Teeming Freshwater Environments

In rivers, ponds, lakes, lagoons, and swamps, many invertebrate species are adapted to life in the water but come from other habitats. Thus, water beetles breathe, not with gills, but with spiracles, the way land insects do. This means they must obtain a reserve of air or come to the surface to breathe. Crustaceans have mechanisms that protect them from losing salt in fresh water. With these adaptations, invertebrates make seemingly calm waters the scene of an intense struggle to survive. •

One Species, Two Environments

Many species of land- and air-dwelling insects lay their eggs in water. After hatching, the larvae undergo metamorphosis in the water. This fact has enabled certain species to prosper by colonizing more than one environment at once. Not only does the same individual inhabit different environments at different stages of its life, but it also has distinct feeding habits and means of breathing during^^l 4 those stages. That fact keeps adults of the species from competing with the young for food.

MAYFLY

Hexagenia sp.

8% OF INSECTS LIVE IN WATER

On the Border

The areas in and around water and close to the water's surface are the scenes of a battle for survival. Most freshwater insects live in this zone.

ADULT

After a few days the pupa's skin splits and the adult mosquito emerges. The adult will live for only a few weeks.

LIFE CYCLE OF THE COMMON MOSQUITO

WATER MEASURER

Hydrometra stagnorum

PUPAE

The larva molts four times as it grows, finally reaching the pupa stage.

In Fresh Water

8% OF INSECTS LIVE IN WATER

On the Border

The areas in and around water and close to the water's surface are the scenes of a battle for survival. Most freshwater insects live in this zone.

FKT^M 1

WHITE-CLAWED CRAYFISH

Austropotamobius pallipes Glands on its antennae excrete water and maintain the balance of salts in its body.

WATER BEETLE LARVA

FKT^M 1

WHITE-CLAWED CRAYFISH

Austropotamobius pallipes Glands on its antennae excrete water and maintain the balance of salts in its body.

WATER BEETLE LARVA

Most parasitic worms are microscopic in size. The ones shown here are highly magnified

DRAGONFL Y NYMPH

Ocean invertebrates live in an osmotic balance between water and the salts the water contains. Invertebrates that live in estuaries or other places where salt water receives currents of fresh water (euryhaline organisms) must keep the concentration of salts in their bodies constant, even when the salinity of the water changes. In fresh water, with its low concentration of salts, crustaceans developed mechanisms to eliminate water and capture salts actively-that is, their bodies expend energy on these functions. For this reason river crustaceans, unlike sea crustaceans, urinate.

The Simplest Life-Forms

ven though most organisms simple invertebrates are unable to move such as sponges, jellyfish, and from one place to another; some even sea anemones look like lack certain tissues or an entire vegetables, they belong to the respiratory or digestive system. Other, animal kingdom. Many of these more developed species, such as squid

THE SIMPLICITY OF THE JELLYFISH

The jellyfish is a very simple animal with a gelatinous consistency and with no respiratory, digestive, or excretory systems. It drifts around in warm ocean waters.

RADIAL SYMMETRY

20-21

JOINTLESS

GENERATING ADDED VALUE g POWERFUL TENTACLES 1

THE SIMPLICITY OF THE JELLYFISH

The jellyfish is a very simple animal with a gelatinous consistency and with no respiratory, digestive, or excretory systems. It drifts around in warm ocean waters.

RADIAL SYMMETRY

20-21

JOINTLESS

GENERATING ADDED VALUE g POWERFUL TENTACLES 1

and octopus, can move about and have become skilled marine predators. Cephalopods are the most highly evolved mollusks. Their heads have highly developed eyes, a mouth with two horn-like jaws, and tentacles with suckers to trap their prey. Some cephalopods live in deep-sea waters, whereas others stay close to shore.

Radial Symmetry any of the numerous invertebrates on Earth live in the ocean. Some, such as polyps and jellyfish, have radial symmetry-that is, their bodies are structured around an axis. A typical echinoderm such as the starfish has tiny, flexible, tube-shaped legs arranged like the spokes of a wheel. The animal uses them to hold onto surfaces and to move. Sponges, on the other hand, are very simple, multiple-celled animals, with many small pores that they use to feed.®

RADIAL SYMMETRY

The body parts are organized around a central axis like the spokes on the wheel of a bicycle. Any plane passing through the body will divide it into two halves, each mirroring the other.

SEA URCHIN

Strongylocentrotus franciscanus

Center

Echinoderms

This phylum includes sea lilies, sea cucumbers, urchins, and starfish. The echinoderms have an internal skeleton made of calcified plates and a locomotion system made up of ambulacral grooves with rows of tube feet. In most echinoderm species, the endoskeleton is made of tiny calcareous plates held together by skin and muscle tissue.

Imaginary axis

Imaginary axis

Center

ECHINODERM

means that this animal's body is covered by a spiny skin.

L CRINOIDEA J Sea lilies

ECHINODERM CLASSES

ECHINOIDEA

Sea udchins

ASTEROIDEA Starfish

L CRINOIDEA J Sea lilies

HOLOTHUROIDEA

Sea cucumbers

OPHIUROIDEA Brittle stars

THERE ARE APPROXIMATELY

7,000

LIVING SPECIES AND 13,000 EXTINCT SPECIES OF ECHINODERMS.

Cnidarians

CLASSIFICATION

Cniclarians are a group of aquatic animals that includes jellyfish, hydras, sea anemones, and corals. Their cells are organized in true tissues. They have specialized cells called cnidoblasts for stinging prey and for defense. Two basic types of cnidarians are polyps and jellyfish.

HYDROZOA: Asexual polyp

SCYPHOZOA: Jellyfish

Mesoglea-

Gastrovascular cavity -

Mouth-

Gastrodermis — Epidermis-

JELLYFISH

Pelagia noctiluca

STINGING CELL

Used for defense

Cnidocyst Nucleus —

GAMETES

Adult jellyfish produce sperm and egg cells during meiosis and then release them.

JELLYFISH

The polyp's body grows and begins to form jellyfish, which pile up like a stack of plates.

Operculum -Cnidocilium

Adult jellyfish

Young jellyfish

DISCHARGING

Rolled-lip tube-

Most common habitat

FERTILIZATION

Fertilization takes place in the waters near the jellyfish, resulting in a zygote.

DISCHARGED

COASTS OF THE UNITED STATES

Unfolded stinging tube

POLYP

The planula larva settles at the bottom, where it attaches to a surface. There it develops a mouth and tentacles, and transforms into a polyp.

BLASTULA

The zygote, after a series of cell divisions, becomes a blastula, or hollow sphere, of cells.

PLANULA

The blastula lengthens and becomes a ciliated larva called a planula.

SPECIES OF CNIDARIANS (COELENTERATES).

Water coming out

Porifera

Are sessile aquatic animals. Most live at the bottom of the ocean, although there are some freshwater species. They are the simplest animals, lacking organs or true tissues, and their cells are independent to a certain extent. They are basically water-filtering bodies formed by one or more cavities. Most porifera have no definite shape, but some have radial symmetry.

Epithelial -cell

Oscula

Spicule

The water with food particles enters through the porocytes.

Direction of water flow

Nucleus

Flagellum

ASCON

SYCON

22 THE SIMPLEST LIFE-FORMS

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