What are the Two Subphyla of the Phylum Chordata?
The phylum Chordata, characterized by the presence of a notochord, is divided into two primary subphyla: Tunicata (Urochordata) and Cephalochordata. These subphyla represent the invertebrate chordates, showcasing evolutionary links to vertebrates.
Introduction to Chordata
The phylum Chordata is a diverse group of animals that includes all vertebrates (animals with backbones) as well as two groups of invertebrate chordates: the Tunicata and the Cephalochordata. What sets chordates apart is the presence, at some point in their development, of four key characteristics: a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail. These features may be present only during embryonic development in some chordates, such as humans, but they define the phylum. Understanding what are the two subphyla of the phylum Chordata? is crucial to grasping the evolutionary relationships within this important animal group.
Defining Characteristics of Chordates
To understand the significance of the Tunicata and Cephalochordata, it’s essential to review the defining characteristics of all chordates:
- Notochord: A flexible, rod-like structure that provides skeletal support. In vertebrates, the notochord is replaced by the vertebral column during development.
- Dorsal Hollow Nerve Cord: A tube of nerve tissue that runs along the back of the animal. In vertebrates, this develops into the brain and spinal cord.
- Pharyngeal Slits: Openings in the pharynx (the region behind the mouth) that filter water for feeding or, in some aquatic vertebrates, develop into gills.
- Post-Anal Tail: A tail that extends beyond the anus, providing propulsion in aquatic chordates.
Subphylum Tunicata (Urochordata)
The Tunicata, also known as urochordates, are marine animals commonly called tunicates or sea squirts. They are characterized by a tunic, a cellulose-like covering that surrounds their body. While adult tunicates appear quite different from typical chordates, their larval form displays all four chordate characteristics. During metamorphosis into the adult stage, many of these characteristics are lost or modified. These are crucial to understanding what are the two subphyla of the phylum Chordata?
Key features of tunicates:
- Larval stage with notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail.
- Adult stage often sessile and filter-feeding.
- Tunic composed of tunicin, a cellulose-like substance.
- Simple nervous system in adults.
- Examples: Sea squirts, salps, larvaceans
Subphylum Cephalochordata
The Cephalochordata are a small group of marine animals commonly called lancelets. These animals are fish-like in appearance and retain all four chordate characteristics throughout their entire life. They are important for understanding the evolution of chordates because they provide a model for the ancestral chordate body plan. Understanding this body plan can help in defining what are the two subphyla of the phylum Chordata?
Key features of cephalochordates:
- Notochord extends the entire length of the body, even beyond the brain.
- Dorsal hollow nerve cord runs the length of the body.
- Numerous pharyngeal slits used for filter feeding.
- Post-anal tail used for swimming.
- Lack a well-defined head or brain.
- Example: Branchiostoma (Amphioxus)
Comparison of Tunicata and Cephalochordata
| Feature | Tunicata (Urochordata) | Cephalochordata |
|---|---|---|
| ——————– | ——————————— | ————————— |
| Notochord | Present in larva, absent in adult | Present throughout life |
| Nerve Cord | Present in larva, reduced in adult | Present throughout life |
| Pharyngeal Slits | Present in both larva and adult | Present throughout life |
| Post-Anal Tail | Present in larva, often absent in adult | Present throughout life |
| Body Covering | Tunic | Skin |
| Habitat | Marine | Marine |
| Lifestyle | Sessile or planktonic | Burrowing, free-swimming |
Evolutionary Significance
The study of tunicates and cephalochordates is vital for understanding the evolutionary origins of vertebrates. Cephalochordates, with their retention of all chordate features throughout life, are often considered to be the closest living relatives of vertebrates. Tunicates, despite their modified adult form, provide insights into the early evolution of the nervous system and the chordate body plan. Exploring what are the two subphyla of the phylum Chordata? helps us paint a clearer picture of the evolutionary pathway that led to vertebrates.
What is the notochord and its function?
The notochord is a flexible, rod-like structure composed of cartilage-like cells. Its primary function is to provide skeletal support to the body. In vertebrates, it serves as a signal center during development and is eventually replaced by the vertebral column. Its presence is a defining feature of chordates.
How do tunicates filter feed?
Tunicates filter feed by drawing water into their pharyngeal slits. The water passes through the slits, and food particles are trapped in mucus. The mucus, along with the trapped food, is then transported to the digestive system. This process allows them to extract nutrients from the surrounding water.
What is the tunic made of?
The tunic of tunicates is composed of tunicin, a type of cellulose-like polysaccharide. This is unusual because cellulose is typically found in plants, not animals. The tunic provides protection and support for the tunicate’s body.
Why are cephalochordates important for understanding vertebrate evolution?
Cephalochordates are considered important for understanding vertebrate evolution because they retain all four chordate characteristics throughout their lives. This makes them a good model for the ancestral chordate body plan. Studying their anatomy and genetics provides insights into the origins of vertebrates.
Do tunicates have a heart?
Yes, tunicates do have a heart, but it is unique. The heart beats in one direction for a period of time and then reverses its direction. This is a peculiar adaptation that is not fully understood, but thought to be related to their unique physiology.
Where are cephalochordates typically found?
Cephalochordates are typically found in shallow, sandy marine environments. They often burrow into the sand, leaving only their anterior end exposed to filter feed. They are relatively rare and patchily distributed.
How does the nervous system differ between tunicates and cephalochordates?
In tunicates, the nervous system is relatively simple, especially in the adult stage. The dorsal nerve cord is reduced to a ganglion. In contrast, cephalochordates have a dorsal nerve cord that runs the entire length of their body, and though they lack a distinct brain, they have sensory structures.
What is the significance of pharyngeal slits?
Pharyngeal slits are openings in the pharynx that are used for filter feeding in invertebrate chordates. In aquatic vertebrates, these slits develop into gills for gas exchange. In terrestrial vertebrates, pharyngeal slits are present during embryonic development but are modified into other structures, such as parts of the ear and jaw.
What is the post-anal tail used for?
The post-anal tail is a tail that extends beyond the anus. It is primarily used for propulsion in aquatic chordates. In terrestrial vertebrates, the tail may be used for balance, communication, or other purposes.
Are tunicates always sessile?
Not all tunicates are sessile (attached to a substrate). Some tunicates, such as salps and larvaceans, are planktonic and drift in the water column. Sessile tunicates are commonly known as sea squirts.
What are some common examples of tunicates and cephalochordates?
A common example of a tunicate is the sea squirt. A common example of a cephalochordate is Branchiostoma, also known as the lancelet or Amphioxus.
Why is understanding these subphyla important?
Understanding what are the two subphyla of the phylum Chordata? gives us valuable insights into the evolutionary history of all chordates, including vertebrates. By studying Tunicata and Cephalochordata, we can better understand the origins of key chordate features and the evolutionary transitions that led to the development of more complex organisms, including ourselves. They bridge the gap and show the transition from invertebrates to vertebrates.