Invertebrate - biology.

« animals with a five-pointed design.

They live in the sea and move with the help of tiny fluid-filled feet—another feature found nowhere else in the animal world. Zoologists recognize several different groups of worms.

The phylum known as flatworms contains the simplest animals possessing heads.

Nerves and sense organs areconcentrated in the head.

Most flatworms are paper-thin and live in a variety of wet or damp habitats, including the digestive systems of other animals.

Roundwormsrepresent another phylum.

They are more complex than flatworms, with cylindrical bodies and mouthparts designed to pierce their food.

Although flatworms havedigestive systems with only one opening, the roundworm digestive system runs from the mouth straight through its body to an excretory opening—a body plan sharedby more advanced invertebrates as well as vertebrates. Although roundworms are extremely abundant, they often go unseen.

So, too, do many worms that live exclusively in the sea, such as spoonworms (phylum Echiura),peanut worms (phylum Sipuncula), and pogonophores (phylum Pogonophora).

Annelids are a large group of worms that contain some more familiar species.

Amongthem are earthworms—annelids that feed by burrowing through the soil.

An earthworm’s body is divided into repeated segments or rings, a feature shared by annelidsas a whole. IV REPRODUCTION AND LIFE CYCLE Invertebrates display a wide variety of methods of reproduction.

Some invertebrates reproduce by asexual reproduction, in which all offspring are genetically identical tothe parent.

Asexual reproduction methods include fragmentation, in which animals divide into two or more offspring, and budding, in which animals sprout buds thatbreak away to take up life on their own.

The majority of invertebrates reproduce sexually.

The genes from two parents recombine to produce genetically uniqueindividuals.

For most invertebrates, sexual reproduction involves laying eggs.

With a few exceptions, such as scorpions and spiders, most invertebrates abandon theireggs as soon as they are laid, leaving them to develop on their own. When invertebrate eggs hatch, the animals that emerge often look nothing like their parents.

Some are so different that, in the past, zoologists mistook them forentirely new species.

Young like this are known as larvae.

As they grow up, larvae change shape, a process known as metamorphosis.

A larval stage enablesinvertebrates to live in different habitats at different stages of their lives.

For example, adult mussels live fastened to rocks, but their larvae live floating amongplankton.

By having larvae that drift with the currents, mussels are able to disperse and find homes with new food sources for their adult life. The change from larva to adult is quite gradual in many invertebrates, such as crabs and lobsters, but in insects it can be much more abrupt.

Caterpillars, the larvae ofbutterflies and moths, often live for several months, but they take just a few days to turn into adults.

During the transition stage, known as the pupa, the caterpillar’sbody is broken down and reassembled, forming an adult insect that is ready to breed. Most invertebrates are short-lived animals, but slow-growing species often break this rule.

Wood-boring beetles can live well into their teens, while queen termites canlive 40 years or more.

But in the invertebrate world, the real veterans live in the sea.

Growth lines on bivalve shells suggest that some clams can live to be 400 yearsold or more.

An age of about 200 years has been claimed for pogonophoran worms living around hydrothermal vents in the darkness of the deep seafloor. V EVOLUTION As the simplest animals, invertebrates date back to the time when animal life first began in ancient shallow seas.

Zoologists are uncertain when this was, because thefirst invertebrates were small and soft-bodied and left no direct fossil remains.

However, some scientists believe that strange patterns preserved in sedimentary rocksdating back to 1 billion years ago may be the fossilized tracks and burrows of ancient invertebrates.

Other scientists, studying genetic material in living animals, believethat the earliest invertebrates may have appeared even earlier and may already have begun to separate into different phyla before 1 billion years ago. The oldest recognized fossils of invertebrates date back to the close of the Precambrian period, about 550 million years ago.

The best known of these fossil finds, fromthe Ediacaran Hills in southern Australia, include animals that look like jellyfish and annelid worms.

Zoologists disagree about their status.

Some think that they mightwell be ancestors of animals alive today, but others believe they belong to a group of invertebrates that eventually became extinct ( see Ediacaran Fauna). With the start of the Cambrian period 542 million years ago, invertebrate life evolved with almost explosive speed.

Due to the appearance of the first invertebrates withexoskeletons, the fossil record provides a rich record of invertebrate life in the Cambrian period.

By the time the Cambrian period ended 488 million years ago, all theinvertebrate phyla alive today were established. Between that time and the present, invertebrates spread through the seas and also invaded land.

Scientists believe that the first land dwellers were almost certainlyarthropods, including the forerunners of wingless insects.

During the Carboniferous period, which began 359 million years ago, flying insects appeared, including giantdragonflies with a wingspan of up to 75 cm (30 in).

But on land the great expansion of invertebrate life occurred during the Cretaceous period, which started 145 millionyears ago.

Flowering plants first evolved in this period, enabling insects to exploit a whole new source of food and triggering a huge growth in insect life that hascontinued to this day. While many invertebrates flourished, some of the most successful groups of invertebrates in the fossil record nonetheless became extinct.

Giant sea scorpions andtrilobites were types of arthropods that thrived for much of the Paleozoic era, about 270 million years ago, but were unable to survive the great mass extinction at theend of the Permian period 251 million years ago.

Ammonites (mollusks related to today’s octopuses and squids) fared better.

They first appeared during the Silurian period about 440 million years ago and lived into the Mesozoic era, only to vanish at the same time as the dinosaurs, about 65 million years ago.

Their intricate massivespiral shells were often superbly preserved as fossils, some measuring almost 2 m (7 ft) across. VI IMPORTANCE OF INVERTEBRATES The continued prominence of invertebrates, measured by their great diversity and abundance, indicates that these animals have adapted to their ecosystems overmillions of years.

In so doing, invertebrates have become necessary to the health of Earth’s ecology.

For instance, all ecosystems support one or more food chains thatform food webs.

Each chain begins with plants, known as primary producers, which convert light energy into food.

Primary producers are eaten by primary consumers,and secondary consumers eat the plant-eating primary consumers.

Decomposers derive their energy from the dead remains of plants and animals.

Invertebratesoccupy several niches in this food web, acting as primary consumers, secondary consumers, and decomposers. Many invertebrates have a direct and invaluable impact on their environment.

For example, the common earthworm burrows deep below the surface, consuming soilalong the way.

Coiled soil masses known as casts are excreted from the worm’s digestive system, making the soil more fertile.

The earthworm’s burrowing actioncontinually moves mineral-rich soil to the surface, which improves plant growth.

The burrowing action also aerates soil, enhancing drainage.

In another example, ashoney bees, butterflies, and moths flit from flower to flower collecting nectar, they inadvertently transport pollen from the male reproductive structure of one flower tothe female reproductive structure of another flower.

Known as pollination, this leads to the fertilization of the plant’s seeds—an essential stage in the process ofreproduction. Other invertebrates form mutually beneficial partnerships with other animals.

For example, some crabs form alliances with sea anemones, which they fasten to their. »