Phylum arthropoda
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Phylum Arthropoda. Similarities between Annelids and Arthropods Arthropods are metameric and their segments have appendages Nervous system with ventral nerve cords. Phylum Onychophora Annelid-like

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  • Phylum Onychophora

  • Annelid-like

  • Segmented; unjointed appendages; similarity in structure of the body wall; segmentally arranged nephridia; pigment-cup ocelli

  • Arthropod-like

  • Reduced coelom, open circulatory system, tracheal system; soft cuticle composed of chitin

Peripatus, a small, nocturnal form found among the leaf-litter of tropical forests of South America.

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The fossil record indicates that onychophorans have not changed much in 500 million years.

The Onychophoran Aysheaia from the Cambrian.

A lobe-limbed, segmented animal. Also note the spines on the legs. The head end has a pair of tapering limbs with spines, and three small projections near the mouth. Traces of the digestive tract can also be seen.

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Phylum Tardigrada changed much in 500 million years.or “water bears” also have features in common with both annelids and arthropods

  • Annelid-like

  • Unjointed (8) legs; annelid-type nervous system

  • Arthropod-like

  • Presence of a cuticle (nonchitinous) that is periodically molted; similar attachment of muscle fibers to exoskeleton

  • One of the most interesting features of tardigrads is their ability to undergo cryptobiosis

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Arthropod Taxonomy: Overview changed much in 500 million years.

The arthropods evolved along four main lines, which most zoologists recognize as 4 distinct subphyla

1. Trilobita - extinct trilobites

2. Chelicerata - horseshoe crabs, spiders, ticks, mites, and some extinct groups

3. Crustacea - crabs, lobsters, shrimps, barnacles

4. Uniramia - insects, centipedes, millipedes

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  • The Arthropod Exoskeleton changed much in 500 million years.

  • Epidermis secretes an external skeleton called the exoskeleton

  • Advantages of possessing an exoskeleton:

    • provides strong support

    • provides rigid levers that muscles can attach to and pull against

    • offers protection

    • serves as a barrier to prevent internal tissues from drying out; important because many arthropods live on land

    • serves as a barrier to prevent infection

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  • Structure of the Exoskeleton changed much in 500 million years.

  • Composed of the polysaccharide chitin and protein - glycoprotein

  • Outer surface called the epicuticle;contains waxes

  • The thicker portion is called the procuticle:

    • exocuticle

    • endocuticle

  • In the exocuticle, the glycoprotein chains are cross linked; process is called tanning






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  • Molting changed much in 500 million years.

  • In order to grow the arthropod must shed its exoskeleton, and secrete a new and larger one - moltingor ecdysis.

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  • Jointed Appendages changed much in 500 million years.

  • Exoskeleton divided into a number of plates and cylinders

  • At the junction point between plates and cylinders, the exoskeleton remains thin and flexible; these are the joints

  • Jointed appendages allows arthropods to move efficiently and quickly

  • Muscles are integral to arthropod movement; they attach to the inner side of the exoskeleton; they often function as a lever system

Arthropod joint

Vertebrate joint

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  • Specialized Arthropod Segments: Reduction in Metamerism changed much in 500 million years.

  • The evolution of the arthropods witnessed a reduction in metamerism

  • The arthropods evolved modified groups of segments (e.g., segments became lost, some fused together

  • The fusion of groups of segments into functional groups is called tagmatization

  • In so doing, various appendages on segments became specialized for functions other than locomotion, e.g. prey capture, filter feeding, sensing various kinds of stimuli, gas exchange, copulation, etc.

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  • Arthropod Respiratory Advances changed much in 500 million years.

  • Special respiratory structures allow the arthropods to metabolize more efficiently and thus move rapidly

  • High metabolic rates require rapid oxygen delivery, and arthropods can accomplish this with respiratory organs that have a large surface area for collecting oxygen quickly

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  • Gills changed much in 500 million years.

  • Many aquatic arthropods (crabs and lobsters) have gills, which are typically modifications of appendages or outgrowths of the body wall - folds of tissue with a large surface area

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  • Tracheae changed much in 500 million years.

  • Gas exchange organs among terrestrial arthropods is usually internal; invaginations of the integument

  • Insects have tracheae,branching networks of hollow air conducting tubes such that air is sent to every cell in every tissue

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  • Book Lungs changed much in 500 million years.

