Biology 2nd Paper (Zoology) · Classes 11-12 · Chapter 1

Vertebrata: Cyclostomata and the Fish Classes.

A hilsa and a shark are both fish, yet one has a bony skeleton and the other is cartilage from end to end. Some fish have no jaws at all, only a round sucking mouth. In this note you will tell five groups of vertebrates apart by their scales, their tails and their gills.

1. Two superclasses, and the jawless ones

In modern classification the subphylum Vertebrata is divided into two superclasses: Cyclostomata and Gnathostomata.

The name Cyclostomata says it all: Greek cyclos means round, stoma means mouth. These animals have no true jaws and no paired appendages. The mouth is round and set with keratinous teeth. In the adult the vertebral column is degenerate or absent. There are two classes: Myxini and Petromyzontida.

2. Hagfish and lamprey

Myxini comes from the Greek myxa, slime. Its fishes are the hagfishes. They are free-living and feed on annelids, molluscs, crustaceans or dying fish. There are about 70 species, all marine, eel-like in appearance.

The body is scaleless, covered with slimy glandular skin, and has no dorsal fin. The mouth is terminal, ringed by four pairs of tentacles. The sides of the pharynx carry 5 to 15 pairs of gill slits. The nasal sac opens into the mouth cavity. There is no larval stage.

Examples: Myxine glutinosa (Atlantic hagfish) and Eptatretus stouti (Pacific hagfish).

In Petromyzontida, Greek petros means stone and myzon means sucking. Its fishes are the lampreys; many are parasites, and there are about 41 species. Lampreys are marine, but they come to freshwater lakes to lay eggs and die within days of spawning. The egg hatches into an ammocete larva, which goes to the sea after metamorphosis.

The adult lamprey is slender and eel-like, scaleless, with one or two dorsal fins. The oral disc works as a sucker, ringed by keratinous teeth. There are seven pairs of gills with separate slits. The nasal sac does not open into the mouth cavity. A larval stage is present.

Examples: Petromyzon marinus and Lampetra tridentatus.

The two classes look alike, so tap through the grid and match up the differences.

3. The jawed ones: Gnathostomata and the cartilaginous fishes

Gnathostomata: Greek gnathos means jaw, stoma means mouth. These vertebrates have true jaws and paired appendages, with cartilaginous or bony skeletons. The superclass has seven classes; this note covers its three fish classes.

Chondrichthyes (Greek chondros = cartilage, ichthys = fish) is the first class of Gnathostomata. It holds about 1000 species of cartilaginous fishes. The class is ancient yet advanced, and mostly marine.

The body is covered with many small placoid scales and the endoskeleton is wholly cartilaginous. On each side of the head 5 to 7 gill slits open separately to the outside. The tail fin is heterocercal: its two lobes are unequal. The mouth and nostrils lie on the ventral side of the head, and the jaws bear many rows of teeth.

Examples: Hydrolagus collei (ratfish), Scoliodon laticaudus (shark), Plesiobatis daviesi (sting ray) and Eusphyrna blochii (hammerhead shark).

4. Ray-finned and lobe-finned fishes

Actinopterygii (Greek actis = rays, pteryx = fin) are the ray-finned fishes. There are 31,045 species, about 96% of all living fishes. They live everywhere from springs to the deep sea.

The skin is glandular, usually covered with cycloid (round) or ctenoid (spiny) scales, though some have none. The endoskeleton is bony. Each side of the head has one gill slit covered by an operculum. The tail fin is homocercal: two equal, rayed lobes. The swim bladder helps the body float in water.

Examples: Tenualosa ilisha (hilsa), Labeo rohita (rui), Channa punctatus (taki) and Anabas testudineus (koi).

Sarcopterygii (sarkos = flesh, pteryx = fin) are the lobe-finned fishes. Scientists think tetrapods and land animals arose from a fish of this class. Only 8 species are alive: lungfishes in Africa, South America and Australia, and two species called coelacanths off the Indian Ocean and Indonesia.

The body is covered with ganoid scales, whose outer layer is ganin. The endoskeleton is bony, with one operculum-covered gill slit on each side of the head. The swim bladder is rich in capillaries, so it helps in both respiration and floating. The tail is diphycercal: the two lobes of the tail fin fuse into one even, flexible fin around the tail.

Examples: Latimeria chalumnae (coelacanth), Neoceratodus forsteri (Australian lungfish) and Protopterus annectens (African lungfish).

The three fish classes differ in tail, scales and gill cover. Put a fish together yourself: the right mix names its class, and a wrong one tells you no such fish is in the book.

5. Try it yourself

Now place fourteen features into the three class boxes. Scales and tail settle most of them.

Four questions separate the five fish classes. Pick a fish and walk the key.

Now name the class from the animal. This is how exams ask it.

Last, six terms on cards: the name on the front, the class and meaning on the back.

6. Check yourself

Three questions in the exam's style. Start with the first.

The second asks you to pick the class from a feature.

Think a little longer over the last one.

The whole note on seven cards.

Source: Biology 2nd Paper (Zoology), Classes 11-12 (NCTB), Chapter 1 “Animal Diversity and Classification”, the “Vertebrata: Cyclostomata and the Fish Classes” part.