Trilobites, Chelicerates, and Myriapods - Unicamdocenti.unicam.it/tmp/2597.pdf · CHAPTER 19...

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CHAPTER 19

Trilobites, Chelicerates, and

Myriapods

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Characteristics

Anthropodization Soft cuticle of the arthropod ancestors was

stiffened by deposition of protein and chitin

Joints had to provide flexibility

Sequence of molts was necessary to allow for growth

Molting required hormonal control

Hydrostatic skeleton function was lost Coelom regressed and was replaced by open

sinuses

Motile cilia were lost


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Phylum Arthropoda Contains over 3/4 of all known species

Approximately 1,100,000 species of arthropods have been recorded

Rich fossil history dating to late Precambrian

Eucoelomate protostomes with well-developed organ systems

Segmented

Molecular analyses indicate annelids and arthropods evolved from different ancestors

Characteristics


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Sizes range from the Japanese crab (four meters in leg span) to the 0.1 mm long follicle mite

Abundance and wide ecological distribution makes them the most diverse animal group

Some are agents of disease and compete with humans for food

Others are beneficial

All modes of feeding occur among arthropods but most are herbivorous

Characteristics


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Relationships among arthropod subgroups

Divided into subphyla based on relationships between subgroups

Groupings among subphyla based on molecular data

Centipedes, millipedes, pauropods, and symphylans are placed in subphylum Myriapoda

Insects are placed in subphylum Hexapoda

Spiders, ticks, horseshoe crabs and their relatives form subphylum Chelicerata

Lobsters, crabs, barnacles, and others form subphylum Crustacea

Characteristics


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Also included in Crustacea are tongue worms

Extinct trilobites are placed in subphylum Trilobita

Relationships are controversial

“Mandibulate hypothesis” Myriapods, hexapods, and crustaceans more

closely related due to a shared mouthpart, the mandible

Molecular evidence of a close relationship between hexapods and crustaceans unites subphylum Crustacea with subphylum Hexapoda in clade Pancrustacea

Characteristics


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Great Diversity and Abundance of Arthropods

Versatile Exoskeleton Cuticle is highly protective and jointed, providing

mobility

Consists of inner thick procuticle and outer thin epicuticle

Procuticle has an exocuticle secreted before a molt and an endocuticle secreted after molting

Both layers of procuticle contain chitin bound with protein

Procuticle is lightweight, flexible, and protects against dehydration

Chitin content varies from 40% of the procuticle in insects to as much as 80% in crustaceans


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Calcium salts responsible for hardness of procuticle of lobsters and crabs

Cuticle is laminated and further hardened by tanning

Cuticle is thin between segments, allowing for movement at the joints

Ecdysis, or molting Process of shedding outer covering and growing a new,

larger one

Arthropods typically molt four to seven times

Weight is a limit to ultimate body size



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Segmentation and Appendages for Efficient Locomotion Primitive pattern

Linear series of similar somites with jointed appendages

Many somites may be fused or combined into specialized groups called tagmata

Appendages often highly specialized for division of labor

Limb segments are hollow levers with internal striated muscles

Appendages may function in sensing, food handling, walking, or swimming



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Air Piped Directly to Cells Terrestrial arthropods

Use an efficient tracheal system for oxygen transport

Aquatic arthropods

Respire via various forms of gills

Highly Developed Sensory Organs Eyes vary from simple light sensitive ocelli to a

compound mosaic eye

Other sensory structures for touch, smell, hearing, balancing, and chemical reception



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Complex Behavior Patterns Arthropods surpass most other invertebrates in

complex and organized activities

Most behavior is innate but some is learned

Use of Diverse Resources through Metamorphosis Many arthropods undergo metamorphic changes

leading to different larval and adult stages

Larvae and adults feed on different organisms and occupy different habitats

Avoid competition



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Subphylum Trilobita

History of an Ancient Group Trilobites arose before the Cambrian,

flourished, and then became extinct 200 million years ago

Trilobed body shape due to a pair of longitudinal grooves

Bottom dwellers and probably were scavengers

Ranging from 2 to 67 centimeters long

Could roll up like pill bugs

Exoskeleton contained chitin strengthened by calcium carbonate


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Body was divided into a cephalon, trunk, and pygidium Cephalon was a fusion of segments

