Animals are a major group of organisms, classified as the kingdom Animalia or Metazoa. In general they are multicellular, capable of locomotion and responsive to their environment, and
feed by consuming other organisms. 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.
Aristotle divided the living world between animals and plants, and this was followed
by Carolus Linnaeus in the first hierarchial 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. Colloquially humans have been considered to be
separate from the animals, but biologically they are definitely included. The name animal comes from the Latin word animalis, of which animalia is the plural, and ultimately from anima, meaning
vital breath or soul.
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 a nervous system, 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, 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 occasionally polyploid. They have a few specialized reproductive cells, which undergo meiosis to produce smaller motile sperm 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 rearrangment 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.
Origin and fossil record
Animals are generally considered to have evolved from flagellate protozoa.
Their closest living relatives are the choanoflagellates, collared
flagellates that have the same structure as certain sponge cells do. Molecular studies place them in a supergroup called the
opisthokonts, which also include the fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal sperm, whereas other eukaryotes tend to
have anterior flagella.
The first fossils that might represent animals appear towards the end of the Precambrian, around 600 million years ago, and are known as the 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
animal phyla with known phyla make a more or less simultaneous appearance during the Cambrian period, about 570 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 very early on. As mentioned, they lack the complex organization found in
most other phyla. The cells are differentiated, but not organized into distinct tissues. Sponges are sessile and typically feed
by drawing in water through pores all over the body, which is supported by a skeleton
typically divided into spicules. The extinct 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 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 phylum Placozoa is similar, but individuals 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, but
in deuterostomes it forms through evagination 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 sea stars, 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 this is less certain.
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 visible to the unaided eye, and the Kinorhyncha,
Priapulida, and Loricifera,
which are all microscopic. 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
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
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 Ectoprocta or moss
animals.
History of classification
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.
Examples
Some well-known types of animals, listed by their common names:
- alpaca, ant, antelope, badger, bear, bee, beetle, bird, bison, butterfly,
cat, coral, chicken, dinosaur, dog, elk, fish, fly, fox, frog, goat, horse, human, jellyfish, lion, lizard, lynx, monkey, octopus, owl, ox, parrot, penguin, pig, rabbit, rat, salamander, scorpion, seahorse, shark, sheep, sloth, snake, spider, squid, starfish, turtle, whale, wolf, worm, zebra
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