Learning Outcomes
i. between invertebrate chordates and vertebrates based on their distinctive characteristics.
ii. Identify the key features of the two subphyla of invertebrate chordates: Urochordata and Cephalochordata.
iii. Recognize the defining characteristics of vertebrates, including the presence of a backbone, internal skeleton, and specialized organs.
iv. Appreciate the evolutionary relationships between invertebrate chordates and vertebrates, understanding their place in the animal kingdom.
i. Invertebrate Chordates: The Ancestral Chordates
The phylum Chordata encompasses a diverse group of animals, including fish, amphibians, reptiles, birds, mammals, and two subphyla of invertebrates: Urochordata and Cephalochordata. These invertebrate chordates, often referred to as tunicates and lancelets, respectively, are considered the closest living relatives of vertebrates.
ii. Urochordata (Tunicates)
Urochordates, commonly known as sea squirts, are marine filter feeders with sac-like bodies. Their body is enclosed in a tunic, a tough, cellulose-like covering. The most distinctive feature of urochordates is their tadpole-like larval stage, which possesses a notochord, a flexible rod-like structure that serves as a support system. However, as urochordates mature, they lose their notochord and become sessile, attaching themselves to rocks or other surfaces.
iii. Cephalochordata (Lancelets)
Cephalochordata, represented by the lancelet, are small, fish-like organisms with a transparent body. They are characterized by a well-developed notochord that runs along the length of their body, providing support and flexibility. Unlike urochordates, lancelets retain their notochord throughout their life cycle. They also possess gill slits, openings on the sides of their pharynx that allow for gas exchange with water.
iv. Vertebrates: The Backbone-Endowed Animals
Vertebrates, the most familiar and diverse group within the phylum Chordata, are distinguished by the presence of a backbone, an internal skeleton composed of bones or cartilage that provides support and protection. This rigid internal skeleton allows for more complex movement and locomotion compared to invertebrate chordates. Vertebrates also possess specialized organs, including a heart for circulating blood, a brain for coordinating movement and behavior, and sense organs for gathering information about the environment.
v. Key Features of Vertebrates
Backbone: A vertebral column, consisting of individual vertebrae, provides support and flexibility.
Internal Skeleton: Bones or cartilage form an internal framework that supports the body and protects internal organs.
Specialized Organs: Dedicated organs, such as a heart, brain, and sense organs, perform specific functions.
Closed Circulatory System: Blood circulates within a network of vessels, delivering oxygen and nutrients to cells and removing waste products.
Bilateral Symmetry: The body is divided into two similar halves along a longitudinal axis.
Paired Limbs: Most vertebrates have paired appendages, such as fins, legs, or wings, for movement.
vi. Evolutionary Relationships
Invertebrate chordates, particularly cephalochordates, are considered the closest living relatives of vertebrates. Studies of fossils suggest that vertebrates evolved from fish-like ancestors that possessed a notochord, gill slits, and a dorsal nerve cord. Over time, these ancestral vertebrates developed a backbone, internal skeleton, and specialized organs, giving rise to the diverse vertebrate groups we see today.
The phylum Chordata encompasses a remarkable diversity of organisms, from the simple invertebrate chordates to the complex vertebrates. Invertebrate chordates, particularly lancelets, provide insights into the evolutionary origins of vertebrates, while vertebrates represent the pinnacle of evolutionary adaptation and complexity in the animal kingdom.
