28.2 Section Objectives – page 747 Arachnids, Crustaceans, and Horseshoe crabs NOTES

Section 28.2 Summary – pages Spiders, scorpions, mites, and ticks belong to the class Arachnida (uh RAK nud uh). Spiders are the largest group of arachnids. What is an arachnid? Spiders and other arachnids have only two body regions—the cephalothorax and the abdomen. Arachnids have six pairs of jointed appendages.

Section 28.2 Summary – pages The first pair of appendages, called chelicerae, is located near the mouth. chelicerae What is an arachnid?

Section 28.2 Summary – pages Chelicerae are often modified into pincers or fangs. Pincers are used to hold food, and fangs inject prey with poison. What is an arachnid?

Section 28.2 Summary – pages Spiders have no mandibles for chewing. Using a process of extracellular digestion, digestive enzymes from the spider’s mouth liquefy the internal organs of the captured prey. The spider then sucks up the liquefied food. What is an arachnid?

Section 28.2 Summary – pages The second pair of appendages, called the pedipalps, are adapted for handling food and for sensing. pedipalps What is an arachnid?

Section 28.2 Summary – pages Although all spiders spin silk, not all make webs. Spider silk is secreted by silk glands in the abdomen. What is an arachnid?

Section 28.2 Summary – pages As silk is secreted, it is spun into thread by structures called spinnerets, located at the rear of the spider. What is an arachnid?

Section 28.2 Summary – pages Ticks and mites differ from spiders in that they have only one body section. tick Ticks, mites, and scorpions: Spider relatives

Section 28.2 Summary – pages The head, thorax, and abdomen are completely fused. Ticks feed on blood from reptiles, birds, and mammals. Ticks, mites, and scorpions: Spider relatives

Section 28.2 Summary – pages Scorpions are easily recognized by their many abdominal body segments and enlarged pincers. Ticks, mites, and scorpions: Spider relatives They have a long tail with a venomous stinger at the tip.

Section 28.2 Summary – pages Crustaceans (krus TAY shuns) are the only arthropods that have two pairs of antennae for sensing. Crustaceans All crustaceans have mandibles for crushing food and typically have two compound eyes.

Section 28.2 Summary – pages Unlike the up-and-down movement of your jaws, crustacean mandibles open and close from side to side. Many crustaceans have five pairs of walking legs. Crustaceans

Section 28.2 Summary – pages The first pair of walking legs are often modified into strong claws for defense. claw legs Crustaceans

Section 28.2 Summary – pages Members of the class Crustacea include crabs, lobsters, shrimps, crayfishes, water fleas, pill bugs, and barnacles. Crustaceans

Section 28.2 Summary – pages Most crustaceans are aquatic and exchange gases as water flows over feathery gills. Sow bugs and pill bugs, two of the few land crustaceans, must live where there is moisture, which aids in gas exchange. Crustaceans

Section 28.2 Summary – pages Horseshoe crabs are members of the class Merostomata. Horseshoe Crabs: Living Fossils Horshoe crabs are considered to be living fossils; Limulus fossils have remained relatively unchanged since the Triassic Period about 220 million years ago.

Section 28.2 Summary – pages Horseshoe crabs are heavily protected by an extensive exoskeleton and live in deep coastal waters. Horseshoe Crabs: Living Fossils They forage on sandy or muddy ocean bottoms for algae, annelids, and mollusks.