This lesson will provide a comprehensive understanding of Phylum Chordata, covering its unique characteristics, classification, and examples of various species within this phylum. By the end of the lesson, students will have a clear understanding of the major groups of chordates and the significant features that define them.
Phylum Chordata
The phylum Chordata is a group of animals that includes some of the most well-known creatures like humans, birds, fish, and frogs. Chordates are distinguished by a few key features that make them unique compared to other animals. These features appear at some point during their development, whether they are present throughout their lives or only during certain stages of growth.
Let’s break this down in a simple way.
Basic Characteristics of Chordates
Chordates share four main characteristics at some point in their life cycle, whether in the embryo or as adults. These characteristics are:
1. Notochord
The notochord is a long, flexible rod-like structure that provides support for the body. Here’s how it works:
- It is made up of vacuolated cells (cells filled with fluid) and is semi-rigid, meaning it is somewhat stiff but still flexible.
- The notochord runs along the length of the body between the gut (digestive system) and the dorsal nerve cord (the nerve cord running along the back).
- In lower chordates (such as tunicates and lancelets), the notochord is present throughout life.
- In vertebrates (like humans, fish, and mammals), the notochord is mostly replaced by the vertebral column (spine) during development, although remnants of the notochord may remain in parts of the spine.
Mnemonic to Remember:
“Notochord is the body’s backbone starter”
This helps remember that the notochord is the initial body structure that gives support before it is replaced by the spine in vertebrates.
2. Pharyngeal Slits
The pharyngeal slits are openings that appear in the pharynx (the throat area) during the embryonic stage. Here’s how they function:
- In all chordates, paired pharyngeal slits develop during the embryonic stage.
- In some chordates (like Amphioxus and fishes), the slits develop into gills, which are used for breathing underwater.
- In other chordates (such as amphibians, like frogs), these slits serve a temporary purpose and may be functional for a part of their life cycle.
- In reptiles, birds, and mammals, the slits are modified into other structures. For example, in humans, these slits develop into parts of the middle ear and other structures in the head and neck.
Mnemonic to Remember:
“Pharyngeal slits, gills or bits!”
This helps remember that these slits can become gills in some animals (like fish), or they can turn into different structures like the ear in humans.
3. Tubular Nerve Cord
The tubular nerve cord is a long, tube-like structure that runs through the body of all chordates. Here’s its function:
- It runs along the dorsal (back) side of the body, just above the notochord.
- In all chordates, this nerve cord develops into the brain at the front end and continues as the spinal cord along the rest of the body. It serves as the body’s central nervous system, controlling various functions and coordinating movements.
Mnemonic to Remember:
“Tubular cord to control your mind!”
This reminds us that the nerve cord becomes the brain and spinal cord, which are crucial for controlling thoughts, actions, and bodily functions.
4. Post-Anal Tail
The post-anal tail is another important feature of chordates:
- All chordates develop a tail that extends beyond the anus.
- In some chordates (like fish), this tail is present throughout life and is used for swimming and movement.
- In others (like humans), the tail is lost or reduced during embryonic development, though remnants can be found in the form of the coccyx (tailbone) at the base of the spine.
Mnemonic to Remember:
“Tails after the anus, help you move around!”
This helps us recall that the tail is a characteristic that helps in movement, especially in animals like fish and other aquatic creatures.
Major Groups of Chordates
Chordates are divided into two major groups: invertebrate chordates and vertebrates.
Invertebrate Chordates
Invertebrates are animals without a backbone. There are two main subgroups of invertebrate chordates:
- Subphylum Urochordata:
- These are also called tunicates or sea squirts.
- The notochord and nerve cord are only present in their free-swimming larvae. As adults, they often lose these structures and become sessile (stationary).
Example: Sea squirts – These animals start life with the typical chordate characteristics but later lose the notochord as they become immobile adults.
- Subphylum Cephalochordata:
- These animals, like Amphioxus (also called lancelets), retain the notochord throughout their life.
- Amphioxus resembles a small fish and is important for studying the evolution of vertebrates.
Vertebrates
Vertebrates are chordates that have a vertebral column (spine) and a cranium (skull). They are more complex and are divided into seven classes, which are grouped into two main categories:
- Agnatha (Jawless Fish):
- These are fish that lack jaws, like lampreys and hagfish.
- Gnathostomata (Jawed Vertebrates):
- These vertebrates have jaws and include fish, amphibians, reptiles, birds, and mammals.
Mnemonic to Remember Major Groups:
“U Can Catch Fish Virtually!”
- U = Urochordata (Tunicates).
- C = Cephalochordata (Amphioxus).
- F = Fish (for Agnatha and Gnathostomata, i.e., Jawless and Jawed vertebrates).
- V = Vertebrates.
This mnemonic helps recall the major groups of chordates and their characteristics, starting from simple organisms to more complex vertebrates.
Summary
Chordates are a diverse and essential group of animals that share some key features, especially during their development. The notochord, pharyngeal slits, tubular nerve cord, and post-anal tail are all important identifiers of chordates. These features are used in different ways across various species, but they all provide the basic structure and function that allow chordates to live and thrive in their environments.
By breaking down these features and using mnemonics, we can remember what makes chordates special, from simple sea squirts to the complex mammals we are familiar with.