Tropical
Marine
Invertebrates
CAS BI 569
Phylum ARTHROPODA
by John R. Finnerty
Phylum Arthropoda
Well over 1 million described
extant species
Four critical bodyplan features.
Chitinous exoskeleton
Segmented body plan
Tagmatization
Jointed paired appendages
Profound changes in “axial
complexity” occur during Arthropod
evolution.
Whither the Coelom?
The rigidity of the body wall (the sclerotized chitinous
exoskeleton) was accompanied by other major changes to
the body plan with important biomechanical ramifications.
peristaltic locomotion using the coelom as a hydrostratic
skeleton is no longer possible
loss of circular body wall muscles
extreme reduction of coelom—remnants found in the
reproductive system and excretory system
new large spacious body cavity, the hemocoel.
A Generalized Arthropod
Exoskeleton composition — Arthropods
Epicuticle
Lipoprotein
Lipids
Procuticle
Chitin (a polysaccharide)
+ associated proteins
Functional Specialization of Segments
Functional Redundancy
Functional Specialization
Fusion of Segments (Tagmatization)
“head”
“thorax”
“abdomen”
Tagmatization
Phylum Tardigrada
“water bears”
Onychophora
Phylum Arthropoda
subphylum TRILOBITOMORPHA
subphylum CHELICERIFORMES
class CHELICERATA
subclass ARACHNIDA (ticks, mites, scorpions, spiders, etc.)
subclass MEROSTOMATA
order Eurypterida (extinct sea scorpions)
order Xiphosura (5 extant species of horseshoe crabs)
class PYCNOGONIDA
subphylum CRUSTACEA
subphylum HEXAPODA
class INSECTA
class ENTOGNATHA (e.g., springtails)
subphylum MYRIAPODA
subphylum Trilobitomorpha
Originated in the Cambrian and went extinct in the
end-Permian extinction, 250 million years ago.
Anterior-posterior furrows divide body into 3 lobes.
Dorso-ventrally flattened.
The anterior section is covered by a head shield.
This head section bears eyes.
Each segment posterior to the mouth has a pair of
biramous appendages.
The outer ramus bears setae, and is specialized for
swimming, breathing, or feeding.
The inner ramus lacks setae, and is specialized for
walking.
In the earliest trilobites, all the appendages were
nearly identical. In later forms, some appendages
differentiated and even lost one ramus.
Trilobite Anatomy
subphylum Cheliceriformes
two tagmata
the anterior PROSOMA (cephalothorax)
the posterior OPISTHOSOMA (abdomen)s
CHELICERAE— the most anterior appendages on the prosoma are
modified into claws (e.g., horseshoe crabs) or fangs (e.g., spiders)
PEDIPALPS — second pair of appendages on prosoma used for
prey manipulation
WALKING LEGS — fours pairs follow the pedipalps on the
prosoma.
Appendages on the opisthosoma are typically modified for
respiration (e.g., the gills of horseshoe crabs of the book lungs of
spiders).
subclass Merostomata, order Eurypterida
“sea scorpions”
originated in the Cambrian (~510 MYA)
perished in the end-Permian extinction
(248 MYA)
possible the largest arthropods that
ever lived, up to 2 meters in length.
Limulus Anatomy
subclass Arachnida
spiders, scorpions, ticks,
mites, whip scorpions
~73,000 species
almost all terrestrial
opisthoma appendages
modified into spinnerets in
spiders or pectenes in
scorpions (a sensory
appendage)
respiration by tracheae or
“book lungs”
Spider Anatomy
subphylum Crustacea
head, thorax, and abdomen
head comprises five
segments
2 pair of antennae
1 pair of mandibles
2 pair of maxillae
thoracic appendages
maxillipeds (food manip.)
pereopods (walking legs)
abdominal appendage
pleopods
Malacostrocan Anatomy
subphylum Hexapoda, class Insecta
very conserved segmental
arrangement
19 segments
five head segments with five
head appendages
antenna
clypeus
mandibles
maxillae
labrum
3 thoracic segments
11 abdominal segments
“for every human alive, there
are an estimated 200 million
insects” —Brusca & Brusca