1. Phylum Arthropoda

2. Similarities between Annelids and Arthropods
Arthropods are metameric and their segments have appendages
Nervous system with ventral nerve cords
Phylum Onychophora
Annelid-like
Segmented; unjointed appendages; similarity in structure of the body wall; segmentally arranged nephridia; pigment-cup ocelli
Arthropod-like
Reduced coelom, open circulatory system, tracheal system; soft cuticle composed of chitin
Peripatus, a small, nocturnal form found among the leaf-litter of tropical forests of South America.

3. The fossil record indicates that onychophorans have not changed much in 500 million years.
The Onychophoran Aysheaia from the Cambrian.
A lobe-limbed, segmented animal. Also note the spines on the legs. The head end has a pair of tapering limbs with spines, and three small projections near the mouth. Traces of the digestive tract can also be seen.

4. Phylum Tardigrada or “water bears” also have features in common with both annelids and arthropods
Annelid-like
Unjointed (8) legs; annelid-type nervous system
Arthropod-like
Presence of a cuticle (nonchitinous) that is periodically molted; similar attachment of muscle fibers to exoskeleton
One of the most interesting features of tardigrads is their ability to undergo cryptobiosis

5. Arthropod Taxonomy: Overview
The arthropods evolved along four main lines, which most zoologists recognize as 4 distinct subphyla
1. Trilobita - extinct trilobites
2. Chelicerata - horseshoe crabs, spiders, ticks, mites, and some extinct groups
3. Crustacea - crabs, lobsters, shrimps, barnacles
4. Uniramia - insects, centipedes, millipedes

6. The Arthropod Exoskeleton
Epidermis secretes an external skeleton called the exoskeleton
Advantages of possessing an exoskeleton:
provides strong support
provides rigid levers that muscles can attach to and pull against
offers protection
serves as a barrier to prevent internal tissues from drying out; important because many arthropods live on land
serves as a barrier to prevent infection

7. Structure of the Exoskeleton
Composed of the polysaccharide chitin and protein - glycoprotein
Outer surface called the epicuticle; contains waxes
The thicker portion is called the procuticle:
exocuticle
endocuticle
In the exocuticle, the glycoprotein chains are cross linked; process is called tanning
epicuticle
exocuticle
procuticle
endocuticle
epidermis

8. Molting
In order to grow the arthropod must shed its exoskeleton, and secrete a new and larger one - molting or ecdysis.

9. Jointed Appendages
Exoskeleton divided into a number of plates and cylinders
At the junction point between plates and cylinders, the exoskeleton remains thin and flexible; these are the joints
Jointed appendages allows arthropods to move efficiently and quickly
Muscles are integral to arthropod movement; they attach to the inner side of the exoskeleton; they often function as a lever system
Arthropod joint
Vertebrate joint

10. Specialized Arthropod Segments: Reduction in Metamerism
The evolution of the arthropods witnessed a reduction in metamerism
The arthropods evolved modified groups of segments (e.g., segments became lost, some fused together
The fusion of groups of segments into functional groups is called tagmatization
In so doing, various appendages on segments became specialized for functions other than locomotion, e.g. prey capture, filter feeding, sensing various kinds of stimuli, gas exchange, copulation, etc.

11. Arthropod Respiratory Advances
Special respiratory structures allow the arthropods to metabolize more efficiently and thus move rapidly
High metabolic rates require rapid oxygen delivery, and arthropods can accomplish this with respiratory organs that have a large surface area for collecting oxygen quickly

12. Gills
Many aquatic arthropods (crabs and lobsters) have gills, which are typically modifications of appendages or outgrowths of the body wall - folds of tissue with a large surface area

13. Tracheae
Gas exchange organs among terrestrial arthropods is usually internal; invaginations of the integument
Insects have tracheae, branching networks of hollow air conducting tubes such that air is sent to every cell in every tissue

14. Book Lungs
Spiders have book lungs, chambers with leaf-like plates for exchanging gases; air flows over the plates and blood flows through them

15. Acute Senses
Arthropods have a well-developed nervous system that is of the same overall design as the annelids; anterior brain and a double, ventral hollow nerve cord.
The sensory receptors of arthropods are usually associated with modifications of the chitinous exoskeleton
The head usually bears various kinds of sense organs (e.g. antennae) with extreme sensitivity

16. Acute Senses cont.
Many arthropods have compound eyes - eyes that are composed of many visual units called facets (ommatidia); capable of color vision and detecting the slightest movements of prey or predators
Some eyes are simple eyes with only a few photoreceptors; however, they are capable of forming crude images

17. Digestive System
Divided into 3 main regions: foregut, midgut, and hindgut
Foregut and the hindgut are lined with chitin
Foregut is involved with ingestion, mechanical breakdown, and storage
Hindgut is involved with water absorption and formation of the feces
Midgut is not lined with chitin; involved with digestion and absorption
Outpockets (e.g. digestive glands) increase the surface area for digestion and absorption

18. Internal Transport and Excretion
Open circulatory system
Many crustaceans possess an excretory organ called the green gland (antennal gland), which filters fluid from the blood
Most insects and spiders have a excretory system called malpighian tubules

19. Reproduction
Sexes are separate; fertilization is external in aquatic forms, internal among the terrestrial forms

20. Arthropod Diversity

21. Subphylum Trilobita A group of extinct marine arthropods.
Ranged in size from a few millimeters up to 75 centimeters.
Body divided into three parts:
cephalon (head), a single plate made up of several fused segments
thorax, consists of a number of segments hinged together
pygidium (tail), also segmented, but like the head, fused together into a single plate.
Compound eye, composed of radially arranged visual units

22. Subphylum Chelicerata
Lack antennae
Body is usually composed of two regions: cephalothorax and abdomen
Cephalothorax is usually covered dorsally by the carapace
Six pairs of appendages: first pair are modified feeding structures - chelicerae
Second pair are called pedipalps
Four additional pairs of appendages are walking legs
No abdominal appendages
Some have compound eyes, usually have simple eyes capable of forming crude images.