Trunk varied in number of somites

Pygidium was fused into a plate

Cephalon bore antennae, compound eyes, a mouth, and jointed appendages

Each body somite except the last had a pair of biramous appendages

One of the branches of biramous appendage was fringed May have been a gill

Subphylum Trilobita


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Subphylum Chelicerata

Characteristics

Chelicerates have six pairs of

cephalothoracic appendages including

chelicerae, pedipalps and four pair of legs

Lack mandibles and antennae

Most suck liquid food from prey


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Class Merostomata: Subclass Eurypterida

Eurypterids (giant water scorpions) were the largest of all fossil arthropods 3 m in length

Fossils date from Ordovician to Permian periods

Resemble both marine horseshoe crabs and terrestrial scorpions

Head: 6 fused segments, 6 pairs of appendages, simple and compound eyes, chelicerae, pedipalps, and 4 pairs of walking legs

Abdomen: 12 segments and spike-like telson



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Class Merostomata: Subclass Xiphosurida, Horseshoe Crabs

Modern horseshoe crab nearly unchanged from ancestors in the Triassic period

5 species in 3 genera survive

Most live in shallow water

Structures Unsegmented carapace covers body in front of a

broad abdomen and a telson

Cephalothorax has 5 pairs of walking legs and a pair of chelicerae

Abdomen bears six pairs of broad, thin, appendages fused in the median line

Book gills exposed on some abdominal appendages

Carapace has 2 compound and 2 simple eyes



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Walk with walking legs and swim with abdominal plates

Feed at night on worms and small molluscs

During mating season, come to shore at high tide to mate

Females burrow into sand to lay eggs

Males follow to add sperm before eggs are covered

Young larvae hatch and return to sea at next high tide

Larvae are segmented and resemble trilobites



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Class Pycnogonida: Sea Spiders All have small, thin bodies

Approximately 1,000 species

Some species duplicate somites, and may have five or six pairs of legs

Males may have a pair of legs (ovigers) to carry developing eggs

Many have chelicerae and palps

Mouth, at the tip of a proboscis, sucks juices from cnidarians and soft-bodied animals



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Most have four simple eyes

Simple dorsal heart

No excretory and respiratory systems

Digestive system sends branches to the legs

Most gonads are in the legs

Occupy all oceans but most common in polar waters

Some suggest that pycnogonids belonged to an early-diverging arthropod lineage



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Class Arachnida Great diversity

Most are free living and more common in warm, dry regions

Structures

Divided into two tagmata: a cephalothorax and an abdomen

Cephalothorax bears a pair of chelicerae, a pair of pedipalps, and 4 pairs of walking legs

No antenna and mandibles



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Most are predaceous and have claws, fangs, poison glands, or stingers

Sucking mouthparts ingest fluids and soft tissues from bodies of their prey

Spiders have spinning glands

A few spiders may have a segmented abdomen, a primitive character

Pedipalps of males are modified, sometimes elaborately, for sperm transfer



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Over 80,000 species have been described

Scorpions appeared on land in the Silurian, mites and spiders by the end of the Paleozoic Era

Most harmless to humans and provide essential control of injurious insects

Some spiders are venomous and can cause pain or death in humans

Ticks may carry human diseases

Mites can be crop pests



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Order Araneae: Spiders

Approximately 40,000 species

Body consists of an unsegmented cephalothorax and abdomen joined by a slender pedicel

Anterior appendages are a pair of chelicerae with terminal fangs

Pair of pedipalps have sensory functions and are used by males to transfer sperm

Basal parts of pedipalps used to handle food

Four pairs of walking legs terminate in claws

All are predaceous, mostly on insects



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Injected venom liquefies and digests the tissues which is sucked into spider’s stomach

Breathe by book lungs and/or tracheae

Book lungs unique to spiders

Parallel air pockets extend into blood-filled chamber

Air enters chamber through a slit in body wall

Tracheae system is less extensive than in insects

Transports air directly to tissues

Tracheal systems of arthropods represent a case of evolutionary convergence



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In spiders and insects, Malpighian tubules serve as excretory structures

Potassium, other solutes, and waste are secreted into tubules

Rectal glands reabsorb the potassium and water, leaving wastes and uric acid for excretion

Conserves water and allows the organisms to live in dry environments

Many spiders have coxal glands, modified nephridia, at the base of legs



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Sensory Systems

Most spiders have eight simple eyes, each with a lens, optic rods, and a retina

Detect movement and may form images

Sensory setae detect air currents, web vibrations, and other stimuli

Spider’s vision usually poor

Awareness of environment depends largely on cuticular mechanoreceptors such as sensory setae



