Arthropods and Echinoderms: Characteristics and Biology

Arachnids

Arachnids, such as spiders, scorpions, and mites, are terrestrial animals that typically live in dry regions. Their bodies are divided into two regions: the cephalothorax and the abdomen.

• Anatomy: They possess two chelicerae for feeding, two palps for defense, and four pairs of legs.
• Physiology: They breathe through tracheae and are primarily carnivorous.
• Reproduction: They are equipped with poison glands to capture prey, have separate sexes, and are oviparous or ovoviviparous.

Crustaceans

Crustaceans include lobsters, shrimp, barnacles, and crabs. While most are marine, some species, such as freshwater crabs or terrestrial woodlice, inhabit different environments.

• Body Structure: The body is divided into the head, thorax, and abdomen, though the head and thorax are often fused into a cephalothorax.
• Sensory and Feeding: The head features two pairs of antennae and a pair of eyes, while the chest contains jaws. They typically have five pairs of legs.
• Respiration and Diet: They breathe through gills located at the base of the legs or through the body surface. Their diet is varied, including carnivores, parasites, and filter feeders.
• Development: They have separate sexes and undergo metamorphosis.

Myriapods

Myriapods have elongated bodies consisting of a head and a long trunk with a variable number of segments.

• Legs: Each segment may have one pair of legs (as seen in centipedes) or two pairs (as seen in millipedes).
• Anatomy: The head possesses a pair of antennae, simple eyes, and mandibles.
• Respiration: They breathe through tracheae and have a varied diet.

Echinoderms

Echinoderms, such as sea urchins, sea cucumbers, and starfish, exhibit radial symmetry as adults, though they possess bilateral symmetry in their larval stage.

• Locomotion: They move using an exclusive system of internal water-filled tubes called the ambulacral system, which ends in suction-cup feet.
• Respiration and Diet: They breathe through their skin and the ambulacral system. They are carnivorous, feeding on small crustaceans and mollusks.
• Reproduction and Regeneration: They have separate sexes, though some are hermaphroditic. After hatching, larvae undergo metamorphosis. Many species, such as starfish, possess a remarkable power of regeneration, allowing them to regrow lost limbs.