Defining Arthropod Characteristics and Body Systems

Arthropod Characteristics: Defining Features

Arthropods are defined by the following characteristics:

• Segmented body structure
• Coelomate (possessing a true coelom)
• Invertebrate
• Bilateral symmetry
• Exoskeleton with jointed appendages

Arthropods were the earliest known invertebrates to have jointed appendages, which allows for flexibility despite having a hard exoskeleton.

The Arthropod Exoskeleton

The exoskeleton is a hard, outer covering made of chitin and protein.

Functions of the Exoskeleton

• Protects and supports internal structures.
• Allows for the attachment of muscles.

Molting

Exoskeletons cannot grow, so they must be replaced throughout life, a process known as molting.

Body Segmentation

Most arthropods exhibit three primary segments: the head, thorax, and abdomen.

• Three segments allow for greater flexibility.
• Some species have two segments (offering more protection due to one less vulnerable joint).
• Some primitive arthropods have a single body segment.

Efficient Respiratory Systems

Arthropods are generally quick and active, necessitating efficient gas exchange systems. Three main types have evolved:

Gills (Aquatic Respiration)

• Aquatic arthropods (like lobsters) exchange O₂ and CO₂ through gills in water.
• Gills function similarly to vertebrate lungs: gases are exchanged as water flows over the surface of the gill tissue. O₂ is absorbed into the body tissue, and CO₂ diffuses into the water.

Tracheal Tubes (Insects)

• Insects utilize a branching network of air passages called tracheal tubes.
• Air movement is facilitated by the contraction and relaxation of muscles surrounding the tubes.
• Openings connecting the tracheal tubes to the outside environment are called spiracles.
• O₂ and CO₂ are exchanged directly with the environment by diffusion.

Book Lungs (Arachnids)

• Spiders and scorpions possess book lungs.
• These are air-filled chambers containing leaf-like plates where gas diffusion occurs.
• The plates are folded layers of hemolymph (arthropod blood)-filled tissue.

Nervous System and Control

The arthropod nervous system includes:

• A brain.
• A double ventral nerve cord.
• Several ganglia, which act as control centers for the specific body sections in which they are located.

Circulatory System

Arthropods utilize an open circulatory system with one or more hearts.

Blood (hemolymph) directly bathes the body tissues, flowing in and out of the heart through vessels.

Digestion and Mandibles

Arthropods possess a complete digestive system, including the mouth, stomach, intestine, anus, and glands that produce enzymes.

Mouthparts often include jaws called mandibles.

Waste Excretion

Terrestrial arthropods excrete wastes through Malpighian tubules, which are located in the abdomen (unlike annelids, where excretory organs are found in each segment).

Reproduction and Sensory Organs

Reproductive Strategies

• Most species have separate male and female members (dioecious).
• Terrestrial species usually utilize internal fertilization.
• Aquatic species usually utilize external fertilization.
• Some species, such as barnacles, are hermaphrodites.

Arthropod Eyes

Arthropods possess two main types of eyes:

• Simple Eye: Has one lens and primarily detects light intensity.
• Compound Eye: Has many lenses, enabling detailed detection of prey, mates, predators, and colors.