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Web-Spinning Habits Spinning silk critical ability for spiders and some

other arachnids

Two or three pairs of spinnerets contain microscopic tubes that run to silk glands

Liquid scleroprotein secretion hardens as it is extruded from spinnerets

Silk threads are very strong and will stretch considerably before breaking

Silk is used for orb webs, lining burrows, forming egg sacs, and wrapping prey



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Reproduction

Before mating, male stores sperm in pedipalps

Mating involves inserting pedipalps into the

female genital openings

A courtship ritual is often required before the

female will allow mating

Eggs may develop in a cocoon in the web or may

be carried by female

Young hatch in about two weeks and may molt

before leaving the egg cocoon



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Are spiders really dangerous? Most people fear spiders without good reason

Spiders are allies of humans in our battle with insects

American tarantulas rarely bite, and bite is not dangerous

Species of black widow spiders are dangerous

Venom is neurotoxic

Brown recluse spider

Hemolytic venom that destroys tissue around the bite

Some Australian and South American spiders are the most dangerous and aggressive



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Order Scorpionida: Scorpions More common in tropical and subtropical

zones but occur in temperate areas

Approximately 1,400 species worldwide

Nocturnal and feed largely on insects and spiders

Sand-dwellers locate prey by detecting surface waves with leg sensillae

Appendages attached to cephalothorax Pair of medial eyes and 2–5 lateral eyes

Preabdomen has 7 segments

Postabdomen has long, slender tail of five segments that ends in a stinging apparatus



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Under the abdomen are comblike pectines

Explore the ground and aid in sex recognition

Stinger on last segment has venom that varies from mildly painful to dangerous

Ovoviviparous or viviparous and produce from 6 to 90 young

Perform complex mating dances

In some species the male stings the female on pedipalp or on edge of cephalothorax



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Order Opiliones: Harvestmen

Harvestmen or daddy longlegs

Common, particularly in tropical regions


Unlike spiders, abdomen and cephalothorax join broadly without a narrow pedicel

Can lose most of their eight long legs without ill effect

Chelicerae are pincerlike

Mostly scavengers



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Order Acari: Ticks and Mites

Medically and economically the most important arachnids

About 30,000 species have been described

Both aquatic and terrestrial Inhabit deserts, polar areas, and hot springs

Most mites are less than 1 millimeter long

Ticks may range up to 2 cm

Complete fusion of cephalothorax and abdomen

No sign of external segmentation



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Mouthparts on tip of the anterior capitulum

Chelicerae on each side help pierce, tear, or grip food

Other mouthparts include pedipalps with a fused base, hypostome, rostrum, and tectum

Adult mites and ticks possess 4 pairs of legs

Transfer sperm directly or by spermatophores

Egg hatches, releasing a six-legged larva Eight-legged nymphal stages follow



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House dust mites Free-living and often cause allergies

Spider mites One of many important agricultural pest mites

that suck out plant nutrients

Chiggers Larval Trombicula mites

Feed on dermal tissues and cause skin irritation

Hair follicle mite Demodex Harmless but other species cause mange in

domestic animals



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Human itch mite

Causes intense itching

Tick species of Ixodes

Carry Lyme disease

Tick species of Dermacentor

Transmit Rocky Mountain spotted fever

Cattle tick

Transmits Texas cattle fever



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Subphylum Myriapoda

Characteristics

Myriopods include

Chilopoda (centipedes)

Diplopoda (millipedes)

Pauropoda (pauropods)

Symphyla (symphylans)

Use trachea to transport

Excretion usually by Malpighian tubules


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Class Chilopoda

Natural History

Centipedes

Found under logs, bark and stones

Carnivorous, eating earthworms, cockroaches, and other insects

House centipede has 15 pairs of long legs Common in bathrooms and damp cellars

Most harmless to humans Few large, tropical centipedes are dangerous


Subphylum Myriapoda


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Characteristics Terrestrial and have flattened bodies with up

to 177 segments

Each segment except the one behind the head and last two

Bears a pair of jointed legs, the last pair of which serves a sensory function

Appendages of first body segment form poison claws

Head has one pair of antennae, a pair of mandibles, and one or two pairs of maxillae

Eyes on either side of the head consist of groups of ocelli

Subphylum Myriapoda


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Salivary glands empty into anterior end of straight digestive tract

Two pairs of Malpighian tubules empty into the hind intestine

Elongated heart has pair of arteries in each somite Ostia provide return flow of hemolymph

