Four distinctive derived characteristics of chordates distinguish them from their ancestors:

A. Notochord, or a rod of vacuolated cells, encased by a firm sheath that lies ventral to the neural tube in vertebrate embryos and some adults.

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

In the subphylum Vertebrata, all members possess the four chordate characteristics at some time in development, but often these structures are altered significantly in adult animals.

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

Enteropneusta (acorn worms). These are 2 cm to 1.5 m long; marine in shallow waters, solitary, live in mud or vegetation; filter-feeders. They have well-developed gill slits, and a stomochord, at one time thought to be homologous with the notochord. They also have a dorsal strand of nerve cells, believed to be the precursor to the dorsal hollow nerve cord. Example genera: Saccoglossus, Dolichoglossus.

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.

Balanoglossus (Fig. 2.9 in text) has some characteristics in common with chordates, such as gill slits and a dorsal nerve cord; however, this species also has a ventral nerve cord, and the nerve cords in general are not hollow like most chordates, but instead are solid. This particular species has a worldwide distribution, lives in shallow sea water, and can range between a few centimeters to up to two meters (6' 6" !!).

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.

The larval form possesses more of the chordate characteristics than the adult form (see figure). Adult is sessile (and sometimes colonial), and must obtain food by siphoning sea water through its body and trapping food particles in the endostyle.

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.

***This example should illustrate to you that although chordates are said to have four basic things in common, these characteristics need not be retained throughout life for an animal to be considered a chordate. Rather, they must only be present in an individual at some time during development.***

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.

Cephalochordates are like vertebrates in having the derived feature of an elongate body as adults, but are still (primitively) filter feeders; that is, they feed while motionless, moving food-laden water by means of cilia on their gill bars.

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."

These observations have led workers to suggest that the freely-swimming mode of locomotion of vertebrates (and cephalochordates) evolved by retaining the form of the larvae of the "ancestors" (hemichordates and urochordates) as the form of the adults of the descendants (cephalochordates and vertebrates). This general phenomenon is called paedomorphosis: the evolutionary retention of larval features of the ancestors as the adult features of the descendants.