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Phylum Arthropoda

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The Phylum Arthropoda is the largest and most diverse of all animal phyla. More than three quarters of the animals on earth are arthropods, and most of these are insects. More than 900,000 species have been described, and if biologists have the opportunity to explore the rain forests before they are completely burned to the ground, a like number will probably be discovered.
introduction
Arthropods have their skeletons on the outside of their bodies, rather than on the inside (exoskeleton)

The phylum's name is based on this feature (Arthropoda means "joint-foot" referring to the external joints in the skeleton that allow movement).

There are both advantages and disadvantages to an exoskeleton

The advantages seem to outweigh any disadvantages, judging from the phylum's success and diversity

Introduction
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Introduction
  • More than anything, the exoskeleton has allowed the arthropods to diversify and move into niches that were previously inaccessible.
  • They were the first beasts to successfully colonize land (the exoskeleton preadapted them for terrestrial habitats since it provided both support and protection from drying out).
  • Insects flew long before anything like a bird existed.
  • We even owe the presence of flowers to insects (most flowers are constructed to attract insects for pollination).
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Introduction
  • There are four major subphyla
    • Subphylum Trilobita (ex. Trilobites – extinct)
    • Subphylum Chelicerata (ex. Spiders)
    • Subphylum Crustacea (ex. Crabs, lobsters, and such)
    • Subphylum Uniramia (ex. Insects . . .)
subphylum trilobita
Subphylum Trilobita
  • Trilobites are the most primitive of the arthropods.
  • Although they are now extinct, their hard exoskeletonsare so well-preserved and plentiful in the fossil record that much is known about their anatomy, diet, embryology, and even behavior (about 4000 species are described).
  • Examination of their appendages and general body structure show that most were marine bottom-dwelling organisms.
  • Trilobites reached their height of distribution and abundance during the Cambrian and Ordovian periods (about 500 million years ago).
subphylum trilobita7
Subphylum Trilobita
  • We understand trilobite diets through the structure of their mouthparts and X-rays of their digestive systems. Since the mouth had no large mandibles, we infer that trilobites were not usually predatory and were restricted to soft food including seaweeds, coelenterates, and mollusks. Food particles were stirred up by the legs and passed forward to the mouth.
  • Some trilobites had long spines on the first leg segment, the gnathobase; these may have been able to tear up larger pieces of food, and probably scavenged for a living.
  • Fossil burrows and tracks have been found that match trilobite bodies very precisely; these show that trilobites could burrow into sediment to feed or to avoid predators.
  • Many trilobites living after the Cambrian developed the ability to roll up, also probably as a defense against predators.
  • When in defense mode, the ridges would have allowed water to enter and flow over the gills, so that the trilobite could breathe while enrolled.
subphylum chelicerata
Members of chelicerata have no antennae and feeding appendages called chelicerae.

(though most have a second appendage called pedipalps)

Their body is divided into two main sections: an anterior cephalothorax and a posterior abdomen.

There are two main classes:

Class Merostomata

Horseshoe Crabs

Class Arachnida

Spiders

Ticks, Mites

Scorpions

Subphylum Chelicerata
subphylum chelicerata class merostomata
Subphylum Chelicerata – Class Merostomata
  • This class is represented by only four species of animals commonly known as "horseshoe crabs”
  • The hosrseshoe crab is called a "living fossil" because it has been on earth for millions of years.
  • They spend most of their time burrowing through soft mud looking for worms, mollusks, and other tasty tidbits.

Their spawning time is the last two weeks in May and the first two weeks in June. They crowd up on the beach at high tide.

subphylum chelicerata class merostomata10
Subphylum Chelicerata – Class Merostomata

The anterior cephalothorax is covered with a horseshoe-shaped carapace, from which the animals derive their common name. Two large compound eyes and a pair of small simple eyes (at the anterior end of the median ridge) are on the dorsal carapace. The abdomen (posterior to the carapace) bears rows of short spines and a single large telson. Despite its formidable appearance, the telson is used neither for offense nor defense, and living animals can be safely handled.

subphylum chelicerata class merostomata11
Subphylum Chelicerata – Class Merostomata
  • Today, we are going to experiment and dissect one!
  • Limulus polyphemus
subphylum chelicerata class arachnida
Subphylum Chelicerata – Class Arachnida
  • This large class of arthropods includes over 60,000 described species, with spiders making up the largest group
  • Other members include:
    • Scorpions
    • Ticks and Mites
  • The major distinguishing characteristics are sections called prosoma and opisthosoma.
  • The prosoma is partially or completely covered with a carapace-like shield (cephalothorax).
  • The opisthosoma (abdomen) may be segmented or unsegmented. The appendages on the opithosoma are absent or modified, being used as spinnerets (spiders) or pectines (probably sensory in function, found in scorpions).
  • Respiration is via tracheae or book lungs;
subphylum crustacea
Subphylum Crustacea
  • Crustaceans are an extremely diverse group of primarily aquatic arthropods with two pair of antennae.
  • The class includes such culinary delights such as shrimp, lobsters, crayfish, and barnacles.
  • Although most are free-living, a few are parasitic.
  • Primitive crustaceans have three main body parts: a head, a thorax, and an abdomen. There is a tendency, however, for the head and thorax to fuse and form a single cephalothorax.
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Subphylum Crustacea
  • Distinctions:
    • All have two pairs of antennae, a pair of mandibles, a pair of compound eyes (usually on stalks), and two pair of maxillae on their heads.
    • The appendages are primitively branched (biramous), and although this condition is modified in many species, adults always have at least some biramous appendages.
    • Crustaceans respire via gills.
subphylum crustacea15
Subphylum Crustacea
  • Today we are dissecting a crayfish (open on top)
subphylum crustacea16
Subphylum Crustacea
  • You’ll see this inside!!! Follow along on your sheet diagram!

I will help you if you ask!!!

subphylum uniramia
Subphylum Uniramia
  • We will cover this better next week . . .
  • They differ from crustaceans by having a single pair of antennae (rather than two) and legs based on a uniramus plan (with unbranched appendages).
  • Two Divisions:
    • Superclass Hexapoda (insects)
    • Superclass Myriapoda
      • Class Chilopoda (centipedes)
      • Class Diplopoda (milipedes)