  • Spiders have book lungs,chambers with leaf-like plates for exchanging gases; air flows over the plates and blood flows through them

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  • Acute Senses changed much in 500 million years.

  • Arthropods have a well-developed nervous system that is of the same overall design as the annelids; anterior brain and a double, ventral hollow nerve cord.

  • The sensory receptors of arthropods are usually associated with modifications of the chitinous exoskeleton

  • The head usually bears various kinds of sense organs (e.g. antennae) with extreme sensitivity

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  • Acute Senses cont. changed much in 500 million years.

  • Many arthropods have compound eyes - eyes that are composed of many visual units called facets (ommatidia); capable of color vision and detecting the slightest movements of prey or predators

  • Some eyes are simple eyes with only a few photoreceptors; however, they are capable of forming crude images

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  • Digestive System changed much in 500 million years.

  • Divided into 3 main regions: foregut, midgut, and hindgut

  • Foregut and the hindgut are lined with chitin

  • Foregut is involved with ingestion, mechanical breakdown, and storage

  • Hindgut is involved with water absorption and formation of the feces

  • Midgut is not lined with chitin; involved with digestion and absorption

  • Outpockets (e.g. digestive glands) increase the surface area for digestion and absorption

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  • Internal Transport and Excretion changed much in 500 million years.

  • Open circulatory system

  • Many crustaceans possess an excretory organ called the green gland (antennal gland),which filters fluid from the blood

  • Most insects and spiders have a excretory system called malpighian tubules

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  • Reproduction changed much in 500 million years.

  • Sexes are separate; fertilization is external in aquatic forms, internal among the terrestrial forms

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Arthropod Diversity changed much in 500 million years.

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Subphylum Trilobita changed much in 500 million years.

  • A group of extinct marine arthropods.

  • Ranged in size from a few millimeters up to 75 centimeters.

  • Body divided into three parts:

    • cephalon (head), a single plate made up of several fused segments

    • thorax, consists of a number of segments hinged together

    • pygidium (tail), also segmented, but like the head, fused together into a single plate.

  • Compound eye, composed of radially arranged visual units

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  • Subphylum Chelicerata changed much in 500 million years.

  • Lack antennae

  • Body is usually composed of two regions: cephalothoraxand abdomen

  • Cephalothorax is usually covered dorsally by the carapace

  • Six pairs of appendages: first pair are modified feeding structures - chelicerae

  • Second pair are called pedipalps

  • Four additional pairs of appendages are walking legs

  • No abdominal appendages

  • Some have compound eyes, usually have simple eyes capable of forming crude images.

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  • Class Meristomata changed much in 500 million years. (e.g., Horeshoe crabs)

  • Marine chelicerates, common off of the Atlantic and Gulf coasts

  • Large dorsal carapace bearing compound eyes

  • Possess chelicerae, pedipalps, and 4 pair of walking legs (all but the last pair are chelate

  • Abdomen terminates in a long tail called the telson; used to turn the animal right side up

  • Possess a series of gill plates called book gills

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  • Class Arachnida changed much in 500 million years.

  • Among spiders, the cephalothorax and abdomen shows no external segmentation; tagma are joined by a narrow pedicel

  • Respiration is accomplished via book lungs, tracheae, or both

  • Usually have 8 simple eyes; at the very least they detect motion; for some of the predatory forms, they are capable of forming crude images

  • Many species have evolved poison glands associated with the chelicerae

Black widow

Brown recluse

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  • Class Arachnida cont. changed much in 500 million years.

  • Many of the spiders and mites are capable of producing silk; produced by silk glands that open to the exterior part of the abdomen through spinnerets

  • Silk is used to build webs for trapping prey, nests which serve as retreats, and egg cases; it is also used to form a dragline

  • Most spiders are predaceous and have all kinds of sensory hairs and relatively well-developed eyes for motion detection

Orb web construction

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  • Prey capture among the spiders changed much in 500 million years.

  • Some species are cursorial predators,those that stalk and ambush their prey; they usually have well-developed eyes

Jumping spider

  • Others are web building spiders,those that construct various kinds of webs made of silk to trap their prey

  • Eyes are not as well developed as cursorial predators, but they have a battery of sensory hairs for detecting vibrations

Grass spider

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  • SubPhylum Crustacea changed much in 500 million years.

