1. Phylum Arthropoda >1,000,000 species!
- 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

2. Arthropod Phylogeny 5 Sub-phyla: Trilobites (extinct since Paleozoic)
(2) Chelicerates (spiders, horseshoe crabs, pycnogonids)
(3) Hexapoda (Insects)
(4) Crustacea (crabs, lobsters, shrimp)
(5) Myriapoda (centipedes, millipedes)

3. 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)

4. Arthropod Cuticle Sensory chetae cuticle cuticle divided into
outer layers for
water retention,
inner for rigidity
hemocoel

5. Arthropod Cuticle epicuticle procuticle hemocoel Sensory cheta
gland cell pore
epicuticle
procuticle
Sensory
cheta
epicuticle has water-retaining
layers of oily lipoproteins, wax
procuticle of chitin, cross-linked
proteins and CaCO3
hemocoel
Chitin: high mol. weight nitrogenous polysaccharide

6. Body Wall
Epidermis 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)

7. 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

8. Molting

9. Arthropod Cross Section
Muscles anchor on inside of cuticle, connect into the jointed
appendages on each segment

10. Open Circulatory System
Heart  arteries  hemocoel  collecting vessels
 pericardium  ostia  heart

11. 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

12. Segmentation + Tagmosis
Head Thorax Abdomen

13. 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

14. Phylum Arthropoda SUB-PHYLA: Trilobitomorpha: Trilobites (extinct)
Cheliceriformes: Chelicerates
Pycnogonids
Crustacea: Crustaceans
Hexapoda: Insects
Myriapoda: Centipedes, Millipedes

15. SubPhylum Trilobitomorpha (Extinct)
Cephalon
Thorax
Pygidium
Most common group of fossil arthropods
- once abundant in oceans
- disappeared by Paleozoic (345 million yr ago)

16. Phylum Arthropoda SUB-PHYLA: Trilobitomorpha: Trilobites (extinct)
Cheliceriformes: Chelicerates (spiders, scorpions, and
horseshoe crabs)
Pycnogonids
Crustacea: Crustaceans
Hexapoda: Insects
Myriapoda: Centipedes, Millipedes

17. Cheliceriformes spiders, scorpions, and horseshoe crabs
~ 65,000 spp.
- 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

18. SubPhylum Cheliceriformes
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
2 groups are marine

19. Pycnogonida: Sea spiders
~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

20. chelicera
1st segment
Ovigerous leg, used by males
for brooding eggs

21. SubPhylum Cheliceriformes
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
2 groups are marine

22. Order Xiphosura – Horseshoe crabs
SubClass Merostomata
Order 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

23. Horseshoe crab: Limulus
pedipalp:
1st walking leg
chelicerae
prosoma
4 pairs of
walking legs
opisthosoma
book gills
anus
telson

24. - once very abundant on beaches in the Atlantic
- heavily harvested to make feed for farm animals

25. SubClass Arachnida >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

26. Phylum Arthropoda SUB-PHYLA: Trilobitomorpha: Trilobites (extinct)
Cheliceriformes: Chelicerates (spiders, scorpions, and
horseshoe crabs)
Pycnogonids
Crustacea: Crustaceans (crab, lobster, shrimp)
Hexapoda: Insects
Myriapoda: Centipedes, Millipedes

27. SubPhylum Crustacea ~75,000 spp.
- 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

28. Crustacean Head 1 2 3 4 5 5-segmented head, each segment with its
own appendages 1 2 3 4 5
thorax limbs,
“borrowed” by
the head

29. Crustacean Head 1st antennae 5 segments = antennules head thorax
2nd
thorax
3rd
If 1st thoracic segment
fuses with head, its
appendages grow as
maxillipeds
- next segment is thus 2nd thoracic

30. External Anatomy of a Crayfish

31. Crustacean limbs Inner branch = endopod Outer branch = exopod
Branches join at base, the protopod
Inner branch = endopod
Outer branch = exopod
body
protopod
exopod
endopod

32. Crustacean limbs Inner branch = endopod Outer branch = exopod
Branches 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
body
endites
epipod
endopod
exopod

33. Arthropod limbs
Epipod
Exopod
Protopod
Endopod
Endites

34. SubPhylum Crustacea 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

35. Class
Malacostraca
Body of 19 segments: head, 5
thorax, 8
abdomen, 6 + telson (tail)
Thorax segments
Abdomen
Pleopods, or
swimming legs
Pereopods, or
walking legs
telson

36. SubPhylum Crustacea seen in today’s lab
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, copepods
Class Branchiopoda - brine shrimp, tadpole shrimp
Class Remipedia - small cave-dwellers; basal lineage
Class Cephalocarida - small benthic detritivores; basal
seen in today’s lab

37. Class Malacostraca (5-8-6 body) SuperOrder Hoplocarida
Order Stomatopoda
= 2nd pereopod
Vicious predators, snagging prey
with raptorial limbs

38. Class Malacostraca (5-8-6 body) SuperOrder Eucarida
Order Euphausiacea (krill)
Krill form feeding swarms, especially at poles
- primary food source for many whales

39. Class Malacostraca (5-8-6 body) SuperOrder Eucarida
decapod =
“10 feet”
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)
8 – 3 =
5 walking legs
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

40. Order Decapoda, Infraorder: Palinura Brachyura (lobsters) (true crabs)
Carapace
Abdomen
5th walking leg
Crab abdomen reduced, folded under thorax
- 5th walking legs modified for swimming

41. Order Decapoda
Infraorder Brachyura (true crabs)

42. 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

43. Both the 1st & 2nd
antennae are
between the eyes
in brachyurans
2nd antennae are
outside the eyes in
anomurans
Anomura Brachyura

44. Order Decapoda
Infraorder Caridea (shrimp)

45. Development in Crabs Crab zoea larva
In decapods, the nauplius stage occurs
inside the egg
Pelagic stages are zoea, then
megalops (transparent juvenile)
Crab zoea larva