Subphylum Urochordata — Core Principles

Core Principles

Urochordates, or Tunicates, are marine chordates characterized by a unique life cycle. Their free-swimming larval stage, resembling a tadpole, possesses all four chordate hallmarks: a notochord (only in the tail), a dorsal hollow nerve cord, pharyngeal gill slits, and a post-anal tail.

This larva undergoes retrogressive metamorphosis, transforming into a sessile or pelagic adult. The adult form is typically sac-like, covered by a cellulose-like tunic, and lacks a notochord and most of the nerve cord.

Adults are filter feeders, drawing water through an incurrent siphon, trapping food with a mucus net secreted by the endostyle in their large pharynx, and expelling water via an excurrent siphon. Key classes include Ascidiacea (sea squirts like *Herdmania*), Thaliacea (salps), and Larvacea (appendicularians like *Oikopleura*).

Their transient chordate features and retrogressive development are crucial for understanding chordate evolution.

Important Differences

vs Subphylum Cephalochordata

Aspect	This Topic	Subphylum Cephalochordata
Notochord	Present only in the tail of the larva; lost in adult.	Extends from head to tail, persists throughout life.
Dorsal Hollow Nerve Cord	Well-developed in larva, degenerates in adult.	Persists throughout life.
Post-anal Tail	Present in larva, lost in adult.	Present throughout life.
Body Covering	Adults covered by a tunic (cellulose-like).	No tunic; covered by a simple epidermis.
Metamorphosis	Undergoes retrogressive metamorphosis.	Undergoes simple metamorphosis or direct development.
Adult Mobility	Mostly sessile or pelagic (free-floating).	Free-swimming, burrowing.
Brain Development	Larval brain degenerates; adult nervous system is simple.	Anterior end of nerve cord slightly enlarged, but no true brain.
Examples	*Herdmania*, *Salpa*, *Oikopleura*	*Branchiostoma* (Amphioxus/Lancelet)

Urochordates and Cephalochordates are both primitive chordates, but they differ significantly in the persistence and extent of their chordate features. Urochordates exhibit these features primarily in their larval tail and undergo retrogressive metamorphosis, leading to a sessile, tunic-covered adult. In contrast, Cephalochordates maintain a notochord extending from head to tail throughout their lives, lack a tunic, and are free-swimming or burrowing. These differences highlight distinct evolutionary paths within the early Chordata, with Cephalochordates often considered closer to the vertebrate lineage due to their more persistent chordate characteristics.