Subphylum Cephalochordata — Core Principles

Core Principles

Subphylum Cephalochordata, exemplified by *Branchiostoma* (lancelets), are small, marine, fish-like invertebrates crucial for understanding chordate evolution. They uniquely retain all four chordate hallmarks—notochord, dorsal hollow nerve cord, pharyngeal gill slits, and post-anal tail—throughout their life.

Their notochord extends anteriorly beyond the nerve cord. They are filter feeders, using numerous pharyngeal slits to trap food particles from water, which enters through the oral hood and exits via the atriopore after passing through an atrium.

They lack a true head, cranium, jaws, paired fins, and a true heart. Their circulatory system is closed but simple, propelled by contractile vessels. Excretion occurs via protonephridia with solenocytes.

The endostyle, a mucus-secreting gland, is homologous to the vertebrate thyroid. Cephalochordates represent a primitive yet complete chordate body plan, serving as a vital link between invertebrates and vertebrates.

Important Differences

vs Subphylum Urochordata

Aspect	This Topic	Subphylum Urochordata
Notochord Persistence	Present throughout life, extends to anterior tip of head.	Present only in larval tail, degenerates in adult.
Adult Body Form	Fish-like, segmented, free-swimming/burrowing.	Sessile, sac-like, often colonial, unsegmented.
Dorsal Hollow Nerve Cord	Present throughout life, forms rudimentary brain.	Present in larva, reduced to a ganglion in adult.
Atrium/Atriopore	Present, water exits via single atriopore.	Present, water exits via atrial siphon.
Excretory Organs	Protonephridia with solenocytes.	Neural gland, pyloric gland, or nephrocytes (no true protonephridia).
Metamerism	Distinct metameric segmentation (e.g., myotomes, gonads).	Absent.

While both Cephalochordata and Urochordata are invertebrate chordates, they exhibit significant differences in their adult body plans and the persistence of chordate features. Cephalochordates maintain all four chordate characteristics throughout their lives, including a notochord extending to the head, and possess a segmented, fish-like body. Urochordates, conversely, are typically sessile, sac-like adults where the notochord and dorsal hollow nerve cord are present only in the free-swimming larval stage, degenerating upon metamorphosis. Their excretory and circulatory systems also show distinct evolutionary paths.