Subphylum Cephalochordata — Definition

Definition

Imagine a small, translucent, fish-like creature, only a few centimeters long, that spends most of its life partially buried in the sand of shallow marine waters. This is a lancelet, a member of the Subphylum Cephalochordata. These fascinating animals are incredibly important in biology because they clearly display all the defining characteristics of chordates, the group that includes us humans, throughout their entire lives.

What are these defining characteristics? Firstly, they have a notochord, a flexible, rod-like structure that runs along their back, providing support. In lancelets, this notochord is particularly special because it extends all the way from the tip of their head (hence 'cephalo' meaning head) to their tail, and it remains present throughout their life. This is unlike humans, where the notochord is largely replaced by the vertebral column during development.

Secondly, they possess a dorsal hollow nerve cord, which is a tube of nervous tissue located above the notochord. This is the precursor to the brain and spinal cord in vertebrates.

Thirdly, they have pharyngeal gill slits, which are a series of openings in the pharynx (throat region). In lancelets, these slits are numerous and are used for filter feeding, trapping tiny food particles from the water as it passes through. They are also involved in respiration.

Finally, they have a post-anal tail, a tail that extends beyond the anus. This tail is muscular and helps in locomotion, though lancelets are not strong swimmers.

Cephalochordates are filter feeders. They draw water into their mouth using cilia, and food particles are filtered out by the pharyngeal slits and then moved into the digestive tract. The filtered water exits through an opening called the atriopore. They have a simple, closed circulatory system, but lack a heart, with blood propelled by contractile vessels. Their excretory system consists of protonephridia with solenocytes, which are primitive kidney-like structures.

These animals are considered living fossils, providing valuable insights into the evolutionary link between invertebrates and vertebrates. Their simple yet complete chordate organization makes them a key study organism for understanding the fundamental body plan of our own lineage.