File: <mollusca.htm> <Index to Invertebrates> <Bibliography> <Glossary> Site Description <Navigate to Home>



Invertebrate Zoology


Kingdom: Animalia, Phylum: Chordata



Phylum: Chordata


Subphylum Urochordata (Tunicata) (tunicates; 3,010 species)

Class Ascidiacea

Class Thaliacea (salps)

Class Appendicularia (larvacea)


Subphylum Cephalochordata (Acraniata) (lancelets; 29+ species)


Subphylum Vertebrata (Craniata) (vertebrates-animals with backbones; 57,690 species)

Class 'Agnatha' paraphyletic (jawless vertebrates; 105+ species)

Subclass Myxinoidea (hagfish; 67 species)

Subclass Petromyzontida (lampreys)

Subclass Conodonta

Subclass Pteraspidomorphi (Paleozoic jawless fish)

Order Anaspida

Order Thelodonti (Paleozoic jawless fish)


Infraphylum Gnathostomata (vertebrates)

Class Placodermi (Paleozoic armoured forms)

Class Chondrichthyes (cartilaginous fish; 890+ species)

Class Acanthodii (Paleozoic "spiny sharks")

Class Osteichthyes (bony fish; 30,000+ species)

Subclass Actinopterygii (ray-finned fish; about 29,500 species)

Subclass Sarcopterygii (lobe-finned fish)


Superclass Tetrapoda (four-legged vertebrates; 18,010+ species)

Class Amphibia (amphibians; 5,995 species) Series Amniota (with amniotic egg)

Class Reptilia (reptiles; 8,200+ species)

Subclass Anapsida (extinct "proto-reptiles" and possibly turtles)

Subclass Synapsida (mammal-like "reptiles"; 4,510+ species, progenitors of


Subclass Diapsida (majority of reptiles, progenitors of birds)


Class Aves (birds; 8,70010,025 species)

Class Mammalia (mammals; 5,700 species)


Bibliography Citations




The Phylum Chordata are animals that are either vertebrates or one of several closely related invertebrates. They are similar by having in at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal appendage or tail. Included here are some groups that resemble invertebrates and may be transferred to that category as more data on their behavior, morphology and DNA is obtained.


Several hypotheses have been proposed for the evolutionary relationships of the Chordata. It is believed that they are monophyletic by having descendants of a single common ancestor. The earliest fossils are from the Early Cambrian period, which includes two species of fish-like animals. Authorities have found it difficult to give a detailed classification within the living Chordata, so that more studies are required to resolve this.




Subphylum Urochordata (Tunicata) (tunicates; 3,010 species)


All members of this subphylum inhabit marine environments. They are also known as sea squirts, which are sessile as adults and permanently fastened.



Body Plan. -- At the upper end of the animal there is an incurrent siphon or oral siphon and en excurrent siphon or atrial siphon. The oral siphon leads to a large pharynx, which possesses a great number of ornate gills that open into the atrium. The U-shaped digestive tract loops into the base and then back again, with the anus opening into the atrium.


Reproduction. -- The animals are all hermaphroditic.


Nervous System. -- There is a single ganglion located between the two siphons.


Body Wall. -- A heavy layer called the "Tunic" composed largely of cellulose forms the body wall.


Embryology. -- The egg develops into a larval form called a tadpole. The tadpole larva has a dorsal nollow nervous system and a notocord in the tail. There are also gill slits. These are all features of the Chordata.


The tadpole settles down and becomes permanently attached by adhesive papillae. Degenerative metamorphosis occurs to the adult stage. It has been suggested that the Vertebrata evolved from the tadpole larva.


Circulation. -- A reversible heart pumps one direction first and then the other direction. There is a respiratory pigment of vanadium.




Subphylum Cephalochordata (Acraniata) (lancelets; 29+ species)



All characteristics of the Chordata appear in this subphylum. The gill slits open into the atrium, there is a well developed notochord and a well developed dorsal, hollow nerve cord. The embryology shows all features of Tunicata, Echinodermata and Enteropneusta.




Sample Examinations Invertebrate Classification