Question

Sketch a simple model of a generic chordate showing and labeling its defining features.

Short answer

A simple model of a generic chordate would include an elongated body structure with a head, body, and tail region. The defining features to label are the notochord (a flexible rod-like structure for support and movement), dorsal nerve cord (hollow, tubular structure above the notochord), pharyngeal slits (small openings connecting the throat to the external environment), post-anal tail (extension of the body beyond the anus), and either endostyle (ciliated, mucus-secreting groove in the pharynx) or thyroid gland (endocrine gland secreting hormones).

Step-by-step solution

Draw the simple body structure of a chordate

Begin by drawing a simple, elongated body structure with a head, body, and tail region. This body plan is for a generic chordate, so don't focus on specific details relating to specific species or groups.

Label the notochord

The notochord is a flexible rod-like structure that provides support and allows for movement. It is one of the primary features that define chordates. Draw a straight line down the dorsal side of the chordate and label it as the notochord.

Label the dorsal nerve cord

The dorsal nerve cord is a hollow, tubular structure located above the notochord. Draw a line parallel to the notochord and label it as the dorsal nerve cord.

Label the pharyngeal slits

Pharyngeal slits are small openings that connect the pharynx (throat) to the external environment. In aquatic chordates, these slits are responsible for filter-feeding and gas exchange, while they serve different functions in terrestrial chordates. Draw a series of small, parallel lines on both sides of the chordate's head, below the notochord, and label them as pharyngeal slits.

Label the post-anal tail

The post-anal tail is an extension of the body that extends posteriorly beyond the anus. This muscular structure is used for propulsion in aquatic chordates and helps with balance in terrestrial chordates. Draw a tapering extension at the posterior end of the chordate that stretches beyond the body, and label it as the post-anal tail.

Label the endostyle or thyroid gland

Chordates have either an endostyle (a ciliated, mucus-secreting groove in the pharynx) or a thyroid gland (an endocrine gland that secretes hormones). Both structures are involved in iodine metabolism, and the endostyle is thought to have evolved into the thyroid gland in vertebrates. Draw a small, oval-shaped structure in the mid-ventral region of the pharynx and label it as either the endostyle or the thyroid gland, depending on your preference for a more invertebrate or vertebrate-like generic chordate. Once you've completed these steps, you should have a simple model of a generic chordate with all its defining features labeled.

Key concepts

Notochord

The notochord is a fundamental feature in the anatomy of chordates. It's a flexible, rod-like structure that runs along the length of the body, providing essential support. This support allows for efficient movement, which is crucial for survival.
In many chordates, the notochord is present only during the embryonic stages. As they develop, it may be replaced or supplemented by the vertebral column, especially in vertebrates such as mammals, birds, and reptiles. Despite this, the notochord is critical as it dictates the basic body plan and differentiates chordates from other animal phyla.

• Acts as a primary axial support.
• Allows for mobility and flexibility.
• Plays a vital role in embryonic development.

Understanding the notochord helps us appreciate how structure and function are intertwined in the animal kingdom.

Dorsal Nerve Cord

Among the defining characteristics of chordates is the dorsal nerve cord, which distinguishes them significantly from non-chordates. It is a tube-like structure situated atop the notochord.
Unlike the nerve cords found in other animal groups, the dorsal nerve cord is hollow and develops into the central nervous system. In humans and other vertebrates, this structure differentiates into the brain and spinal cord.

• Implements coordination and control of body functions.
• Is crucial for sensory integration and processing.
• Plays a role in the development of complex nervous systems.

The presence of the dorsal nerve cord is essential in forecasting the complex nervous systems observed in more evolved chordates.

Pharyngeal Slits

Pharyngeal slits originally served a variety of functions in early chordates, primarily related to feeding and respiration. They are found in the pharyngeal region and open to the external environment.
In aquatic chordates, such as fish, these slits function in gas exchange and sometimes filter feeding. Terrestrial chordates, however, have adapted them for various uses. In vertebrates like humans, the slits contribute to the formation of structures such as parts of the ear and tonsils during embryonic development.

• Facilitate gas exchange in aquatic habitats.
• Adapt into different structures in terrestrial animals.
• Aid in filter-feeding in certain species.

Understanding pharyngeal slits helps uncover the evolutionary innovations that allow chordates to thrive in diverse environments.

Post-Anal Tail

The post-anal tail is a unique feature that extends beyond the posterior end of the digestive tract. This characteristic is seen in all chordates at some stage of their life cycle.
In many aquatic species, the tail is used for locomotion, helping creatures move swiftly through the water. For terrestrial animals, it often aids in balance, communication, and other functions. Although some chordates, such as humans, may lose a prominent tail post-embryonically, the initial presence is a testament to our shared evolutionary ancestry.

• Provides propulsion in aquatic environments.
• Facilitates balance in land-based species.
• Demonstrates evolutionary significance.

The post-anal tail underscores the versatility and adaptability observed in chordate lineages.

Endostyle

The endostyle is a lesser-known, yet vital, chordate feature. Found in the pharynx, it's primarily involved in feeding and iodine metabolism in some invertebrate chordates.
In species such as tunicates and amphioxus, the endostyle secretes mucus to trap food particles. Through evolutionary processes, it is believed that the endostyle has given rise to the thyroid gland found in vertebrates, which plays a critical role in regulating growth and metabolism through hormone secretion.

• Assists in particle feeding in marine environments.
• Crucial for iodine metabolism.
• Evolutionary precursor to the thyroid gland in vertebrates.

Understanding the endostyle offers insights into the evolutionary transitions leading to more advanced endocrine systems in complex organisms.