- Body divided into segments, or somites, regionally fused into specialized groups by tagmosis (i.e., 5 segments form head)
- Each body segment has a pair of jointed appendages
- Cuticle forms well-developed exoskeleton, made up of plates called sclerites
- Growth by ecdysis (hormone-induced molting)- no cilia on larvae or adults
- Main body cavity = hemocoel, with open circulatory system; coelom reduced to region around gonads
- exoskeleton used as attachment point for muscles
Phylum Arthropoda >1,000,000 species!
Arthropod Phylogeny
5 Sub-phyla:
(1) Trilobites (extinct since Paleozoic)
(2) Chelicerates (spiders, horseshoe crabs, pycnogonids)
(3) Hexapoda (Insects)
(4) Crustacea (crabs, lobsters, shrimp)
(5) Myriapoda (centipedes, millipedes)
Arthropod Success 1: Exoskeleton
Modification of segmented body plan, by growth of a hard external covering = cuticle, or exoskeleton
protection, osmoregulation - a major pre-adaptation for the transition to life on land
Loss of circular muscles, rigid body coelom not needed for hydrostatic skeleton replaced w/ open circulatory system + hemocoel (like in molluscs, but independently evolved)
Growth must proceed through a series of molts, controlled by the hormone ecdysone (where clade Ecdysozoa gets name)
Arthropod Cuticle
hemocoel
Sensorychetae
cuticle
cuticle divided into outer layers for water retention, inner for rigidity
Arthropod Cuticle
Chitin: high mol. weight nitrogenous polysaccharide
hemocoel
Sensorycheta
gland cell pore
procuticle
epicuticle
epicuticle has water-retaining layers of oily lipoproteins, wax
procuticle of chitin, cross-linked proteins and CaCO3
Body WallEpidermis is a single layer of epithelial cells that secrete the cuticle in layers
Outer layer is epicuticle, with water-repellant hydrophobic layers - outermost layer: lipoproteins (fat + protein) - middle layer: waxy layer of fats, wax esters - inner layer: protein
Inner layer is the procuticle, of protein + chitin; hardened by:
(1) sclerotization = cross-linking of proteins into a 3D matrix (2) mineralization = depositing calcium carbonate in procuticle of crustaceans (i.e., crab shell)
Molting
Stages between molts are termed instars
- this is when actual tissue growth occurs, although there’s no size increase until after the molt
Cuticle is weakened enzymatically, then animal crawls out
After molting, animal sucks in air/water to inflate new cuticle, which then hardens
Molting
Arthropod Cross Section
Muscles anchor on inside of cuticle, connect into the jointed appendages on each segment
Heart arteries hemocoel collecting vessels
pericardium ostia heart
Open Circulatory System
Arthropod Success 2: Tagmosis
Success as a group is due largely to diversity of body form, possible because of specialization of segments, regions, + appendages
Tagmosis = segments are grouped together + specialized for particular functions greater efficiency
- head, thorax and abdomen are tagmata, regions specialized for performing different tasks
- arise from spatially restricted expression of Hox genes and other developmental regulatory genes
Segmentation + Tagmosis
Head Thorax Abdomen
Arthropod Success 3: Jointed Appendages
How do you move without cilia? Jointed appendages (limbs) + specialized muscles to move the limb pieces, or podites
- extrinsic muscles connect to body wall- intrinsic muscles are contained entirely inside the limb
Ancestral condition, found in crustaceans, is to have biramous limbs: each limb has 2 branches
Uniramous (1 branch) - insects
Biramous - crustaceans
SUB-PHYLA:
Trilobitomorpha: Trilobites (extinct)
Cheliceriformes: Chelicerates Pycnogonids
Crustacea: Crustaceans
Hexapoda: Insects
Myriapoda: Centipedes, Millipedes
Phylum Arthropoda
Most common group of fossil arthropods- once abundant in oceans- disappeared by Paleozoic (345 million yr ago)
SubPhylum Trilobitomorpha (Extinct)
Cephalon
Thorax
Pygidium
SUB-PHYLA:
Trilobitomorpha: Trilobites (extinct)
Cheliceriformes: Chelicerates (spiders, scorpions, and horseshoe crabs)
Pycnogonids
Crustacea: Crustaceans
Hexapoda: Insects
Myriapoda: Centipedes, Millipedes
Phylum Arthropoda
~ 65,000 spp.Cheliceriformes spiders, scorpions, and horseshoe crabs
- Body of 2 tagmata: cephalothorax + abdomen
- 1st pair of appendages = chelicerae (look like fangs)
- no antennae
- Gas exchange by book gills, book lungs or tracheae
- Separate sexes
Phylum Arthropoda SubPhylum Cheliceriformes
Class Pycnogonida – “Sea Spiders”
Class Chelicerata SubClass Merostomata
Order Eurypterida – extinct giant sea scorpions Order Xiphosura – Horseshoe crabs
SubClass Arachnida- scorpions, spiders, ticks, mites
SubPhylum Cheliceriformes
2 groups are marine
~1,000 spp.
