Arthropod Biology: Structure, Respiration, and Diversity
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Arthropod Characteristics
Arthropods are coelomate, segmented, invertebrate animals exhibiting bilateral symmetry. They possess an exoskeleton and jointed appendages. They were the earliest invertebrates to have jointed appendages.
Respiration Methods
Respiration varies among groups:
- Gills: Work similarly to vertebrate lungs. Water flows over the surface of lung tissue; O2 is absorbed, and CO2 diffuses out.
- Land Tracheal Tubes: A branching network of air passages moved by muscular contraction and relaxation. Air enters through spiracles (openings), and gas diffuses throughout.
- Book Lungs: Found in spiders and scorpions. These consist of leaflike plates folded like pages, where hemolymph (blood tissue) circulates.
Senses and Nervous System
Movement, sound, and chemicals are detected by antennae.
Communication often occurs via pheromones. Most possess one pair of large compound eyes; some have simple eyes (detecting light/dark). Compound eyes have many lenses, enabling them to see prey and mates.
They have one brain, several ganglia, and two ventral nerve cords. They have an open circulatory system with one or more hearts. Excretion occurs via Malpighian tubes in the abdomen. Some exhibit parthenogenesis, developing asexually from unfertilized eggs.
Diversity of Arthropods
Arachnida
Named for their appendages. This group includes mites and ticks. They possess a cephalothorax and an abdomen. They have chelicerae (mouthparts) and pedipalps (used for handling and sensing). Silk is made in the abdomen by spinnerets.
- Ticks: Have one fused body section and cause pain when they bite.
- Scorpions: Have many abdominal pincers (venomous stinger).
Crustacea
Possess gills and are aquatic. They are the only arthropods to have two pairs of antennae and two compound eyes. The first pair is often used for defensive claws. They typically have 2–3 body sections and require moist environments.
Other Classes
- Chilopoda (Centipedes): Use Malpighian tubes for waste removal and tracheal tubes for respiration; they are carnivores.
- Diplopoda (Millipedes): Are herbivores or detritivores.
- Merostomata: Include horseshoe crabs, which are considered living fossils from the Precambrian era.
- Insecta: The largest group. They have three body segments, usually one pair of wings, and six legs. They often mate only once. Their phylogeny likely evolved from annelids, showing varied life cycles.
Echinoderm Characteristics
Echinoderms are easily distinguished by their tube feet, spiny hard skin, water vascular system, ossicles (calcium carbonate plates), and endoskeleton. Larvae are bilateral, but adults are typically radial, allowing them to sense in 360 degrees.
Water Vascular System
Water enters through the madreporite (aboral side), filtering large particles. The system uses hydraulic pressure. Tube feet allow for diffusion, attachment, and independent movement.
Feeding and Nervous System
Some are predators, capable of everting their stomach; others are grazers. They lack central control; sensory input comes from eyespots in rays, with a central nerve ring located near the mouth.
Classes of Echinoderms
- Asteroida: Sea stars, typically possessing five rays.
- Ophiuroidea: Brittle stars; they regenerate rays, and their tube feet are used only for feeding.
- Echinoidea: Sea urchins and sand dollars; they have gills (or podia) on the aboral surface.
- Holothuroidea: Sea cucumbers; they use tube feet and tentacles for movement.
- Crinodea: Sea lilies.
Early ancestors of echinoderms were bilateral and sessile. Free-swimming bilateral forms existed during the Paleozoic era.