Four distinctive derived characteristics of chordates distinguish them from their ancestors:
B. Hollow nerve cord that lies dorsal to the notochord
C. Pharyngeal pouches
D. Endostyle - elongated groove in the pharynx floor of protochordates that may develop as the thyroid gland in chordates
These four characteristics may be found in some of the ancestors of chordates and are commonly placed in an informal grouping called Protochordates. These serve as living representations of the missing fossils in vertebrate evolution.
Phylum Hemichordata - acorn worms and pterobranchs
Hemichordates are a group of organisms that show an affinity to the chordates, but are lacking some key characteristics of chordates. They include two groups
• Pterobranchia (pterobranchs). These are tiny, deep-sea, colonial, moss-like animals. There is no trace of dorsal nerve cord or notochord, and only one pair of gill slits in species of the genus Cephalodiscus.
In addition, this species also lacks a notochord. It does have a structure called the stomochord , or a diverticulum (blind sac) that is made up of cells that resemble those found in the notochord, but has a different developmental pathway. So, it should be clear why these species are called Hemichordates, and are not included with the true chordates.
Subphylum Urochordata (Phylum Chordata) - tunicates/sea squirts
Urochordates are all marine, and are enclosed in a tough cellulose-like tunic (hence the common name tunicate Tunicates are members of the true chordates, and represent some of the most primitive ancestors of the Subphylum Vertebrata (see Fig, 2.14 - 2.18 in text). Most of the 2000 species belong to the taxon Ascidiacea (sea squirts). This group undergoes complete metamorphosis from a mobile larva to a sessile adult, resorbing the tail and notochord. Some are solitary, most are colonial. The few remaining species of tunicates belong to the taxa Thaliacea and Appendicularia (larvaceans). Thaliaceans lack a tail and notochord; they have no known larval stage. They are small, free-swimming, pelagic barrel-shaped animals that use jet propulsion. Appendicularians do not metamorphose, and are able to reproduce as free-swimming larvae.
Tunicates differ strongly in appearance between the adult and larva.
• Larvae are tadpole-like and free-living, and have an endostyle, gill slits, dorsal nerve cord, and notochord. The larval stage lasts only a few days, and ends when the larva attaches to a substrate and metamorphoses into an adult.
Subphylum Cephalochordata (Phylum Chordata) - Amphioxus
The last group are the cephalochordates, which are usually represented by one organism - Branchiostoma lanceolatum , commonly called Amphioxus (which means "sharp at both ends"). See Fig. 2.22 - 2.24 in text.
Amphioxus are 2-3 inches in length, and live on seashores throughout the temperate zone. Fish-like in appearance, it has a laterally compressed dorsal fin, but it does not have complete organs, or any bony structures.
Amphioxus shows some cephalization, in that the primary feeding structures are concentrated at the anterior end, and it has a pigment spot on the anterior end that may be used for orienting toward light.
Origin of Free-Swimming Vertebrates
In contrast to protochordates (hemichordates, urochordates, and cephalochordates), vertebrates are actively-feeding, predatory organisms that move by lateral undulation of an elongate body.
• Hemichordates and most urochordates are also filter-feeders, moving water through their gill slits, but are sessile as adults. When ascidian tunicates metamorphose, the notochord is resorbed.
• Note, however, that ascidian and larvacean urochordates have a free-swimming larval stage (with a notochord); ascidians metamorphose to sessile adults, but larvaceans become sexually mature as mobile "larvae."