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Orange elephant ear sponge,
Agelas clathrodes, in foreground. Two corals in the background: a
sea fan,
Iciligorgia schrammi, and a sea rod,
Plexaurella nutans.
Animals are a major group of multicellular organisms, of the kingdom Animalia or Metazoa. Their
body plan becomes fixed as they develop, usually early on in their development as embryos, although some undergo a process of metamorphosis later on in their life. Most animals are motile - can move spontaneously and independently.
Etymology
The word "animal" comes from the Latin word animal, of which animalia is the
plural, and is derived from anima, meaning vital breath or soul. In everyday colloquial usage, the word usually refers to
non-human animals. The biological definition of the word refers to all members of the Kingdom
Animalia. Therefore, when the word "animal" is used in a biological context, humans are included.
Characteristics
Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and usually multicellular[1] (although see Myxozoa), which separates them from bacteria and most protists. They are heterotrophic,[2] generally digesting food in an internal chamber, which separates them from
plants and algae. They are also distinguished from plants, algae,
and fungi by lacking cell walls.[3] All animals are motile,[4] if only at certain life stages. Embryos pass through a blastula
stage, which is a characteristic exclusive to animals.
Structure
With a few exceptions, most notably the sponges (Phylum Porifera), animals have bodies
differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve
tissue, which sends and processes signals. There is also typically an internal digestive chamber, with one or two openings. Animals with this sort of organization are called metazoans, or
eumetazoans when the former is used for animals in general.
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix
composed of collagen and elastic glycoproteins. This may
be calcified to form structures like shells, bones, and spicules. During development it forms a relatively
flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other
multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also,
unique to animal cells are the following intercellular junctions: tight junctions,
gap junctions, and desmosomes.
Reproduction and development
Nearly all animals undergo some form of sexual reproduction. Adults are diploid or
polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile spermatozoa or larger non-motile
ova. These fuse to form zygotes, which develop into new
individuals.
Many animals are also capable of asexual reproduction. This may take place
through parthenogenesis, where fertile eggs are produced without mating, or in some
cases through fragmentation.
A zygote initially develops into a hollow sphere, called a blastula, which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location
and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first
invaginates to form a gastrula with a digestive chamber,
and two separate germ layers - an external ectoderm and an internal endoderm. In most cases,
a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.
Most animals grow by indirectly using the energy of sunlight. Plants use
this energy to convert sunlight into simple sugars using a process
known as photosynthesis. Starting with the molecules carbon dioxide (CO2) and water
(H2O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose (C6H12O6) and releases oxygen
(O2). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants
(or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used
directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for
motion. This process is known as glycolysis.
Animals who live close to hydrothermal vents and cold
seeps on the ocean floor are not dependent on the energy of sunlight. Instead, chemosynthetic archaea and eubacteria
form the base of the food chain.
Origin and fossil record
Animals are generally considered to have evolved from a flagellated eukaryote. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain
sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few
small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.
The first fossils that might represent animals appear towards the end of the Precambrian,
around 575 million years ago, and are known as the Ediacaran or Vendian biota. These are
difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups,
and it is possible they are not really animals at all. Aside from them, most known animal phyla make a more or less simultaneous
appearance during the Cambrian period, about 542 million years ago. It is still disputed
whether this event, called the Cambrian explosion, represents a rapid divergence
between different groups or a change in conditions that made fossilization possible.
Groups of animals
The sponges (Porifera) diverged from other animals early. As mentioned above, they lack the
complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct
tissues. Sponges are sessile and typically feed by drawing in water through pores. Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.
Among the eumetazoan phyla, two are radially symmetric and have digestive chambers with a single opening, which serves as both
the mouth and the anus. These are the Cnidaria, which include sea
anemones, corals, and jellyfish, and the
Ctenophora or comb jellies. Both have distinct tissues, but they are not organized into
organs. There are only two main germ layers, the ectoderm and endoderm, with only
scattered cells between them. As such, these animals are sometimes called diploblastic. The
tiny Placozoans are similar, but they do not have a permanent digestive chamber.
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding
and sensory organs. The body is triploblastic, i.e. all three germ layers are
well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also
an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however - for
instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely
simplified body structures.
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong
to four major lineages:
- Deuterostomes
- Ecdysozoa
- Platyzoa
- Lophotrochozoa
In addition to these, there are a few small groups of bilaterians with relatively similar structure that appear to have
diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed
from the Bilateria as well.
Deuterostomes
Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes the
initial opening (the archenteron) develops into the mouth, and an anus forms separately. In
deuterostomes this is reversed. In most protostomes cells simply fill in the interior of the gastrula to form the mesoderm,
called schizocoelous development, but in deuterostomes it forms through invagination of the
endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos
undergo different cleavage.
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are
the Echinodermata and Chordata. The former are radially
symmetric and exclusively marine, such as starfish, sea
urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and
mammals.
In addition to these, the deuterostomes also include the Hemichordata or acorn worms.
Although they are not especially prominent today, the important fossil graptolites may belong
to this group.
The Chaetognatha or arrow worms may also be deuterostomes, but more recent studies
suggest protostome affinities.
Ecdysozoa
The Ecdysozoa are protostomes, named after the common trait of growth by moulting or
ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders,
crabs, and their kin. All these organisms have a body divided into repeating segments, typically
with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the Nematoda or roundworms, the second largest animal phylum.
Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important
parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, which are
invisible to the unaided eye, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a
pseudocoelom.
The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since
in both embryos develop with spiral cleavage.
Platyzoa
Bedford's flatworm,
Pseudobiceros bedfordi
The Platyzoa include the phylum Platyhelminthes, the
flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more
complex ancestors.
A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms lack a coelom, as do their closest relatives, the microscopic Gastrotricha.
The other platyzoan phyla are microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida,
Micrognathozoa, and possibly the Cycliophora. These groups
share the presence of complex jaws, from which they are called the Gnathifera.
Lophotrochozoa
Roman snail,
Helix pomatia
The Lophotrochozoa include two of the most successful animal phyla, the
Mollusca and Annelida. The former includes animals such as
snails, clams, and squids, and the
latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of
trochophore larvae, but the annelids were considered closer to the arthropods, because they
are both segmented. Now this is generally considered convergent evolution, owing to
many morphological and genetic differences between the two phyla.
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a fan of cilia around the mouth, called a lophophore. These were traditionally grouped together as the lophophorates, but it now appears they are
paraphyletic, some closer to the Nemertea and some to the Mollusca and Annelida. They include
the Brachiopoda or lamp shells, which are prominent in the fossil record, the
Entoprocta, the Phoronida, and possibly the
Bryozoa or moss animals.
Model organisms
Because of the great diversity found in animals, it is more economical for scientists around the world concert their efforts
on a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how
animals function in general. Because they are easy to keep and breed, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organism, and among the first lifeforms to be genetically sequenced. This was facilitated by the
severely reduced state of their genomes, but the double-edged sword here is that with many
genes, introns and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent
of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan genome projects currently in progress. Analysis of the starlet sea
anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to
the Eumetazoa.[5]
An analyse of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the
eumetazoan animals were more comlex than previously assumed. [6]
Other model organisms belonging to the animal family include mouse (Mus musculus) and
zebrafish (Danio rerio}.
History of classification
Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus in the first
hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have
been restricted somewhat. For instance, microscopic protozoa were originally considered animals
because they move, but are now treated separately.
In Linnaeus' original scheme, the animals were one of three kingdoms, divided into
the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current
understanding of the group, though there is some variation from source to source.
See also
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