- marine, intertidal to abyssal depths; worldwide distribution
- benthic, live on seaweeds or other invertebrates
- sucking proboscis on 1st head segment used to feed on soft- bodied invertebrates
- males: brood eggs on ovigers, special leg appendages
- females: hollow legs filled with eggs
Pycnogonida: Sea spiders
1st segment Ovigerous leg, used by males for brooding eggs
chelicera
Phylum Arthropoda SubPhylum Cheliceriformes
Class Pycnogonida – “Sea Spiders”
Class Chelicerata SubClass Merostomata
Order Eurypterida – extinct giant sea scorpions Order Xiphosura – Horseshoe crabs
SubClass Arachnida- scorpions, spiders, ticks, mites
SubPhylum Cheliceriformes
2 groups are marine
SubClass MerostomataOrder Xiphosura – Horseshoe crabs
5 living species (all others extinct)
- inhabit shallow marine waters
- burrow just under sand surface, prey on buried animals like bivalves
- small chelicerae
- limited distribution: Limulus polyphemus restricted to east coast of North America
- distinctive telson, or tail spine
Horseshoe crab: Limulus
prosoma
opisthosomabook gills
telson
anus
cheliceraepedipalp:1st walking leg
4 pairs ofwalking legs
- once very abundant on beaches in the Atlantic
- heavily harvested to make feed for farm animals
>60,000 spp.
- opisthosomal (abdominal) appendages absent, or modified as spinnerets for spinning silk proteins into webs
- no compound eyes
- gas exchange by tracheae or book lungs
…Scorpions, spiders, mites, ticks
SubClass Arachnida
SUB-PHYLA:
Trilobitomorpha: Trilobites (extinct)
Cheliceriformes: Chelicerates (spiders, scorpions, and horseshoe crabs)
Pycnogonids
Crustacea: Crustaceans (crab, lobster, shrimp)
Hexapoda: Insects
Myriapoda: Centipedes, Millipedes
Phylum Arthropoda
SubPhylum Crustacea- body of 3 tagmata: 5-segmented head, thorax, abdomen
- cephalic shield or carapace present to protected dorsum
- mandibles, modified limbs, act as jaws; 2 pairs of antennae
- biramous limbs (2 forks)
- “gills” actually legs modified for gas exchange
- excretion by nephridia (glands near antennae)
- simple ocelli + compound eyes (often stalked)
- nauplius larva, which molts and goes through several instars
~75,000 spp.
Crustacean Head
23
54
15-segmented head, each segment with its own appendages
thorax limbs, “borrowed” by the head
Crustacean Head
If 1st thoracic segment fuses with head, its appendages grow as maxillipeds
- next segment is thus 2nd thoracic
5 segments 1st antennae= antennules
2nd3rd
1st
thorax
head
External Anatomy of a Crayfish
Crustacean limbsBranches join at base, the protopod
Inner branch = endopod
Outer branch = exopodprotopod
endopod
exopod
body
Crustacean limbsBranches join at base, the protopod
- extensions on outer side = epipods- often flattened; function as gills, gill cleaners
- extensions towards the body = endites- often form a spiny,
grinding surface
Inner branch = endopod
Outer branch = exopod
endopodexopod
body
epipodendites
Arthropod limbs
Protopod
Epipod
Endopod
Exopod
Endites
Class Malacostraca - crabs, shrimps, lobsters
Class Maxillopoda - ostracods, barnacles, copepods
Class Branchiopoda - brine shrimp, tadpole shrimp
Class Remipedia - small cave-dwellers; basal lineage
Class Cephalocarida - small benthic detritivores; basal
SubPhylum Crustacea
Class Malacostraca
Pereopods, orwalking legs
Pleopods, orswimming legs
Thorax segmentsAbdomen
telson
Body of 19 segments: head, 5 thoraxthorax, 8
abdomen, 6 + telson (tail)
Class Malacostraca – 19 body segments (5-8-6 body)
3 major Super-orders:
Hoplocarida - stomatopods (mantis shrimps)
Eucarida - krill, crabs, shrimps, lobsters
Peracarida - isopods, amphipods, mysids
Class Maxillopoda - ostracods, barnacles, copepodsClass Branchiopoda - brine shrimp, tadpole shrimpClass Remipedia - small cave-dwellers; basal lineageClass Cephalocarida - small benthic detritivores; basal
SubPhylum Crustacea
seen in today’s lab
Vicious predators, snagging prey with raptorial limbs
Class Malacostraca (5-8-6 body) SuperOrder Hoplocarida Order Stomatopoda
= 2nd pereopod
Class Malacostraca (5-8-6 body) SuperOrder Eucarida
Order Euphausiacea (krill)
Krill form feeding swarms, especially at poles - primary food source for many whales
Class Malacostraca (5-8-6 body) SuperOrder Eucarida
Order Euphausiacea (krill)Order Decapoda (14,000 spp.)
InfraOrder: Caridea (shrimps) Astacidea (crayfish) Palinura (lobsters) Brachyura (true crabs)
Anomura (hermits, king crabs) Thalassinidea (ghost shrimp)
3 maxillipeds, leaving 5 pairs of pereopods (hence name) - at least one pair usually modified as chelae, or claws
Large carapace covering head, branchial chamber
decapod =“10 feet”
Order Decapoda, Infraorder: Palinura Brachyura (lobsters) (true crabs)
Crab abdomen reduced, folded under thorax
- 5th walking legs modified for swimming
Abdomen
Carapace
5th walking leg
Order DecapodaInfraorder Brachyura (true crabs)
Order Decapoda Infraorder Anomura (hermit + king crabs)
- 3 to 4 pairs of walking legs
- 5th leg reduced, often gill cleaner
sand crab
hermit
King crab
Both the 1st & 2ndantennae are betweenthe eyes in theanomurans
Order DecapodaInfraorder Caridea (shrimp)
Development in Crabs
In decapods, the nauplius stage occurs inside the egg
Pelagic stages are zoea, then megalops (transparent juvenile)
Crab zoea larva
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