Invertebrate Chordates: Features, Diversity, and Phylogeny

Invertebrate Chordates: Defining Characteristics

Invertebrate chordates possess several key features, including:

• A notochord
• A dorsal hollow nerve cord
• Gill slits (present at some point in development)
• Muscle blocks (present at some point in development)

They also exhibit fundamental characteristics shared by many complex animals:

• Bilateral symmetry
• A well-developed coelom
• Segmentation

The Notochord

The notochord is a long, semi-rigid, rod-like structure located between the digestive system and the dorsal hollow nerve cord.

• Invertebrate chordates retain this feature throughout life.
• In vertebrates, it is replaced by a backbone during development.
• It develops just after gastrulation from mesoderm tissue on what becomes the dorsal side of the embryo.
• The support provided by the notochord allows for powerful side-to-side movement.

Dorsal Hollow Nerve Cord

This structure develops from ectoderm that rolls into a hollow tube.

• In most chordates, cells in the posterior section become the spinal cord, while cells in the anterior section become the brain.
• Nerves connect the nerve cord to the muscle blocks.

Gill Slits (Pharyngeal Slits)

Gill slits are paired openings located on the pharynx behind the mouth.

• They are primarily used to strain food from water.
• Vertebrates either lose these features or they are significantly reduced during development.

Muscle Blocks

Muscle blocks are modified body segments consisting of stacked muscle layers (resembling cooked fish).

• They are anchored to the notochord, providing a firm structure to pull against.
• They aid in the movement of the tail (all chordates have a tail at some point in development).
• The tail extends beyond the anus in most chordates.

Diversity of Invertebrate Chordates

Two subphyla of the phylum Chordata are classified as invertebrates:

1. Subphylum Urochordata – the tunicates (also known as sea squirts)
2. Subphylum Cephalochordata – the lancelets

Subphylum Urochordata: Tunicates (Sea Squirts)

Tunicates, or sea squirts, often look and feed like sponges (*Porifera*).

• Larval Stage: Larvae resemble tadpoles, are free-swimming only for a short time, and do not feed.
• Adult Stage: Tunicates secrete a protective tunic around their bodies. Only the gill slits in adults suggest their chordate relationship.
• Defense Mechanism: When disturbed, they contract muscles and squirt water out, earning them the name "sea squirt." (Feeding and body structure details are typically covered in a coloring plate or diagram.)

Subphylum Cephalochordata: Lancelets

Lancelets are small, streamlined, common marine animals, typically about 5 cm long as adults.

• They superficially resemble fish but possess only one layer of skin, lacking pigment or scales.
• They are filter feeders that retain all chordate features throughout their life.
• They remain buried in mud or sand with their anterior end exposed.
• They lack a distinct head but have light-sensing cells on the anterior end.
• A hood with tentacles covers the mouth, directing water current and food particles.

Phylogeny of Chordates

Invertebrate chordates probably share a sessile animal ancestor with echinoderms and vertebrates.

Fossils of lancelet-like organisms dating back 550 million years suggest that these invertebrates predate the evolution of vertebrates.