Phylum Arthropoda

This is the most extensive phylum in the animal kingdom. It is composed of 3/4 of all known species. Approximately 900,000 species of arthropods have been recorded. Included are spiders, scorpions, ticks, mites crustaceans, millipedes, centipedes, insects, and some others. There is an extended fossil record.

Arthropods are eucoelomate protostomes with well-developed organ systems. Most species have combined or fused somites and appendages into functional groups called tagmata for specialized purposes.

Although all types - carnivorous, omnivorous, and symbiotic - occur, the majority are herbivorous. In diversity of ecological distribution, the arthropods have no rivals.


1) Bilateral symmetry; metameric body divided into tagmata consisting of head and trunk.

2) Jointed appendages; primitive, one pair to each somite. Often modified for specialized functions.

3) Exoskeleton of cuticle (protective noncellular organic layer secreted by external epithelium) containing protein, lipid, chitin, and often calcium carbonate secreted by underlying epidermis and shed (molted) at intervals.

4) Complex muscular system with exoskeleton for attachment, striated muscles for rapid actions, smooth muscles for visceral organs; no cilia.

5) Reduced coelom in adult; most of body cavity consisting of hemocoel (sinuses in tissues) filled with blood.

6) Complete digestive system; mouthparts modified form appendages and adapted for different methods of feeding.

7) Open circulatory system with dorsal contractile heart, arteries, and hemocoel.

8) Respiration by body surface, gills, tracheae (air tubes), or book lungs.

9) Paired excretory glands called coxal, antennal, or maxillary glands present in some, homologous to metameric nephridial system of annelids; some with other excretory organs (malpighian tubules).

10) Nervous system w/ dorsal brain connected by a ring around gullet to a double nerve chain of ventral ganglia; fusion of ganglia in some species; well-developed sensory organs.

11) Sexes usually separate, with paired reproductive organs and ducts; usually internal fertilization; oviparous of ovoviviparous; often metamorphosis; parthenogenesis in few forms.


Similarities between Arthropoda and Annelida

1) External segmentation

2) Segmental arrangement of muscles

3) Ventral nerve cord w/ metamerically arranged ganglia and dorsal cerebral ganglia

4) Spiral cleavage (found in some arthropods)


Differences between Arthropoda and Annelida

1) Fixed number of segments (in adults)

2) Usually lack intersegmental septa

3) Pronounced tagmatization (compared w/ limited tagmatization in annelids)

4) Coelomic cavity reduced; main body cavity a hemocoel

5) Open circulatory system

6) Special mechanisms (gills, tracheae, book lungs) for respiration

7) Exoskeleton containing chitin

8) Jointed appendages

9) Compound eyes (also present in a few annelids) and other well-developed sense organs

10) Absence of cilia

11) Metamorphosis in many cases


Arthropod success:

1) versatile exoskeleton

a) cuticle : outer covering secreted by underlying epidermis.

1) endocuticle : inner, thicker cuticle. contains chitin bound w/ protein. this makes it flexible and lightweight, while still protective.

2) epicuticle : outer, thinner cuticle. composed protein and lipid. Protein is stabilized and hardened by tanning, adding further protection.

Both are composed of several layers.

May be soft and permeable or may form a veritable coat of armor. Between body segments and between segments of appendages it is thin and flexible, creating movable joints and permitting free movements. It may also line foregut and hindgut, line and support trachea, and be adapted for biting mouthparts, sensory organs, copulatory organs, and ornamental purposes.

It does, however, impose important restrictions on growth. To grow, an arthropod must shed its outer covering at intervals and grow a larger one - called ecdysis or molting. Weight limits ultimate body size.

2) Segmentation and appendages more efficient locomotion.

a) Somites provided w/ pair appendages

1) Arrangement modified: segments and appendages specialized for adaptive functions. EX: limb segments essentially hollow levers moved by internal muscles. Jointed appendages equip w/ sensory hairs and also been adapted for sensory functions, food handling, swift and efficient walking legs, and swimming appendages

3) Air goes directly to cells.

a) Highly efficient tracheal system of air tubes. Deliver oxygen directly to tissues and cells and makes high metabolic rate possible. Also helps limit body size

b) Aquatic breathe through some form gill also quite efficient

4) Highly developed sense organs

a) Have great variety. Range from compound (mosaic) eye to simpler senses of touch, smell, hearing, balancing, chemical reception, and so on.

5) Complex behavior patterns

a) Exceed most other inverts in complexity and organization of their activities. Innate (unlearned) behavior unquestionably controls much of what they do, but learning also plays an important part in most of them

6) Reduced competition through metamorphosis

a) Many arthropods pass through metamorphic changes, including larval form quite different from adult in structure. Larval form often adapted for eating different kind of food from that of adult. Therefore less competition w/in species.