Zoology Terms Starting With N
Zoology Glossary: N
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Nematocyst
/ neh-MAT-oh-sist / · Greek nema, thread; kystis, bladder
Nematocyst is the explosive stinging capsule found inside specialized cells called cnidocytes in jellyfish, sea anemones, and other cnidarians, containing a coiled, barbed thread that fires outward at extraordinary speed to inject venom into prey or predators.
Each nematocyst is a pressurized capsule held within a single cnidocyte. When triggered by chemical or mechanical contact, the lid of the capsule opens and the coiled thread explosively everts outward, reaching speeds exceeding 5 meters per second and delivering its full discharge in under 700 nanoseconds, making it one of the fastest cellular mechanisms in the animal kingdom. The thread can penetrate the cuticle of small crustaceans, inject paralyzing venom, and in some species wrap around prey to restrain it.
A single tentacle of the Portuguese man o’ war (Physalia physalis) may carry hundreds of thousands of nematocysts arranged in batteries that fire in coordinated sequence.
The venom of the box jellyfish (Chironex fleckeri) is among the most potent of any marine animal and can cause cardiac arrest in humans within minutes of a significant sting. Researchers have identified at least three distinct toxin proteins in its nematocyst venom, each targeting different physiological systems.
Cell Specialization →Jellyfish tentacles sting by biting or injecting venom with teeth. Nematocysts fire microscopic tubules that pierce tissue mechanically and deliver venom through the hollow thread, with no teeth involved at any stage.
A sea anemone (order Actiniaria) uses nematocysts along its tentacles to capture small fish and crustaceans that brush against them. Each cnidocyte fires only once and must be replaced by a new cell migrating from the body wall, a process that takes roughly 48 hours.
Nematode
/ NEM-uh-tohd / · Greek nema (thread) + eidos (form)
Nematode is a member of the phylum Nematoda, a group of unsegmented roundworms with a pseudocoelom and a complete digestive tract, found in virtually every habitat on Earth as free-living decomposers, predators, or parasites of plants and animals.
Nematodes are among the most numerically abundant animals on Earth; a single handful of fertile soil can contain millions of individuals, and free-living species such as Caenorhabditis elegans feed on bacteria, fungi, and decaying organic matter, driving nutrient cycling in terrestrial ecosystems. Parasitic species cause substantial harm: Ascaris lumbricoides infects an estimated 800 million to 1 billion people worldwide, and root-knot nematodes (Meloidogyne spp.) cost global agriculture billions of dollars annually in crop losses. Despite their ecological and medical importance, nematodes share a remarkably uniform body plan, a smooth, cylindrical tube within a tube, with no circulatory or respiratory organs.
Their cuticle is shed four times during development, and the timing of these molts is controlled by steroid hormones similar to those governing ecdysis in arthropods.
C. elegans was the first multicellular organism to have its entire genome sequenced, completed in 1998, and remains the only animal whose complete connectome, all 302 neurons and roughly 7,000 synaptic connections, has been fully mapped.
All nematodes are parasites. The vast majority of nematode species are free-living decomposers that break down organic matter and recycle nutrients in soil and aquatic sediments.
Caenorhabditis elegans is a free-living soil nematode only about 1 millimeter long that scientists use extensively to study development, aging, and genetics. Its transparent body contains exactly 959 somatic cells in the adult hermaphrodite, allowing researchers to track individual cell divisions and migrations under a light microscope. This fixed cell lineage made the species a landmark model organism.
Nocturnal
/ nok-TUR-nul / · Latin nocturnus (of the night)
Nocturnal is an activity pattern in which an animal is mainly active during darkness and rests during daylight, often supported by sensory adaptations for low-light conditions.
Nocturnal animals show visual adaptations for low-light function, including large eyes, wide pupils, a reflective tapetum lucidum behind the retina, and retinas dominated by rod photoreceptors rather than cones. Non-visual senses are often enhanced as well: barn owls (Tyto alba) can locate prey by sound alone in complete darkness, using asymmetrically positioned ears that detect the vertical angle of a sound source to within one degree. Bats navigate and hunt using echolocation, emitting ultrasonic pulses above 20 kilohertz and interpreting the returning echoes to resolve objects as fine as a wire 0.1 millimeters in diameter.
Nocturnality may have helped early mammals avoid diurnal dinosaur predators during the Mesozoic, a hypothesis supported by the relatively limited color vision retained in most modern mammals compared with reptiles and birds.
The tarsier (genus Tarsius) of Southeast Asia has eyes so large relative to its skull that they cannot rotate in their sockets. To compensate, tarsiers can rotate their heads nearly 180 degrees in each direction, much like owls.
Nocturnal animals never come out in daylight. Many nocturnal species will emerge during the day when disturbed, when food availability shifts, or during overcast conditions that reduce ambient light below their threshold for daytime avoidance.
A barn owl (Tyto alba) can detect and strike a mouse moving beneath a layer of snow using hearing alone, with no visual input. Its left and right ear openings sit at different heights on the skull, a difference of roughly 15 millimeters that lets the brain calculate the elevation of a sound source with exceptional precision.
Notochord
/ NOH-toh-kord / · Greek noton (back) + chorde (string)
Notochord is a flexible, rod-like structure composed of vacuolated cells running along the dorsal axis of all chordate embryos, providing mechanical support and signaling cues that pattern the developing body.
The notochord defines membership in the phylum Chordata and organizes the surrounding tissues during embryonic development by secreting signaling molecules, including Sonic hedgehog protein, that direct the formation of the neural tube and somites above it. In most vertebrates, the notochord is progressively replaced by the vertebral column as cartilage or bone forms around it, though remnant notochord cells persist in the nucleus pulposus at the center of each intervertebral disc, where they retain a water-absorbing, shock-absorbing function throughout life. Degeneration or herniation of the nucleus pulposus, the condition commonly called a slipped disc, is therefore a direct clinical consequence of notochord-derived tissue failing under mechanical stress.
In the invertebrate chordate amphioxus (Branchiostoma lanceolatum), the notochord persists into adulthood and provides the primary axial support for swimming.
Chordoma, a rare bone tumor that arises from remnant notochord cells, can develop anywhere along the spine or at the base of the skull. Because these cells are present in adults, chordomas can appear decades after birth, most commonly in people between 50 and 60 years of age.
The notochord is the same structure as the backbone. The notochord is a flexible cellular rod present in embryos, while vertebrae are discrete mineralized or cartilaginous elements that develop separately around or adjacent to it.
Lancelets (Branchiostoma lanceolatum) retain a fully functional notochord throughout their adult lives, unlike vertebrates, which replace most of it with vertebrae. The notochord runs nearly the full 5-centimeter body length and stiffens during muscle contractions, allowing undulating swimming through sandy seafloor sediments. This adult condition resembles the ancestral chordate body plan.