Chapter 31 : Chordates

Phylum : Chordata

50,000 species. They are coelomate, and deuterostomes.
All chordates have four characteristics :
1) notochord - long flexible rod between the digestive system and nerve cord.
( in vertebrates it is replaced by the vertebrae ).
2) dorsal nerve cord : develops into the central nervous system - the spinal cord & brain.
3) pharyngeal gill slits : at least in the embryo.
4) tail that extends beyond the anus : at least in the embryo.

Subphylum Cephalochordata

Evolved 500 million years ago. Invertebrates - common name : lancelets, or amphioxus. They have no head or brain. Thin, fish-like body supported by the notochord. They are filter feeders : cilia on gill slits move water. Usually burrow tail first into sand. Lancelets
Example : Branchiostoma

Subphylum Urochordata

Invertebrates - common name : tunicates or sea squirts. The larva is tadpole like, adult is rounded and sessile. Filter feeders, water enters the incurrent siphon and leaves the excurrent siphon, pulled by cilia around the gill slits.
Outer layer ( tunic ) is made from the carbohydrate tunicin. Sea squirts

Most species are poisonous (Julius Caesar's wife poisoned several of his rivals by feeding them tunicates). They are hermaphrodite. A simple heart pumps blood in both directions. In 2002 the genome of a sea squirt was sequenced. Examples : Molgula
Trididemnin ( source of anti-cancer drug Didemnin ).

Subphylum Vertebrata

The oldest fossil vertebrate was recently discovered - over 500 million years old: Fossil. Roughly 50,000 species.

Vertebrates have a hard skeleton of cartilage or bone, replacing the notochord. Very strong cephalization ( an obvious head). The front of the nerve cord becomes the brain. The brain is protected by the cranium ( skull ).
Vertebrates have a closed circulatory system. They are dioecious (separate male and female).

Class Agnatha ( jawless fish )

Skeleton of cartilage, but notochord still present in adult. No fins, long, eel-like body, round mouth (no jaws). Gill pouches empty through pores, not slits. Examples : Petromyzon ( marine lamprey ) : It lays eggs in freshwater, larva look like Branchiostoma and filter feed. Adults in the sea attach to fish as parasites. Lampreys devastated the fishing in the Great Lakes when they entered through man-made canals.

Myxine ( hagfish ) : Hagfish feed on dead or dying fish, located by smell. It produces slime to deter predators.

Class Chondrichthyes ( Cartilaginous fish )

They have jaws, and a skeleton of cartilage. This class includes sharks and rays. They have paired fins for swimming. An oily liver gives them buoyancy. Thay have hard skin with placoid ( backwards facing ) scales.

Predators with good sense organs : They have internal fertilization. The reproductive, digestive, excretory systems end in a single cloaca. Photos Example: Torpedo (ray).

Class Osteichthyes ( bony fish )

30,000 species (largest class of vertebrates). They have a skeleton of hard bones. Bony fish can move their gills, so can breath while stationary.

Three types of scales :
Ganoid : thick, non overlapping.
Cycloid : thin, overlapping.
Ctenoid : thin, overlapping, with teeth to reduce drag.

Bony fish have color vision ( unlike sharks ), so are often brightly colored. Photos

They have a swim bladder (filled with air or water like the ballast tanks in a submarine) for buoyancy. The swim bladder evolved from the lungs of early fish. External fertilization (males release sperm into the water near the eggs). Fish are being used in biomonitoring to detect toxic chemicals in the water supply. Example : Bothus ( flounder ).

Class Amphibia

Amphibians (literally "two lives" ) have two different stages : larval ( tadpole ) and adult. External fertilization, so need water for breeding, but adults can live on land: the first land vertebrates. Fossils date back about 350 million years. They have 4 limbs ( tetrapod ), and evolved from fish living in shallow water. Fossils

Gas exchange is through the skin and lungs (skin is kept wet). Skin produces magainins ( antibiotics ) and fungicides as well as toxins.
In frogs the tongue is connected to front of mouth, so it can flick out a long way to catch food.

Order : Anura ( Frogs and toads )
Frog calls are specific to each species. Frog calls. Frogs have 4 toes on the front feet and 5 on the hind feet. Frog legs

Poison arrow frogs ( Dendrobates ) are brightly colored to warn predators of their toxin. Poison arrow
Pipa toads protect eggs under the skin of the females back.
Rhinoderma frogs : males protect young in their vocal sacs.

Many species of amphibian are declining in numbers, due to habitat loss, new predators or fungal infection.

Last edited November 2008, by David Byres, dbyres@fccj.edu