Arthropods

Question 1

How are the phylum arthropoda described?

Answer 1

1,000,000 spp
Represented in virtually in every habitat
Especially successful in terrestrial environments
Protostomes
Segmented
Exoskeleton

Question 2

How is a generalised arthropod described?

Answer 2

Nervous system similar to annelids (dorsal brain)
Circulatory system open circuit
Hemocyanin is usual respiratory pigment
Compound eyes are common
Jointed cuticle (exoskeleton) with flexible membrane at joints
Cuticle extends into front
Segments often specialised to form distinct body regions- Tagmata

Question 3

How are the cuticles structured in the arthropoda?

Answer 3

Cuticle of each segment with four plates (but often modified)
Rigid cuticle necessitates periodic moulting
Coelom very reduced in adults

Question 4

How does moulting work in the arthropoda?

Answer 4

New exoskeleton is formed below the old exoskeleton
Some materials are re-absorbed from the old exoskeleton
Exoskeleton begins to split along specific lines of weakness
Body swells as it emerges from the splits in the old exoskeleton
Body remains soft to enable it to expand further. Vulnerable to predation (tend to hide)

Question 5

How are the arthropods appendages described?

Answer 5

Jointed appendages specialised for different functions
Movement of jointed appendages by antagonistic muscle system
Primitive condition of one pair appendages per segment
Appendages branched (biramous) in many arthropods
Muscle faster acting than annelid muscles

Question 6

What sub-phyla exist in the phylum Arthropoda?

Answer 6

Sub phyla:
- Trilobita- the trilobites (extinct)
- Chelicerata- spiders, scorpions etc
- Crustacea- crabs, barnacles, shrimp etc
- Uniramia- millipedes, centipedes, insects

Question 7

How are the Trilobita described?

Answer 7

Extinct since end of paleozoic (-240 mya)
3900 spp described
Mostly 3-10cm
Evidence of burrowing, epibenthic, pelagic and nektonic forms

Question 8

How are the Trilobita structured?

Answer 8

Three lobes- refers to the axial and two lateral lobes
Body composed of cephalon, thorax and pygidium

Question 9

What did the Trilobita have that was the first appearance of that structure?

Answer 9

Compound eyes- first appearance in fossil record

Question 10

What does each segment of the Trilobita bear and what are they capable of?

Answer 10

Each segment bears a pair of branched appendages
These are fairly similar on different segments
Some capable to rolling into protective

Question 11

How are the Chelicerata described?

Answer 11

Lack antennae
Body divided into cephalothorax and abdomen
1st pair appendages are chelicerae and have a feeding function
2nd pair appendages are pedipalps- modified for different functions
Normally followed by 4 pairs of legs

Question 12

What classes exist in the Chelicerata?

Answer 12

Class Merotomata- horseshoe crabs and sea scorpions (extinct)
Class Arachnida- spiders, scorpions, mites etc
Class Pycnogonida- sea spiders

Question 13

How are the class Merostomata described?

Answer 13

Five or six pairs abdominal appendages modified as gills
Spike-like telson at posterior end of body

Question 14

What sub classes exist in the class Merostomata?

Answer 14

Sub class Xiphosura- horseshoe crabs
Sub class Eurypterida- sea scorpions (extinct)

Question 15

How are the sub class Xiphosura described?

Answer 15

Four living species
Compound eyes (no other living chelicerates have these)
Live in shallow water in soft sediments
Large carapace gives protection when ploughing through sediment

Question 16

How are the sub class Eurypterida described?

Answer 16

Extinct since end of the paleozoic (240 mya)
Similar body plan to horseshoe crabs (cephalothorax is smaller and abdomen divided into separate segments)
Legs show various modifications
Last pair of legs modified as paddles- active swimmers
Predatory- one species >2m long
Marine and brackish water- possibly terrestrial
May be ancestral to arachnida

Question 17

How are the class Arachnida described?

Answer 17

Very large group but mostly terrestrial
Spiders, scorpions, mites, ticks and several other groups of spider like or scorpion-like terrestrial arthropods
Marine representatives found amongst the mites in the family Halacaridae
-200 spp- intertidal to abyssal- epibenthic and interstitial- often more prominent component of meiofauna

Question 18

How are the class Pycnogonida described?

Answer 18

1000 spp
Aberrant group- relationship to other chelicerates uncertain
Narrow body, anterior proboscis, four pairs of long legs
Often found on hydroids and bryozoans
Carnivorous, detrital feeders or herbivorous
No gas exchange or excretory organs

Question 19

How are the Uniramia described?

