The Phylum Chordata and Its Groups Help
Introduction to The Phylum Chordata and Its Groups
The Phylum Chordata can be subdivided into a number of different groups. The major classifying factor is the answer to the following question: “Does the particular chordate species being considered keep a notochord in its back throughout its lifetime, or does it replace the notochord with a full vertebral column after it has passed through the embryo stage?”
The Sea Squirts And Lancelets: No “jointed Backbone”
Figure 12.2 reveals that two of the major groups of chordates keep a notochord in their back throughout their lives. These two groups are called the Urochordates ( YOUR -oh- kor -dates) and the Cephalochordates ( SEF -uh-low- kor -dates).
The Urochordates are animals that literally have a “cord” ( chord ) within their “tail” ( uro -). By this it is meant that these creatures have a swimming larval stage with a notochord in its tail section. The main examples of Urochordates are the sea squirts . Interestingly enough, the adult stage of the sea squirt has a body shaped like a “U,” and it remains anchored to a rock at the bottom of the sea. At one end lies an intake siphon , which sucks in seawater and filters out plankton and other tiny creatures during feeding. And at the other end lies an output siphon , which squirts or shoots out a jet of water whenever the animal is bothered by a predator.
The Cephalochordates are literally a group of “cord-heads” ( cephalo -)! The implication of this odd name is displayed in Figure 12.2 (B) for the lancelets ( LANS -lits). The lancelets are slender, fish-like marine animals that lie partially buried in the sand under shallow water. Their thin, tapered body is pointed at both ends, making it look much like a small spear or “little lance.” Because it has no skull, the lancelet doesn’t have a brain, either! Thus, the notochord extends all the way up into its head, officially making it a “cord-head” (Cephalochordate). The lancelet is a type of suspension feeder , meaning that it feeds on small particles suspended in seawater, which it draws into its mouth with the help of waving, hair-like tentacles ( TEN -tuh-kuls).
Both sea squirts and lancelets have notochords in their backs during at least part of their life cycles, but they are still officially invertebrates. Many biologists think that they are important evolutionary “bridging species” between the other invertebrates (which don’t even have a notochord) and the true vertebrates (whose primitive notochord has been replaced by a bony vertebral column).
The Vertebrates – Chordates With A “jointed Backbone”
In contrast to the Urochordates and Cephalochordates, the Vertebrata ( ver -teh- BRA -tuh) or vertebrates are chordates with both a jointed (segmented) backbone and a brain case or cranium ( KRAY -nee-um) (see Figure 12.3). You may recall (Chapter 11) that the “jointed backbones” are technically called the vertebrae. The cranium is just a formal name for the “skull” ( crani ) “present” (- um ) at the top of the vertebral column. Owing to their high degree of cephalization (concentration of sensory, motor, and other nerve functions within the head region), vertebrates have a brain in their cranium. The vertebrates generally have a notochord only as part of their embryonic stage of development. As their bodies mature, the notochord is replaced by a vertebral column (linked series of jointed vertebrae).
The dorsal nerve cord, now protected by a vertebral column, is called the spinal cord. The skull (brain case or cranium), vertebral column, and ribs make up what is called the axial ( AX -ee-ul) skeleton. The reason for this name is the pattern set by the bones or cartilage ( KAR -tih- lj ), commonly known as “gristle,” which house the brain and spinal cord. This ordered bone/cartilage pattern forms a longitudinal ( long -jih- TWO -duh-nal) axis, or “lengthwise axle,” around which the vertebrate body can turn or pivot.
Finally, vertebrates have a closed circulatory system. The heart serves as a pump, which sends the blood coursing out through the entire body. After the body tissues are supplied with oxygen, glucose, and other nutrients, the blood (now filled with tissue waste products) returns back to the heart through a closed loop of vessels.
Practice problems for these concepts can be found at: Chordata Test
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