Chordata
A phylum under kingdom Animalia, chordates are highlighted by the presence of four characteristics: notochord, a longitudinal, flexible rod located between the digestive tube and the nerve cord; post-anal tail, a muscular bony tail extending past the anus; dorsal hollow nerve cord, developed from an ectoderm rolling into a hollow neural tube; and pharyngeal slits or clefts, arches found along the outer surface of the pharynx.
Vertebrates make up majority of phylum Chordata, but not all chordates are vertebrates.
Basal Chordates
Branchiostoma lanceolatum
Comprised of the subphyla Cephalochordata and Urochordata (sometimes referred to as Tunicata), basal chordates are also known as the protochordates or invertebrate chordates. They have poor fossil records attributed to difficulty in interpreting what little discovered fossils there are due to their soft bodies.
Cephalochordata
As all chordates, cephalochordates (also called lancelets or amphioxi) possess the notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. They have cilia that draw seawater into their mouth and a net of mucus that removes any tiny food particles that pass through it.
They are gonochoristic, having separate sexes, and undergo external fertilization, releasing the eggs and sperm into the water. Larvae are similar in structure to adults, although smaller and more asymmetrical, before metamorphosing into their adult form.
Due to the absence of a brain or a cranium, cephalochordates are sometimes referred to as acraniates. They do not have a heart.
The common ancestor of chordates are said to may have looked like lancelets.
Urochordata
Urochordata is the sister group of Vertebrata and are known as the closest relatives of vertebrates. Unlike cephalochordates, they are poor models for the last tunicate-vertebrate ancestor attributed to their rapid rates of evolution.
As all chordates, tunicates possess the notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail. These are more prominent in larvae, with their nervous system dissolving in adulthood. The term tunicate is derived from the tunic, the hard, leathery outer covering in which their bodies are embedded.
Most tunicates are hermaphroditic, allowing for both sexual and asexual reproduction. There exist both solitary and colonial tunicates, with solitary tunicates having a preference for sexual reproduction and colonies having a preference for asexual reproduction through budding.
They are marine filter-feeders with habitats ranging from shallow waters to the deep sea. Most are sessile or immobile. They possess two openings or siphons: the incurrent or oral siphon, through which water enters; and the excurrent or atrial siphon, through which water and other waste exits.
Examples of Basal Chordates
Rhopalaea crassa in Dinagat, Libjo, Philippines
Cephalochordata has only one class Leptocardii, comprised of around 30 species of lancelets. With three extant genera, the largest is Branchiostoma with some 11 species. Branchiostoma lanceolatum (shown here) is often used as a model organism in studying the evolution of the vertebrate.
Urochordata has some 3,000 species and is traditionally classified into three extant classes: Ascidiacea (ascidians or sea squirts), Thaliacea, and Appendicularia. Thaliacea contains the order Salpida, commonly known as salps.
Rhopalaea crassa (shown above) is a solitary sessile species of tunicate in class Ascidiacea, commonly found in crevices and between coral branches in the Indo-West Pacific. Its tunic ranges from yellow-green to orange-yellow.
Basal Chordates in the Philippines
Oxycorynia fascicularisa in Ticao, San Miguel, Philippines
There have been cephalochordates reported in the 1970s in Lucena Anchorage, Quezon, Philippines identified to be the species Branchiostoma belcheri and Epigonichthys cultellus. Little is known about cephalochordates in the Philippines.
Urochordates are more commonly found in Philippine waters than cephalochordates. These include species of the class Ascidiacea, more commonly known as sea squirts, and Thaliacea, commonly known as salps.
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