Pair of spiracles in each somite allows air to diffuse through branched air tubes of the tracheae

Arthropod nervous system includes a portion that serves as a visceral nervous system

Subphylum Myriapoda


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Reproduction

Sexes separate with unpaired gonads and

paired ducts

Some lay eggs and others are viviparous

Young resemble adults and do not undergo

metamorphosis

Subphylum Myriapoda


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Class Diplopoda

Natural History

Millipedes

Less active than centipedes Walk with graceful rather than wriggling motion

Most eat decayed plants but a few eat living plant tissue

Most are slow moving and roll into a coil for defense

Some secrete toxic or repellant fluids from special repugnatorial glands on side of body

More than 10,000 species of worldwide

Subphylum Myriapoda


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Characteristics

Cylindrical bodies with 25 to more than 100 segments

Short thorax consists of 4 segments, each bearing one pair of legs

Head has 2 clusters of simple eyes and a pair each of antennae, mandibles, and maxillae

Each abdominal somite has 2 pairs of spiracles opening into air chambers and tracheal air tubes

Two genital apertures located toward anterior end

Subphylum Myriapoda


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Reproduction

Appendages of seventh segment specialized

as copulatory organs

After copulation, female lays eggs in a nest

and guards them

Larvae have only one pair of legs per

segment

Subphylum Myriapoda


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Class Pauropoda

Life History Live in moist soil, leaf litter, decaying

vegetation, or under bark and debris

Least well known of myriapods

Characteristics Soft-bodied, small (2 mm or less)

Approximately 500 species

Head lacks true eyes, has branched antennae, and a pair of sense organs

Subphylum Myriapoda


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12 trunks segments bear 9 pairs of legs but none on the last 2 segments A tergal plate covers each of the two segments

Lack tracheae, spiracles, and circulatory system

Probably most closely related to diplopods

Subphylum Myriapoda


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Class Symphyla

Life History and Reproduction

Live in humus, leaf mold, and debris

Male Scutigerella places a spermatophore at

end of a stalk

Female stores the sperm in special pouches

Removes and smears eggs with sperm before

attaching them to moss or lichen

Young hatch with only 6 or 7 pairs of legs

Subphylum Myriapoda


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Characteristics

Small (2–10 mm) with centipede-like bodies

Soft-bodied with 14 segments 12 segments bear legs and one bears a pair of

spinnerets

Antennae are long and unbranched

About 160 species are known

Eyeless with sensory pits at base of antennae

Tracheal system connects to a pair of spiracles on the head and tracheal tubes to the anterior only

Subphylum Myriapoda


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Phylogeny and Adaptive Diversification

Phylogeny

Relationships between subphyla are debated

Taxon of Pancrustacea, which includes hexapods and crustaceans, is well-supported

Phylogenies using molecular data rarely support grouping Myriapoda with Pancrustacea

There is support for placement of Myriapoda as the sister taxon for Cheliceratae


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Biologists assume that the ancestral arthropod had a segmented body with one pair of legs per segment

Evolution caused adjacent segments to fuse and to make body regions

Hox gene studies indicate that the first five segments fused to form the head tagma in all four extant subphyla

In spiders, Hox gene studies indicate that the entire prosoma corresponds to the head of other arthropods



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Sea spiders remain within subphylum Chelicerata because Hox gene studies have found that their head appendage arose from the region of the head that corresponds to the second segment

Genetic studies have been helpful in understanding the evolution of uniramous and biramous appendages Molecular evidence repeatedly places hexapods

with crustaceans even though hexapods have uniramous appendages and crustaceans have biramous appendages.



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Leads to the question: Did uniramous appendage development evolve more than once?

Numbers of appendages per segment is another variable character among arthropods that lends itself to more testing



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Adaptive Diversification

In contrast to annelids, arthropods have pronounced tagmatization by fusion of somites

Those with primitive characters have appendages on each somite Derived forms are specialized

Modification of exoskeleton and appendages allowed variation in feeding and movement

Adaptations made possible by cuticular exoskeleton and small size fostered high diversity



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Classification

Subphylum Trilobita


Class Merostomata

Class Pycnogonida

Class Arachnida

Subphylum Myriapoda

Class Diplopoda

Class Chilopoda

Class Pauropoda

Class Symphyla

Subphylum Crustacea

Subphylum Hexapoda


Trilobites, Chelicerates, and Myriapods - Unicamdocenti.unicam.it/tmp/2597.pdf · CHAPTER 19...

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