The trait that is common to all invertebrates is the absence of a
vertebral column (backbone): this creates a distinction between invertebrates and vertebrates. The distinction is one of convenience only; it is not based on any clear biologically
homologous trait, any more than the common trait of having wings functionally unites insects, bats, and birds, or than not having wings unites
tortoises,
snails and
sponges. Being animals, invertebrates are heterotrophs, and require sustenance in the form of the consumption of other organisms. With a few exceptions, such as the
Porifera, invertebrates generally have bodies composed of differentiated tissues. There is also typically a digestive chamber with one or two openings to the exterior.
Morphology and symmetry The
body plans of most
multicellular organisms exhibit some form of
symmetry, whether radial, bilateral, or spherical. A minority, however, exhibit no symmetry. One example of asymmetric invertebrates includes all
gastropod species. This is easily seen in
snails and
sea snails, which have helical shells.
Slugs appear externally symmetrical, but their
pneumostome (breathing hole) is located on the right side. Other gastropods develop external asymmetry, such as
Glaucus atlanticus that develops asymmetrical
cerata as they mature. The origin of gastropod asymmetry is a subject of scientific debate. Other examples of asymmetry are found in
fiddler crabs and
hermit crabs. They often have one claw much larger than the other. If a male fiddler loses its large claw, it will grow another on the opposite side after
moulting.
Sessile animals such as
sponges are asymmetrical alongside
coral colonies (with the exception of the individual
polyps that exhibit radial symmetry);
Alpheidae claws that lack pincers; and some
copepods,
polyopisthocotyleans, and
monogeneans which parasitize by attachment or residency within the
gill chamber of their
fish hosts).
Nervous system Neurons differ in invertebrates from mammalian cells. Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH. The first invertebrate in which a neuron cell was identified was the medicinal
leech,
Hirudo medicinalis. Learning and memory using nociceptors have been described in the sea hare,
Aplysia. Mollusk neurons are able to detect increasing pressures and tissue trauma. Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs,
nematodes and arthropods.
Respiratory system . The largest tracheae run across the width of the body of the cockroach and are horizontal in this image. Scale bar, 2 mm. of the cockroach. Scale bar, 2.0 mm. One type of invertebrate respiratory system is the open
respiratory system composed of
spiracles, tracheae, and
tracheoles that
terrestrial arthropods have to transport
metabolic gases to and from tissues. The distribution of spiracles can vary greatly among the many
orders of insects, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it. The tracheae are invaginations of the cuticular
exoskeleton that branch (
anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm. The smallest tubes, tracheoles, penetrate cells and serve as sites of
diffusion for
water,
oxygen, and
carbon dioxide. Gas may be conducted through the respiratory system by means of active
ventilation or passive diffusion. Unlike vertebrates, insects do not generally carry oxygen in their
haemolymph. A tracheal tube may contain ridge-like circumferential rings of
taenidia in various
geometries such as loops or
helices. In the
head,
thorax, or
abdomen, tracheae may also be connected to air sacs. Many insects, such as
grasshoppers and
bees, which actively pump the air sacs in their abdomen, are able to control the flow of air through their body. In some aquatic insects, the tracheae exchange gas through the body wall directly, in the form of a
gill, or function essentially as normal, via a
plastron. Despite being internal, the tracheae of arthropods are shed during moulting (
ecdysis).
Hearing Reproduction Like vertebrates, most invertebrates reproduce at least partly through
sexual reproduction. They produce specialized
reproductive cells that undergo
meiosis to produce smaller, motile
spermatozoa or larger, non-motile
ova. These fuse to form
zygotes, which develop into new individuals. Others are capable of
asexual reproduction, or sometimes, both methods of reproduction. Extensive research with model invertebrate species such as
Drosophila melanogaster and
Caenorhabditis elegans has contributed much to our understanding of
meiosis and reproduction. However, beyond the few model systems, the modes of reproduction found in invertebrates show incredible diversity. In one extreme example, it is estimated that 10% of
orbatid mite species have persisted without sexual reproduction and have reproduced asexually for more than 400 million years. Social interaction is particularly salient in
eusocial species but applies to other invertebrates as well. Insects recognize information transmitted by other insects.
Phyla '' from the
Pliocene of
Cyprus The term invertebrates covers several phyla. One of these are the sponges (
Porifera). They were long thought to have diverged from other animals early. They lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges typically feed by drawing in water through pores. Some speculate that sponges are not so primitive, but may instead be secondarily simplified. The
Ctenophora and the
Cnidaria, which includes
sea anemones,
corals, and
jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. 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, they are sometimes called
diploblastic. The
Echinodermata are radially symmetric and exclusively marine, including
starfish (Asteroidea),
sea urchins, (Echinoidea),
brittle stars (Ophiuroidea),
sea cucumbers (Holothuroidea) and
feather stars (Crinoidea). The largest animal phylum is also included within invertebrates: the Arthropoda, including insects,
spiders,
crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. In addition, they possess a hardened exoskeleton that is periodically shed during growth. Two smaller phyla, the
Onychophora and
Tardigrada, are close relatives of the arthropods and share some traits with them, excluding the hardened exoskeleton. The
Nematoda, or roundworms, are perhaps the second largest animal phylum, and are also invertebrates. 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
Kinorhyncha,
Priapulida, and
Loricifera. These groups have a reduced coelom, called a pseudocoelom. Other invertebrates include the
Nemertea, or ribbon worms, and the
Sipuncula. Another phylum is
Platyhelminthes, the flatworms. These were originally considered primitive, but it now appears they developed from more complex ancestors. Flatworms are
acoelomates, lacking a body cavity, as are their closest relatives, the microscopic
Gastrotricha. The
Rotifera, or rotifers, are common in aqueous environments. Invertebrates also include the
Acanthocephala, or spiny-headed worms, the
Gnathostomulida,
Micrognathozoa, and the
Cycliophora. Also included are two of the most successful animal phyla, the Mollusca and Annelida. The former, which is the second-largest animal phylum by number of described species, 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. Among lesser phyla of invertebrates are the
Hemichordata, or acorn worms, and the Chaetognatha, or arrow worms. Other phyla include
Acoelomorpha,
Brachiopoda,
Bryozoa,
Entoprocta,
Phoronida, and
Xenoturbellida. == Classification ==