Phylum ARTHROPODAby John R. Finnerty

CAS BI 569

TropicalMarine

Invertebrates

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.

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.

Whither the Coelom?

A Generalized Arthropod

Exoskeleton composition — Arthropods

Lipids

Chitin (a polysaccharide)+ associated proteins

Epicuticle

Procuticle

Lipoprotein

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

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.

subphylum Trilobitomorpha

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

arrangement19 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