urochordata

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Subphylum Urochordata

Subphylum Urochordata (also called Tunicata) includes tunicates, salps, and larvaceans. This subphylum, along with subphylum Cephalochordata, is informally classified under protochordates. Protochordates are “primitive” chordates that are not vertebrates. Most adult tunicates are sessile and do not show chordate characteristics. However tunicates have a free-swimming larval stage that exhibits all chordate characteristics. Tunicates get their name from the outer covering, called a tunic, which cloaks their body.

The first two photos show pictures of tunicate larva. The next three photos show pictures of adult tunicates.

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Subphylum Cephalochordata

Subphylum Cephalochordata includes lancelets. This subphylum, along with subphylum Urochordata, is informally classified under protochordates. Protochordates are “primitive” chordates that are not vertebrates. Lancelets are marine filter feeders and show all five chordate characteristics throughout their life. These animals have finger-like projections near their mouth called cirri. These structures act as a coarse filter to screen out particles too large to consume. 

Phylum Chordata

Chordates are defined by five distinct characteristics:

1) The presence of a notochord. A notochord is a flexible rod that is the main axial supporting structure in all embryo and some adult chordate animals.

2) The presence of a dorsal, hollow nerve chord. The nerve chord is located dorsal to the notochord.

3) The pharyngeal apparatus. This embryonic structure consists of pharyngeal arches, pharyngeal pouches, and pharyngeal groves. These structures are used for feeding and respiration. In lower chordates parts of this structure develop into gill slits that are used to filter water and collect food particles. More derived chordates have modified pharyngeal arches into structures of the head and neck. (A common misconception is that human embryos have gill slits. This is incorrect terminology. Human embryos have the pharyngeal apparatus that do not actually develop into slits.)

4) The presence of a post-anal tail. This structure is an extension of the notochord and nerve chord past the anus.

5) The presence of the endostyle. This structure is an elongated groove in the pharynx floor of protochordates (”primitive chordates” that are not vertebrates) that develops into thyroid gland in chordates. These structures play a role in regulating metabolism.

It is important to note that all five traits are present throughout the development of a chordate; however, not all five traits may be present at the same time. This phylum includes the subphylum Urochordata, subphylum Cephalochordata, and subphylum Vertebrata.