  • Possess 2 pair of antennae: First pair is homologous to those of insects; second pair is unique to the crustaceans

  • Second antennae have various functions, including sensory, locomotion or feeding.

  • The head bears a pair of compound eyes and 3 pairs of mouthparts: a pair of mandibles, and 2 pairs ofmaxillae; used for food handling

  • Trunk varies considerably among classes

  • Primitively, the first three pairs of thoracic segments are maxillipeds;function in handling food

  • Also, there are usually 5 pairs of appendages strengthened for walking (walking legs) and protection (chelipeds,pincer-like claws)

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  • SubPhylum Crustacea cont. changed much in 500 million years.

  • Abdomen is also highly variable, but it is primitively large

  • Groups with a well-developed abdomen usually possess six pairs of appendages: Five pairs of structures called swimmerets (=pleopods);one pair of structures called uropods,

  • Uropods together with the terminal telson form a tail fan than can serve as rudders during locomotion

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  • SubPhylum Crustacea cont. changed much in 500 million years.

  • Primitively many of the appendages of the crustaceans are biramous: there is an outer exopodand an inner endopod

  • They usually have an extremely hardened exoskeleton, which is impregnated with calcium carbonate - carapace

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  • SubPhylum Crustacea cont. changed much in 500 million years.

  • The primitive larva of the crustaceans is called the nauplius larva

  • It has an unsegmented body, a frontal eye, and 3 pairs of appendages, representing the 2 pairs of antennae and the mandibles

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Crustacean Diversity changed much in 500 million years.


Water flea


Fairy shrimp

Fish louse

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Barnacles are Crustaceans! changed much in 500 million years.

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  • SubPhylum Uniramia changed much in 500 million years.

  • Single pair of antennae

  • First pair of feeding appendages are mandibles

  • There are one or 2 pairs of maxillae

  • Number of legs vary from 3 pair to many pairs; they are unbranched or uniramous

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  • SubPhylum Uniramia cont. changed much in 500 million years.

  • Class Chilopoda (Centipedes)

  • Serial segmented, flattened body and each segment has a pair of jointed appendages

  • Active predators, killing their prey with poison claws,which are modified legs on first segment

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  • SubPhylum Uniramia cont. changed much in 500 million years.

  • Class Diplopoda (Millipedes)

  • Serially segmented, rounded body with 2 smaller pairs of legs per segment

  • Slow moving; feed on decaying plants

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  • Class Insecta changed much in 500 million years.

  • Body is divided into 3 parts: the head, thorax and the abdomen.

  • Head has one pair of antennae, a pair of compound eyes and several sets of simple eyes

  • Mouthparts: a pair of mandibles, and two pairs of maxillae

  • One pair of maxillae are fused together to form a lower lip - labium

  • An upper lip - labrum - formed from an extension of the head

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  • Class Insecta cont. changed much in 500 million years.

  • Mouthparts are highly modified depending on the group you are discussing

  • Mosquitoes have pointed mouthparts for piercing and sucking; grasshoppers have mouthparts that are well adapted for chewing; butterflies for siphoning; flies for sponging

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  • Class Insecta con’t changed much in 500 million years.

  • Thorax is composed of 3 segments and each one has a pair of legs; the last two segments also have a pair of wings.

  • Wings of insects are modified portions of the exoskeleton

  • 1st pair is usually tough and leathery and fold over the inner pair for protection.

  • Abdomen does not have appendages; terminal portions do harbor the reproductive structures

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  • Most insects undergo changed much in 500 million years.metamorphosis

  • Incomplete Metamorphosis (Hemimetabolous)

  • Early developmental stages are very similar to the adults; only the wings and the reproductive structures gradually develop

  • The immature stages are called nymphs

  • Thus development is egg----> nymphs ----> adult

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Complete Metamorphosis (Holometabolous) changed much in 500 million years.

  • Each of the developmental stages is structurally and functionally very different

  • The egg develops into an immature larva; eats voraciously

  • Followed by a transitional stage - pupa,contained within cocoon

  • Metamorphosis occurs within the pupal exoskeleton, yielding a sexually mature adult