tend to curl up ventrally like an armadillo.

Leeches range in size from little pale species half an
inch long when grown, to the huge blackish members
of the horse-leech group (Hcemopis) a foot or more in
length. Many of them are beautifully colored with
soft green and yellow tints. The much branched
alimentary canal, when filled with food, shows through
the skin of the more transparent species in a pattern
that is highly decorative.

Leeches eat mainly animal food. They are para-
sites on large animals or foragers on small animals or
scavengers on dead animals. Very commonly one finds
the parasites attached to the thinner portions of the
skins of turtles, frogs, fishes and craw-fishes. There is
no group in which the boundary between predatory and
parasitic habits is less distinct than in this one; many



176



Aquatic Organisms



leeches will make a feast of vertebrate blood, if occasion
offers, or in absence of this will swallow a few worms
instead.

The mouth of leeches is
adapted for sucking, in some
cases it is armed for making
punctures, as well: hence the
food is either more or less fluid
substances like blood or the
decomposing bodies of dead
animals, or else it consists of
the soft bodies of animals
small enough to be swallowed
whole.

The eggs of leeches are
cared for in various ways:
commonly one finds certain
of them in minute packets,
attached to stones. Others
(Hcemopis, etc.) are stored in
larger capsules and hidden
amid submerged trash. Oth-
ers are sheltered beneath the
body of the parent, and the
young are brooded there for
a time after hatching, as
shown in the accompanying
figure. Nachtrieb (12) states
that they are so carried "until
the young are able to move
about actively and find a host
for a meal of blood.' 1

Leeches are doubtless fed upon by many carnivorous
animals. They are commonly reported to be taken
freely by the trout in Adirondack waters. In Bald Moun-
tain Pond they swim abundantly in the open water.




FIG. 84. A clepsine leech
(Placobdella rugosa), over-
turned and showing the
brood of young protected
beneath the body. (From
the senior author's General
Biology).



Rotifers



177



The Rotifers constitute a large group of minute
animals, most characteristic of fresh- water. They
abound in all sorts of situations, and present an extra-
ordinary variety of forms and habits. Their habits
vary from ranging the open lake to dwelling symbioti-
cally within the tissues of water plants ; from sojourning
in the cool waters of peren-
nial springs, to running a
swift course during the tem-
porary existence of the most
transient pools. They even
maintain themselves in rain-
spouts and stone urns, where
they become desiccated with
evaporation between times of
rain.

Rotifers are mainly micro-
scopic, but a few of the larger
forms are recognizable with
the unaided eye. Often they
become so abundant in pools
as to give to the water a tinge
of their own color. Grouped
together in colonies they be-
come rather conspicuous.
The spherical colonies of Cono-

chilus when attached to leaf-tips, as in the accom-
panying picture, present a bright and flower-like
appearance. Entire colonies often become detached,
and then they go bowling along through the water,
in a most interesting fashion, the individuals jostling
each other as they stand on a common footing, and
all merrily waving their crowns of cilia in unison. Often
a little roadside pool will be found teeming with the
little white roiling spheres, that are quite large enough
to be visible to the unaided eye.




FIG. 85. Three colonies of the
rotifer, Conochilus, attached
to the tips of leaves of the
pond-weed, Nais.



Aquatic Organisms



Melicerta is a large sessile rotifer that lives attached
to the sterns of water-plants and when undisturbed
protrudes its head from the open end of the tube, and
unfolds an enormous four-lobed crown of waving cilia.
It is a beautiful creature. Our picture shows the cases
of a number of Melicertas, aggregated together in a

cluster, one case serving as a
support for the others.

The crown of cilia about the
anterior end of the body is the
most characteristic structure
possessed by rotifers. It is
often circular, and the waving
cilia give it an aspect of rota-
tion, whence the group name.
It is developed in an extra-
ordinary variety of ways as
one may see by consulting in
any book on rotifers the figures
of such as Stephanoceros, Flos-
cularia, Synchceta, Trochos-
phcera and Brachionus.

The cilia are used for driv-
ing food toward the mouth
that lies in their midst, and
for swimming. Most of the
forms are free-swimming, and
many alternately creep and
swim.

Brachionus (fig. 87) shows
well the parts commonly found in rotifers. The body
is inclosed in a lorica or shell that is toothed in front
and angled behind. From its rear protrudes a long
wrinkled muscular "foot," with two short "toes"
at its tip. This serves for creeping. The lobed
crown of cilia occupies the front. Behind the quad-




FIG. 86. Two clusters of rotifers
(Melicerta), the upper but
little magnified. Only the
cases (none of the animals)
appear in the photographs.



Rotifers



179



rangular black eyespot in the center of the body
appears the food communicating apparatus (mastax),
below which lie ovaries and alimentary canal. Any
or all the external parts may be wanting in certain




FIG. 87. A rotifer (Brachionus entzii] in dorsal and ven-
tral views. (After France").

rotifers. The smaller and simpler forms superficially
resemble ciliate infusoria, but the complex organization
shown by the microscope will at once distinguish them.
Rotifers eat micro-organisms smaller than them-
selves. They reproduce by means of eggs, often
parthenogenetically. The males in all species are
smaller than the females and for some species males
are not known.



i8o



Aquatic Organisms



Molluscs A large part of the population of lake and
river beds, shores, and pools is made up of molluscs.

They cling, they
climb, they bur-
row, they float
they do every-
thing but swim in
the water. They
are predominantly
herbivorous, and
constitute a large
proportion of the
producing class
among aquatic
animals. Two great
groups of molluscs
are common in
fresh water, the
familiar groups of
mussels and snails.




Fresh-water mus-
sels (clams, or
bivalves) abound
in suitable places,
where they push
through the mud
or sand with their
muscular protrusi-
ble foot, and drag
the shell along in
a vertical position
leaving a channel-
like trail across the bottom. They feed on micro-organ-
isms.

The two commonest sorts of fresh-water mussels are
roughly distinguished by size and reproductive habits



FIG. 88. A living mussel, Anodonta, with foot
retracted and shell tightly closed. A copious
growth of algae covers the portion of the
shell that is exposed above the mud in loco-
motion: the remainder is buried in oblique


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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 10 of 26)