Invertebrate Diversity: Annelids, Molluscs, Chelicerates, Crustaceans, and Hexapods

Comprehensive vocabulary flashcards covering the morphology, physiology, phylogenetic relationships, and life histories of Hexapoda, Chelicerata, Myriapoda, Crustacea, Mollusca, Annelida, and Nematoda.

Subphylum Hexapoda

A pancrustacean subphylum named for possessing six uniramous legs, featuring three distinct tagmata (head, thorax, and abdomen) with all locomotor appendages attached to the thorax.

Class Entognatha

A hexapod group containing small soil- and litter-dwelling taxa (Protura, Diplura, Collembola) whose mouthpart bases are enclosed within the head capsule.

Furcula

A forked, tail-like spring organ found in Collembola (springtails) that snaps downward against the substrate to fling the animal into the air.

Class Insecta

The largest animal group, comprising approximately $1.1\text{ million}$ described species, characterized by ectognathous (exposed) mouthparts and three thoracic segments carrying six legs, with most possessing wings.

Insect Tracheal System

A gas exchange network consisting of branched, thin-walled internal tubes that deliver oxygen directly to tissues, communicating with the outside environment via closeable spiracles.

Spiracles

External respiratory openings located along the insect cuticle that can open and close to regulate gas exchange and minimize water loss.

Insect Malpighian Tubules

Excretory structures that reabsorb water and potassium while conserving fluids, evolving independently from (and non-homologous to) the identically named tubules of arachnids.

Ametabolous Development

Direct postembryonic development in wingless insects (such as silverfish and springtails) where the young emerge resembling miniature adults without undergoing distinct metamorphosis.

Hemimetabolous Development

Gradual insect metamorphosis involving an egg, wingless aquatic or terrestrial nymphs, and a winged adult, observed in grasshoppers, cicadas, mantids, and dragonflies.

Holometabolous Development

Complete metamorphosis occurring in approximately $88\%$ of insect species, progressing through egg, feeding larva, quiescent pupa, and winged adult stages to prevent resource competition between life stages.

Drones

Haploid male honey bees that develop parthenogenetically from unfertilized eggs in the hive.

Subphylum Trilobita

Extinct marine arthropods arising before the Cambrian ($\sim 540\text{ mya}$) and ending in the Permian ($\sim 245\text{ mya}$), with a dorsoventrally flattened body divided longitudinally into three lobes and reinforced with calcium carbonate.

Mandibulate Hypothesis

A phylogenetic framework grouping Myriapoda, Crustacea, and Hexapoda into the clade Mandibulata based on the shared derived presence of chewing jaws (mandibles).

Subphylum Chelicerata

Arthropods lacking antennae and mandibles, organized into two tagmata (cephalothorax/prosoma and abdomen/opisthosoma) with six pairs of appendages: one pair of chelicerae, one pair of pedipalps, and four pairs of walking legs.

Chelicerae

The pincer-like or fang-bearing first pair of anterior appendages characteristic of chelicerate arthropods.

Class Merostomata

A chelicerate class containing horseshoe crabs (subclass Xiphosurida) characterized by an unsegmented horseshoe-shaped carapace, book gills, and an elongated posterior telson.

Class Pycnogonida

Marine sea spiders featuring a greatly reduced abdomen, an elongated proboscis for sucking cnidarian juices, and in some groups, duplicated segments yielding extra pairs of legs.

Pedicel

The narrow, slender stalk connecting the cephalothorax and abdomen in spiders (order Araneae).

Order Opiliones

Arachnids known as harvestmen ($\sim 5{,}000\text{ species}$) whose cephalothorax and abdomen are broadly joined without a pedicel; they lack venom glands and defend themselves using noxious gland secretions.

Order Acari

The most species-rich arachnid order ($\sim 40{,}000\text{ species}$), comprising mites and ticks, characterized by complete fusion of the cephalothorax and abdomen without external segmentation.

Class Chilopoda

Centipedes: fast, agile, carnivorous myriapods possessing one pair of legs per trunk segment, chewing mandibles, and venom claws modified from appendages to subdue prey.

Class Diplopoda

Millipedes: slow-moving, herbivorous myriapods bearing one pair of legs per thoracic segment, two pairs of legs per abdominal segment, and chemical defenses such as hydrogen cyanide.

Subphylum Crustacea

Primarily aquatic mandibulates possessing two pairs of antennae, one pair of mandibles, two pairs of maxillae, and ancestrally biramous appendages.

Biramous Appendage

A two-branched arthropod limb structure consisting of a basal protopod (composed of coxa and basis) supporting an outer exopod and an inner endopod.