Answer 19

Single pair of antennae and appendages are unbranched
Massively diverse group but almost entirely terrestrial
Includes Myriapods (centipedes and millipedes) and the insects
Evolved on land- none are fully marine
May visit intertidal zone to feed but few live there
Apterygotes (insects lacking wings) are notable exception

Question 20

What are some examples of the Uniramia?

Answer 20

Heteroptera- Halobates spp.
The most fully marine known insects- live water surface of open sea in warm climates, predatory

Apterygotes- Thysanura (silverfish) e.g. Petrobius spp.

Diptera- Chironomid (midge) larvae

Question 21

How are the crustacea described?

Answer 21

38,000 spp- most are marine
Very diverse structurally and ecologically
Two pairs antennae (unique in arthropods)
One pair mandibles and two pairs maxillae
Carapace commonly present
Appendages typically biramous but often highly modified
Gills are normally present and associated with appendages
Two types of eye- pair of compound eyes and small median naupliar eye

Question 22

What classes exist in the crustacea?

Answer 22

Remipedia
Cephalocarida
Branchiopoda
Ostracoda
Copepoda
Mystacocarida
Tantulocarida
Branchiura
Cirripedia
Malacostraca

Question 23

How are the class Remipedia?

Answer 23

Primitive body plan- numerous un-fused segments each with a pair similar biramous appendages
First described in 1981
All from anchialine caves
Elongate body resembling a polychaete
Active free-swimming predators

Question 24

How are the class Cephalocarida described?

Answer 24

Primitive body plan- first 8 segments each bearing a pair of similar biramous appendages
First described in 1953
Benthic sediments
Horseshoe shaped head and elongate trunk of 20 similar segments
Selective deposit feeders on sediment surface
Meiofaunal

Question 25

How are the class Branchiopoda described?

Answer 25

Almost entirely freshwater- flattened leaf-like appendages are characteristic
Includes order Cladocera- Important components marine plankton

Question 26

How are the class Branchiopoda structured?

Answer 26

Trunk covered by bivalved carapace
2nd antennae modified for swimming, 1st antennae vestigial
4 to 6 pairs of trunk appendages
Median compound

Question 27

How are the class Ostracoda described?

Answer 27

Characteristic hinged bivalved carapace
Carapace often has CaCO3
Trunk reduced, no more than 2 trunk appendages
Some planktonic, most benthic
Various feeding modes

Question 28

How are the class Copepoda described?

Answer 28

Usually small with tapered body- thorax has 5 pairs biramous appendages, abdomen has no appendages, 1st antennae longer than 2nd antennae, no compound eye, naupliar eye present

Question 29

How many orders exist in the class Copepoda and how are they important?

Answer 29

Several orders- distinguished on basis of articles of 1st antennae
Ecologically important in plankton

Question 30

What are the various modes of feeding in the class Copepoda?

Answer 30

Suspension feeders, herbivores, predators etc- many (>1000) species parasitic and of economic importance
Also important in sediments as major component of meiofauna
Vast majority of these are of the Order Hapacticoida which include >50% of copepod species

Question 31

How are the class Mystacocarida described?

Answer 31

Discovered in 1943
Meiofaunal and similar in appearance to copepods
Both pairs of antennae and mandibles are long and appear similar to those of nauplius larvae

Question 32

How are the classes Tantulocarida, Branchiura and Pentastomida described?

Answer 32

Highly specialised parasites- thought to be related to copepods
Tantulocarida- ectoparasites of deep water crustaceans- look like copepods without legs

Branchiura- ectoparasites of fish, covered by carapace, antennae reduced and modified for attachment

Pentastomida- worm like respiratory parasites of reptiles

Question 33

How are the class Cirripedia described?

Answer 33

The barnacles- include acorn barnacles, goose barnacles and parasitic forms
Body is enclosed by a bivalve carapace (within the external plates)
Both sets of antennae reduced or absent
Most are suspension feeders, some are commensal, others are parasitic

Question 34

Where do the class Cirripedia reside?

Answer 34

Acorn barnacles- often numerically dominant on exposed rocky shores
Cyprid larvae settle on rock in spring or early summer
Acorn barnacles are tolerant of wave exposure and scour

Question 35

How do the class Cirripedia feed?

Answer 35

Most are suspension feeders, some are commensal, others parasitic

Question 36

What are goose barnacles?

Answer 36

Class Cirripedia
Stalked barnacles often attached to free-floating debris (one group found on rock)