All Of The Following Are Characteristics Of Arthropods Except

All of the Following are Characteristics of Arthropods Except…

Arthropods are incredibly diverse, making up over 80% of all known animal species. From the tiny mites in your carpet to the colossal Japanese spider crab, this phylum boasts an astonishing array of creatures. Understanding their defining characteristics is crucial to appreciating their ecological significance and evolutionary success. This article will delve into the key features that unite arthropods, highlighting what doesn't fit within this incredibly successful group. We will explore the common misconceptions and delve into the exceptions that prove the rule, emphasizing why certain characteristics, while seemingly arthropod-like, actually point to other lineages.

Defining Characteristics of Arthropods

Before we explore the exceptions, let's establish the fundamental traits that generally define arthropods:

1. Segmented Body Plan:

Arthropods possess a segmented body, meaning their bodies are divided into repeating units. These segments can be fused together to form larger body regions like the head, thorax, and abdomen (as seen in insects), or remain more distinct (as in centipedes). This segmentation is reflected in their nervous, circulatory, and muscular systems. The repetition of segments provides structural support and allows for specialization of different body regions.

2. Exoskeleton:

Arguably the most defining feature of an arthropod is its exoskeleton, a hard, external covering made primarily of chitin, a tough polysaccharide. This exoskeleton provides protection from predators, prevents desiccation (water loss), and provides attachment points for muscles. However, this rigid structure necessitates molting, a process where the arthropod sheds its exoskeleton to allow for growth.

3. Jointed Appendages:

The "arthro-" in "arthropod" literally means "jointed." Arthropods possess jointed appendages, which are limbs, antennae, or other structures that are segmented and articulated. These appendages are highly adaptable and perform a wide range of functions including locomotion, feeding, sensing, and reproduction. The jointed nature allows for a greater range of motion compared to non-jointed appendages.

4. Ventral Nerve Cord:

Arthropods possess a ventral nerve cord, meaning their main nerve cord runs along the underside (ventral) of their body. This is in contrast to vertebrates, which have a dorsal nerve cord. This ventral nerve cord is typically segmented, reflecting the segmented body plan. Ganglia, or clusters of nerve cells, are present in each segment, allowing for localized control and coordination of movement and other functions.

5. Open Circulatory System:

Arthropods typically have an open circulatory system, meaning their blood (hemolymph) is not confined to blood vessels but rather flows through a body cavity called the hemocoel. The heart pumps hemolymph into the hemocoel, where it bathes the organs and tissues directly. This system is less efficient at delivering oxygen compared to a closed circulatory system, but it is less demanding in terms of structural complexity.

What is NOT a Characteristic of Arthropods?

Now, let's address the question directly: Many features might seem arthropod-like, but are actually absent in true arthropods or are characteristic of other animal groups. Let's examine some examples:

1. Internal Skeleton:

Internal skeletons are characteristic of vertebrates, not arthropods. Arthropods rely on their external exoskeleton for structural support and protection. While some arthropods may have internal skeletal elements for support within their appendages, they lack the complex internal bony skeleton found in vertebrates.

2. Bilateral Symmetry with a Dorsal Nerve Cord:

While arthropods exhibit bilateral symmetry (meaning their body can be divided into two mirror-image halves), the location of their nerve cord is ventral, not dorsal. A dorsal nerve cord is a defining characteristic of chordates, which includes vertebrates. This fundamental difference underscores the significant evolutionary divergence between arthropods and chordates.

3. Closed Circulatory System:

As mentioned earlier, arthropods typically have an open circulatory system. A closed circulatory system, where blood is always contained within vessels, is a characteristic of more complex animals like vertebrates and some annelids. The open system of arthropods reflects their overall simpler body plan and physiological requirements.

4. Three Germ Layers: (with a caveat)

While arthropods do develop from three embryonic germ layers (ectoderm, mesoderm, and endoderm), this is a characteristic of most animals, not just arthropods. The specific fates of these germ layers and the resulting tissues and organs are what differentiate arthropod development from that of other animals. This statement wouldn't be something you'd classify as an exception in and of itself. Instead, the specific arrangement and derivatives of the germ layers are unique to arthropods.

5. Lack of Molting:

Molting, or ecdysis, is an essential process for arthropods to grow because their exoskeleton doesn't expand. Animals that lack an exoskeleton do not molt. The presence of molting strongly suggests an arthropod lineage, whereas its absence indicates a different animal group.

6. Vertebrae:

The presence of vertebrae, the individual bones that make up the backbone, is a defining feature of vertebrates. Arthropods lack vertebrae completely.

7. Hair or Fur:

Hair or fur is a characteristic of mammals, not arthropods. While some arthropods might have bristle-like structures, these are fundamentally different from mammalian hair in their structure and function.

8. Feathers:

Feathers are a unique characteristic of birds, another vertebrate group completely separate from arthropods. While some arthropod wings may appear feathery at a glance, their structure and development are significantly different from bird feathers.

9. Scales (in the reptilian sense):

Reptilian scales are made of keratin, providing protection and preventing water loss. While some arthropods have hardened scales in their exoskeleton (though generally composed of chitin), the underlying structure and developmental origin are different from reptilian scales.

Misconceptions and Related Phyla:

It's important to address some common misconceptions related to arthropod characteristics and other closely related phyla:

Onychophora (Velvet Worms):

These intriguing creatures share some superficial similarities with arthropods, such as segmented bodies and appendages. However, they lack a true exoskeleton and possess a unique hydrostatic skeleton. Their appendages are also less articulated than those of true arthropods. Their phylogenetic position remains debated, but they are generally considered a sister group to arthropods.

Tardigrada (Water Bears):

Water bears are microscopic animals that exhibit some arthropod-like features. However, they lack jointed appendages, possessing instead lobopods (un-jointed leg-like structures). Their exoskeleton-like cuticle is also structurally different from that of arthropods. They are considered to be a separate lineage, though their evolutionary relationship to arthropods is still being investigated.

Myriapoda (Centipedes and Millipedes):

While clearly arthropods, these groups illustrate the variability within the phylum. Their bodies are highly segmented, with numerous pairs of legs. They demonstrate that arthropod body plans can be quite diverse, although they still retain the key characteristics mentioned earlier.

Conclusion: The Defining Unity of Arthropods

In conclusion, while a wide array of features might appear superficially similar to those found in arthropods, it's the combination of segmented body plan, exoskeleton, jointed appendages, ventral nerve cord, and open circulatory system that truly defines this remarkably successful phylum. Understanding the exceptions and related phyla clarifies the unique evolutionary trajectory of arthropods and their dominant role in terrestrial and aquatic ecosystems. While there is still much to be discovered about their evolutionary history and diversity, the fundamental characteristics remain the key to identifying and understanding this incredible group of animals.