Chordates and Vertebrates

Introduction to Chordates

Chordates are a diverse phylum of animals that share several key anatomical features. This group includes both invertebrate and vertebrate species, and their evolutionary history is central to understanding the development of complex animal body plans.

• Phylum Chordata includes animals with a notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail at some stage of development.

• Chordates first appeared approximately 521 million years ago during the early Cambrian period.

• Major groups include Cephalochordata (lancelets), Tunicata (sea squirts), and Vertebrata (vertebrates).

Major Outcomes of Study

• Understand hypotheses regarding the origins of vertebrates from chordate ancestors.

• Describe the evolution of the vertebrate body plan, including the development of a head and mineralized skeleton.

• Explore the major groups of vertebrates, both living and extinct, and their evolutionary history.

Chordate Characteristics

Defining Features of Chordates

All chordates possess four primary characteristics at some stage in their life cycle. These features are essential for classification and understanding their evolutionary relationships.

• Notochord: A flexible, rod-shaped structure that provides support. In vertebrates, it is replaced by the vertebral column during development.

• Dorsal Hollow Nerve Cord: A tube of nervous tissue located above the notochord, which develops into the central nervous system (brain and spinal cord) in vertebrates.

• Pharyngeal Slits or Clefts: Openings in the pharynx that function in filter-feeding, respiration, or develop into other structures (e.g., jaws, ears in humans).

• Post-anal Tail: An extension of the body past the anus, used for locomotion and balance in many species.

Example: In humans, the notochord becomes the cartilage between vertebrae, pharyngeal slits develop into jaw and ear structures, and the post-anal tail is present during embryonic development as the coccyx (tail bone).

Classification of Chordates

Subphyla of Chordata

The phylum Chordata is divided into three main subphyla, each with distinct characteristics and evolutionary significance.

• Cephalochordata: Includes lancelets (e.g., Branchiostoma), which retain all chordate features throughout life and lack a vertebral column.

• Tunicata (Urochordata): Includes sea squirts and salps. Larvae possess all chordate traits, but adults are often sessile and lose most features except the pharyngeal slits.

• Vertebrata: Includes fish, amphibians, reptiles, birds, and mammals. Vertebrates have a backbone and a more complex body plan.

Comparison of Chordate Subphyla

Evolutionary Relationships

Chordates and Echinoderms

Chordates are closely related to echinoderms (e.g., sea urchins) through a common ancestor. Both groups are deuterostomes, meaning the first opening in embryonic development becomes the anus.

• Deuterostome Development: Characterized by the blastopore forming the anus first, not the mouth.

• Shared features include bilateral symmetry, a head with sensory organs, and a one-way digestive system.

Cephalochordates (Lancelets)

Structure and Function

Lancelets are small, fish-like marine animals that retain all chordate characteristics throughout life. They lack a vertebral column and have a simple nervous system.

• Body Plan: Notochord extends the length of the body; dorsal nerve cord; numerous gill slits for respiration and feeding.

• Locomotion: Use segmented muscles for swimming; burrow into sand with head exposed.

• Feeding: Filter-feeders; water enters mouth, passes through pharyngeal slits, food particles are trapped and sent to the stomach.

Tunicata (Sea Squirts)

Life Cycle and Adaptations

Sea squirts are marine invertebrate chordates that exhibit all chordate traits during their larval stage. Adults are often sessile and lose most chordate features except for pharyngeal slits.

• Larval Stage: Free-swimming, possess notochord, dorsal nerve cord, pharyngeal slits, and post-anal tail.

• Adult Stage: Sessile, often colonial, covered by a protective tunic; retain only pharyngeal slits for filter-feeding.

• Feeding: Water enters through incurrent siphon, passes through pharyngeal slits, food is filtered and sent to the stomach, waste exits via excurrent siphon.

Vertebrates

Major Groups of Vertebrates

Vertebrates are distinguished by the presence of a backbone and a complex nervous system. They represent the most diverse and advanced group within the phylum Chordata.

• Fish: Aquatic vertebrates with gills and fins.

• Amphibians: Animals that live both in water and on land; undergo metamorphosis.

• Reptiles: Vertebrates with scaly skin; lay shelled eggs.

• Birds: Warm-blooded vertebrates with feathers and wings; capable of flight.

• Mammals: Warm-blooded vertebrates with hair or fur; produce milk for their young.

Summary Table: Major Vertebrate Groups

Key Terms and Concepts

• Notochord: Flexible rod providing support in chordate embryos.

• Dorsal Hollow Nerve Cord: Precursor to the central nervous system.

• Pharyngeal Slits: Openings in the pharynx for filter-feeding or respiration.

• Post-anal Tail: Tail extending beyond the anus, used for movement.

• Deuterostome: Animal whose anus forms before the mouth during development.

Reflection

Not all chordates are vertebrates. Two chordate subphyla, Cephalochordata and Tunicata, are invertebrates. All vertebrates, however, are chordates, as they possess the defining chordate characteristics during development.

Additional info: The notes have been expanded to provide academic context and definitions for key terms, as well as logical grouping and clarification of fragmented points.