Serial Homology

The structural correspondence of repeatedly arranged, segmental appendages within the same organism that derive from the same ancestral embryonic plan, as seen in the diverse specialized limbs of crayfish.

Antennal Glands

Paired excretory structures located at the base of crustacean antennae (called green glands in decapods) that function primarily in fluid and ionic balance rather than nitrogenous waste elimination.

Nauplius

The free-swimming ancestral larval form common to many crustaceans, characterized by an unsegmented body, three pairs of appendages, and a single median eye.

Ecdysis

The physiological and physical shedding of the old, rigid chitinous cuticle to accommodate organismal body growth in ecdysozoans.

Subclass Pentastomida

Parasitic crustaceans (tongue worms) that inhabit the respiratory systems of vertebrates, featuring a wormlike body with lost circulatory, excretory, and respiratory organs and producing millions of eggs.

Class Remipedia

Cave-dwelling marine crustaceans bearing $25\text{ to }38$ uniform trunk segments with paired biramous swimming limbs, representing the first recognized venomous crustaceans.

Order Isopoda

A malacostracan group that includes the only truly terrestrial crustaceans (sow bugs and pill bugs), as well as marine and freshwater species and ectoparasites.

Class Polyplacophora

Marine molluscs (chitons) possessing seven to eight overlapping dorsal shell plates, a flat muscular foot, and an algal-scraping radula, specialized for rocky intertidal zones.

Osphradia

Chemoreceptive sensory organs located in the mantle cavity of certain molluscs (such as chitons and gastropods) designed to sample incoming water currents.

Gastropod Torsion

A developmental $180^{\circ}$ counterclockwise twisting of the visceral mass in veliger larvae that shifts the anus and mantle cavity from posterior to anterior positions directly over the head.

Gastropod Coiling

The spiral winding of the gastropod shell and visceral mass that shifts weight for compact carrying, often leading to the loss of the right gill, atrium, and kidney to prevent fouling.

Operculum

A hard, proteinaceous or calcified plate on the dorsal foot of many gastropods that covers and seals the shell aperture when the body withdraws.

Cnidosacs

Specialized chambers at the distal ends of cerata/papillae in nudibranch sea slugs that store functional nematocysts sequestered from cnidarian prey for the slug's own defense.

Class Bivalvia

Sedentary, laterally compressed filter-feeding molluscs (also called Pelecypoda) enclosed within a two-valved shell, entirely lacking a head and a radula.

Glochidium

A specialized bivalve larval stage found in freshwater mussels that attaches to the gills or fins of a host fish as a temporary parasite.

Class Cephalopoda

Active, predatory marine molluscs (squids, octopuses, nautiluses, cuttlefish) characterized by beaklike jaws, arms and tentacles, a funnel for jet propulsion, and a closed circulatory system.

Hectocotylus

A specialized, modified arm in male cephalopods used to extract a spermatophore from his mantle cavity and transfer it to the female.

Chromatophores

Elastic, pigment-containing cells in cephalopod integument controlled by muscular and nervous action to facilitate rapid camouflage and signaling.

Metamerism

The serial repetition of homologous body units (segments) along the longitudinal axis, separated internally by transverse septa in annelids.

Setae

Tiny, chitinous bristles projecting from the annelid body wall that provide traction during peristaltic movement or anchor worms within their burrows.

Parapodia

Paired lateral, fleshy epidermal outgrowths on annelid segments (prominent in Errantia) used for locomotion, burrow pumping, and respiratory gas exchange.

Family Chaetopteridae

Sedentary, parchment-tube-dwelling polychaetes that utilize highly modified, fan-like parapodia to pump water through a U-shaped burrow and trap food in a mucous net.

Clade Errantia

A clade of mostly marine, actively motile polychaete annelids possessing well-differentiated heads with sensory organs and prominent parapodia with internal setae.

Clitellum

A thickened glandular ring of secretory cells on the epidermis of earthworms and leeches that secretes a cocoon for egg deposition and fertilization.

Class Hirudinida

Leeches: annelids possessing exactly $34$ segments, anterior and posterior suckers, no setae or parapodia, and anticoagulant enzymes secreted during fluid feeding.

Nematode Locomotion

A distinctive side-to-side thrashing movement caused by the contraction of purely longitudinal muscles acting antagonistically against a high-pressure hydrostatic pseudocoelom and cuticle.

Ascaris lumbricoides

A large parasitic nematode infecting humans through ingestion, wherein females produce up to $200{,}000$ eggs per day that pass out with host feces.