Lab Practical 3: Animal Kingdom - Arthropoda

Phylum Arthropoda

• Most diverse and abundant animal phylum.
• Features a rigid exoskeleton and jointed appendages adapted for various functions including locomotion and defense.
• Has segmented bodies, coelomates, and an open circulatory system.
• Exoskeleton composed of chitin, offering protection and a moisture barrier while facilitating muscle attachment.
• Shed exoskeleton to grow; primary study focuses on appendage variation.
• Divided into four subphyla: Chelicerata, Myriapoda, Crustacea, Hexapoda.

Subphylum Crustacea

• Characterized by mandibles for mouthparts and biramous appendages (two-branched).
• Includes crabs, lobsters, shrimp, and isopods.
• Possess specialized appendages for various functions.

Subphylum Chelicerata

• Mouthparts known as chelicerae, functioning as pincers or fangs.
• Comprises horseshoe crabs and spiders; structured with two body segments: cephalothorax and abdomen.
• Arachnids have a cephalothorax with one pair of chelicerae, one pair of pedipalps, and four pairs of legs.
• Distinguishing feature includes the absence of antennae.

Subphylum Hexapoda

• Features mandibles for mouthparts and uniramous appendages (single-branched).
• Dominated by insects, representing the most species-rich group among eukaryotes.
• Typically have six legs and two pairs of wings.
• Inhabit nearly all terrestrial ecosystems and fill various ecological roles.
• Exhibits two forms of metamorphosis and includes ten significant insect orders.

Subphylum Myriapoda

• Characterized by mandibles and a head followed by numerous body segments with paired uniramous appendages.
• Includes millipedes and centipedes.

Class Merostomata (Horseshoe Crabs)

• Shaped carapace covers the cephalothorax; flexible joints separate it from the abdomen.
• Features five pairs of walking appendages; anterior pair modified as pedipalps.
• Chelicerae located anterior to pedipalps and degenerated legs, known as chilaria, are present.
• Abdominal appendages modified into book gills for gas exchange.

Adaptation and Structure

• The chitinous covering functions as a skeleton, aiding in muscle attachment and protection.
• Horseshoe crab's external features like carapace and dorsal eyes facilitate burrowing and scavenging in sandy environments.
• "Book" gills appear like pages in a book, assisting in respiration.

Class Arachnida

• Most diverse class of chelicerates, primarily terrestrial.
• Cephalothorax has fangs modified from chelicerae for prey capture.
• Pedipalps serve to manipulate food and sense the environment; possess four pairs of walking legs.
• Scorpions are identified as early terrestrial arthropods.

Scorpion Features

• Scorpions appear menacing due to their stinger and large pedipalps.
• Pedipalps are larger than scorpion chelicerae, which are hook-shaped.

Spider Characteristics

• Most spiders have eight eyes, often paired but varying in size.
• Body segmentation is evidenced by the segmented appearance of legs.
• Hairy texture in many spider species serves a sensory function.

Class Crustacea Anatomy

• Includes crayfish, crabs, and shrimps; inhabits marine and freshwater environments.
• Biramous appendages and two pairs of antennae; characterized by compound eyes.
• Mandibles are opposing and derived from anterior appendages; cephalothorax covered by carapace.
• Five anterior pairs of appendages serve specific roles ranging from sensory to locomotion.
• Swimmerets or pleopods assist in reproduction; uropods surround the telson.

Adaptive Features of Crayfish

• The diversity in mouthpart shapes allows crayfish to consume various food sizes and textures, beneficial for their scavenger diet.