Common Characteristics of the Arthropods Help
Introduction to Arthropods
Arthropods, like other invertebrates, contain no jointed backbones. But they are set distinctly apart, however, by their “jointed” (arthro-) “feet” (pod)!
As Figure 11.1 shows, the general body plan shared by all arthropods can be represented by the anatomy of a lobster. The arthropods are a huge phylum of invertebrates with bilateral symmetry whose bodies are clearly divided into jointed segments, and whose legs are hollow and also divided into jointed segments. Possessing bilateral symmetry, the arthropods have pairs of matching right–left antennae (an- TEN -ee) or feelers, legs, and in many insects, wings.
The Segmented Body
The lobster is covered by a hard exoskeleton ( eks -oh- SKEL -eh-ten) – an “outer” (exo-) “hard dried body” (skeleton). Since the exoskeleton is on the surface of the animal, it is also called the cuticle ( KYOO -tih-kl) or “little skin.” The exoskeleton (cuticle) can even be thought of as a “coat of mail,” a type of armor! Why is this? The reason is that the lobster exoskeleton contains large amounts of chitin ( KY -tin). Chitin also covers the surfaces of crabs, beetles, and crickets. Chitin is a tough, horny substance providing protection to the invertebrate body, much as if the lobster were an ocean-dwelling knight wearing a “coat of mail” ( chitin )!
The chitin-armored lobster body has two pairs of antennae (short and long), two feeding mouthparts, as well as two large pincers, attached to either side of the head. A thorax ( THOH -racks) or “chest” piece forms a second segment behind the head. The head and thorax are often lumped together by biologists and named as a single large segment called the cephalothorax ( sef -uh-low- THOR -aks) or “head” ( cephal )-and-“chest” piece. Four hollow, segmented legs are attached to either side of the cephalothorax and allow the lobster to easily walk over the ocean floor.
Behind the thorax is the third body segment – a much-longer abdomen ( AB -duh-mun). The abdomen is literally the long “belly” of the beast. But for we lobster-eating humans, it is usually just considered the tail! The exoskeleton protecting the abdomen is subdivided into a number of hinged plates, allowing the lobster to use its swimming appendages (uh- PEN -dih-jes) or “hangers-on” to scoot its body rapidly along.
An Open-ended Circulatory System
Arthropods do have hearts, but these hearts pump a fluid called hemolymph ( HE -moh- limpf ), rather than blood. The circulatory system is an open one. This means that the heart pumps the hemolymph into vessels and sinuses which are not closed off. Nevertheless, the hemolymph still provides the body tissues with an adequate supply of oxygen and other nutrients.
A Dependence Upon Molting
Because many arthropods have a rigid exoskeleton, yet they continue to grow in size throughout most of adulthood, they are forced to periodically molt ( MOHLT ) or shed. After the exoskeleton is molted (shed), the lobster or other arthropod is temporarily soft-bodied and vulnerable to hungry predators. But the head of the lobster bears two eyes, each perched upon a movable stalk, that lets it readily see approaching predators (and hopefully escape being eaten!).
Practice problems for these concepts can be found at: Arthropod Test