Invertebrates in the context of Paraphyletic

Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (from Latin quattuor for "four", and pes, pedis for "foot"). Quadruped animals are found among both vertebrates and invertebrates.

A facultative anaerobic organism is an organism that makes ATP by aerobic respiration if oxygen is present, but is capable of switching to fermentation if oxygen is absent.

Some examples of facultatively anaerobic bacteria are Staphylococcus spp., Escherichia coli, Salmonella, Listeria spp., Shewanella oneidensis and Yersinia pestis. Certain eukaryotes are also facultative anaerobes, including pupfish, fungi such as Saccharomyces cerevisiae and many aquatic invertebrates such as nereid polychaetes.

A squid (pl. squid) is a mollusc with an elongated soft body, large eyes, eight arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting these criteria). Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.

Squid diverged from other cephalopods during the Jurassic and radiated at the beginning of the Late Cretaceous, and occupy a similar role to teleost fish as open-water predators of similar size and behaviour. They play an important role in the open-water food web. The two long tentacles are used to grab prey and the eight arms to hold and control it. The beak then cuts the food into suitable size chunks for swallowing. Squid are rapid swimmers, moving by jet propulsion, and largely locate their prey by sight. They are among the most intelligent of invertebrates, with groups of Humboldt squid having been observed hunting cooperatively. They are preyed on by sharks, other fish, sea birds, seals and cetaceans, particularly sperm whales.

The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word βένθος (bénthos), meaning "the depths". Organisms living in this zone are called benthos and include microorganisms (e.g., bacteria and fungi) as well as larger invertebrates, such as crustaceans and polychaetes.

Organisms here, known as bottom dwellers, generally live in close relationship with the substrate and many are permanently attached to the bottom. The benthic boundary layer, which includes the bottom layer of water and the uppermost layer of sediment directly influenced by the overlying water, is an integral part of the benthic zone, as it greatly influences the biological activity that takes place there. Examples of contact soil layers include sand bottoms, rocky outcrops, coral, and bay mud.

In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at specific times of the year, or at specific points in its life cycle.

In medieval times, it was also known as "mewing" (from the French verb "muer", to moult), a term that lives on in the name of Britain's Royal Mews where the King's hawks used to be kept during moulting time before becoming horse stables after Tudor times.

Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores (seed predators) feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable. Granivores are found across many families of vertebrates (especially mammals and birds) as well as invertebrates (mainly insects); thus, seed predation occurs in virtually all terrestrial ecosystems.

Seed predation is commonly divided into two distinctive temporal categories, pre-dispersal and post-dispersal predation, which affect the fitness of the parental plant and the dispersed offspring (the seed), respectively. Mitigating pre- and post-dispersal predation may involve different strategies. To counter seed predation, plants have evolved both physical defenses (e.g., shape and toughness of the seed coat) and chemical defenses (secondary compounds such as tannins and alkaloids). However, as plants have evolved seed defenses, seed predators have adapted to plant defenses (e.g., ability to detoxify chemical compounds). Thus, many interesting examples of coevolution arise from this dynamic relationship.

Whether invertebrates can feel pain is a contentious issue. Although there are numerous definitions of pain, almost all involve two key components. First, nociception is required. This is the ability to detect noxious stimuli which evokes a reflex response that moves the entire animal, or the affected part of its body, away from the source of the stimulus. The concept of nociception does not necessarily imply any adverse, subjective feeling; it is a reflex action. The second component is the experience of "pain" itself, or suffering—i.e., the internal, emotional interpretation of the nociceptive experience. Pain is therefore a private, emotional experience. Pain cannot be directly measured in other animals, including other humans; responses to putatively painful stimuli can be measured, but not the experience itself. To address this problem when assessing the capacity of other species to experience pain, argument-by-analogy is used. This is based on the principle that if a non-human animal's responses to stimuli are similar to those of humans, it is likely to have had an analogous experience. It has been argued that if a pin is stuck in a chimpanzee's finger and they rapidly withdraw their hand, then argument-by-analogy implies that like humans, they felt pain. It has been questioned why the inference does not then follow that a cockroach experiences pain when it writhes after being stuck with a pin. This argument-by-analogy approach to the concept of pain in invertebrates has been followed by others.

The ability to experience nociception has been subject to natural selection and offers the advantage of reducing further harm to the organism. While it might be expected therefore that nociception is widespread and robust, nociception varies across species. For example, the chemical capsaicin is commonly used as a noxious stimulus in experiments with mammals; however, the African naked mole-rat, Heterocephalus glaber, an unusual rodent species that lacks pain-related neuropeptides (e.g., substance P) in cutaneous sensory fibres, shows a unique and remarkable lack of pain-related behaviours to acid and capsaicin. Similarly, capsaicin triggers nociceptors in some invertebrates, but this substance is not noxious to Drosophila melanogaster (the common fruit fly).Criteria that may indicate a potential for experiencing pain include:

Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are gastropods, i.e. they are sea snails (marine gastropod mollusks) that, over evolutionary time, have either entirely lost their shells or have seemingly lost their shells due to having a significantly reduced or internal shell. The name "sea slug" is often applied to nudibranchs and a paraphyletic set of other marine gastropods without apparent shells.

Sea slugs have an enormous variation in body shape, color, and size. Most are partially translucent. The often bright colors of reef-dwelling species imply that these animals are under constant threat of predators. Still, the color can warn other animals of the sea slug's toxic stinging cells (nematocysts) or offensive taste. Like all gastropods, they have small, razor-sharp teeth called radulas. Most sea slugs have a pair of rhinophores—sensory tentacles used primarily for the sense of smell—on their head, with a small eye at the base of each rhinophore. Many have feathery structures (cerata) on the back, often in a contrasting color, which act as gills. All species of genuine sea slugs have a selected prey animal on which they depend for food, including certain jellyfish, bryozoans, sea anemones, plankton, and other species of sea slugs.

The forest floor, also called detritus or duff, is the part of a forest ecosystem that mediates between the living, aboveground portion of the forest and the mineral soil, principally composed of dead and decaying plant matter such as rotting wood and shed leaves. In some countries, like Canada, forest floor refers to L, F and H organic horizons. It hosts a wide variety of decomposers and predators, including invertebrates, fungi, algae, bacteria, and archaea.

The forest floor serves as a bridge between the above ground living vegetation and the soil, and thus is a crucial component in nutrient transfer through the biogeochemical cycle. Leaf litter and other plant litter transmits nutrients from plants to the soil. The plant litter of the forest floor (or L horizon) prevents erosion, conserves moisture, and provides nutrients to the entire ecosystem. The F horizon consists of plant material in which decomposition is apparent, but the origins of plant residues are still distinguishable. The H horizon consists of well-decomposed plant material so that plant residues are not recognizable, with the exception of some roots or wood.