23. Class Meristomata (e.g., Horeshoe crabs)
Marine chelicerates, common off of the Atlantic and Gulf coasts
Large dorsal carapace bearing compound eyes
Possess chelicerae, pedipalps, and 4 pair of walking legs (all but the last pair are chelate
Abdomen terminates in a long tail called the telson; used to turn the animal right side up
Possess a series of gill plates called book gills

24. Class Arachnida
Among spiders, the cephalothorax and abdomen shows no external segmentation; tagma are joined by a narrow pedicel
Respiration is accomplished via book lungs, tracheae, or both
Usually have 8 simple eyes; at the very least they detect motion; for some of the predatory forms, they are capable of forming crude images
Many species have evolved poison glands associated with the chelicerae
Black widow
Brown recluse

25. Class Arachnida cont.
Many of the spiders and mites are capable of producing silk; produced by silk glands that open to the exterior part of the abdomen through spinnerets
Silk is used to build webs for trapping prey, nests which serve as retreats, and egg cases; it is also used to form a dragline
Most spiders are predaceous and have all kinds of sensory hairs and relatively well-developed eyes for motion detection
Orb web construction

26. Prey capture among the spiders
Some species are cursorial predators, those that stalk and ambush their prey; they usually have well-developed eyes
Jumping spider
Others are web building spiders, those that construct various kinds of webs made of silk to trap their prey
Eyes are not as well developed as cursorial predators, but they have a battery of sensory hairs for detecting vibrations
Grass spider

27. SubPhylum Crustacea
Possess 2 pair of antennae: First pair is homologous to those of insects; second pair is unique to the crustaceans
Second antennae have various functions, including sensory, locomotion or feeding.
The head bears a pair of compound eyes and 3 pairs of mouthparts: a pair of mandibles, and 2 pairs of maxillae; used for food handling
Trunk varies considerably among classes
Primitively, the first three pairs of thoracic segments are maxillipeds; function in handling food
Also, there are usually 5 pairs of appendages strengthened for walking (walking legs) and protection (chelipeds, pincer-like claws)

28. Abdomen is also highly variable, but it is primitively large
SubPhylum Crustacea cont.
Abdomen is also highly variable, but it is primitively large
Groups with a well-developed abdomen usually possess six pairs of appendages: Five pairs of structures called swimmerets (=pleopods); one pair of structures called uropods,
Uropods together with the terminal telson form a tail fan than can serve as rudders during locomotion

29. SubPhylum Crustacea cont.
Primitively many of the appendages of the crustaceans are biramous: there is an outer exopod and an inner endopod
They usually have an extremely hardened exoskeleton, which is impregnated with calcium carbonate - carapace

30. The primitive larva of the crustaceans is called the nauplius larva
SubPhylum Crustacea cont.
The primitive larva of the crustaceans is called the nauplius larva
It has an unsegmented body, a frontal eye, and 3 pairs of appendages, representing the 2 pairs of antennae and the mandibles

31. Crustacean Diversity Ostracod Water flea Copepod Fairy shrimp
Fish louse

32. Barnacles are Crustaceans!

33. SubPhylum Uniramia Single pair of antennae
First pair of feeding appendages are mandibles
There are one or 2 pairs of maxillae
Number of legs vary from 3 pair to many pairs; they are unbranched or uniramous

34. Class Chilopoda (Centipedes)
SubPhylum Uniramia cont.
Class Chilopoda (Centipedes)
Serial segmented, flattened body and each segment has a pair of jointed appendages
Active predators, killing their prey with poison claws, which are modified legs on first segment

35. Class Diplopoda (Millipedes)
SubPhylum Uniramia cont.
Class Diplopoda (Millipedes)
Serially segmented, rounded body with 2 smaller pairs of legs per segment
Slow moving; feed on decaying plants

36. Class Insecta
Body is divided into 3 parts: the head, thorax and the abdomen.
Head has one pair of antennae, a pair of compound eyes and several sets of simple eyes
Mouthparts: a pair of mandibles, and two pairs of maxillae
One pair of maxillae are fused together to form a lower lip - labium
An upper lip - labrum - formed from an extension of the head

37. Class Insecta cont.
Mouthparts are highly modified depending on the group you are discussing
Mosquitoes have pointed mouthparts for piercing and sucking; grasshoppers have mouthparts that are well adapted for chewing; butterflies for siphoning; flies for sponging

38. Wings of insects are modified portions of the exoskeleton
Class Insecta con’t
Thorax is composed of 3 segments and each one has a pair of legs; the last two segments also have a pair of wings.
Wings of insects are modified portions of the exoskeleton
1st pair is usually tough and leathery and fold over the inner pair for protection.
Abdomen does not have appendages; terminal portions do harbor the reproductive structures

39. Most insects undergo metamorphosis
Incomplete Metamorphosis (Hemimetabolous)
Early developmental stages are very similar to the adults; only the wings and the reproductive structures gradually develop
The immature stages are called nymphs
Thus development is egg----> nymphs ----> adult

40. Complete Metamorphosis (Holometabolous)
Each of the developmental stages is structurally and functionally very different
The egg develops into an immature larva; eats voraciously
Followed by a transitional stage - pupa, contained within cocoon
Metamorphosis occurs within the pupal exoskeleton, yielding a sexually mature adult