Agnatha and Gnathostomata, Practice Problems and FAQs

Everyone wants to look good and you know that the facial bones are very important for the maintenance of the shape of our face. But facial bones are different in different groups of animals. That is why the faces of different animal groups are different. In the facial bones, the jaw bones are very important. Most of the animals have lower jaw bones and upper jaw bones. The lower jaw (mandible), which also shapes the lower face and chin, supports the bottom row of teeth. The bone that moves when the mouth opens and shuts is this one. The upper jaw's (maxilla) function is to support the nose, hold the upper teeth, and sculpt the centre of the face.

So now just think about your face without the jaw bones. Then we won't be able to chew food, or breathe and our face will look different from other humans. So now you understand the importance of the jaw bones. Right?

                              Fig: Facial bones

But there are animals without jaw bones. How do they survive then? How will their faces look? I am sure now you are a bit curious about this. Those animals without the jaw bones are present under the subphylum Vertebrata. So with the presence and absence of jaw bones, the subphylum Vertebrata is divided into two. They are the Agnatha and Gnathostomata. So we will discuss more about these divisions in this article.

Table of contents

• Subphylum Vertebrata
• Classification of subphylum Vertebrata
• Classification of division Agnatha
• Classification of division Gnathostomata
• Practice Problems
• FAQs

Subphylum Vertebrata

Subphylum Vertebrata belongs to phylum Chordata. It got its name from the vertebral column present in the animals and categorised under Vertebrata. The vertebral column develops at the adult stage from the notochord which is a rod-like structure of mesodermal origin found in the embryonic stage. Vertebrates are triploblastic and possess a well-developed organ system. Vertebrates are further classified into two divisions. They are Agnatha and Gnathostomata.

Classification of Subphylum Vertebrata

Subphylum Vertebrata is divided into two divisions, Agnatha and Gnathostomata as represented below. Agnatha are those animals which lack jaws and Gnathostomata are those with jaws. Agnatha possesses only one class called Cyclostomata. Gnathostomata is divided into two super classes called Pisces and Tetrapoda. There are six classes in Gnathostomata and they are Chondrichtyes, Osteichthyes, Amphibia, Reptilia, Aves and Mammals.

                                          Fig: Classification of Vertebrata

Classification of division Agnatha

Division Agnatha includes the most primitive vertebrates. Along with the jaws they also lack exoskeleton and paired appendages. They are further classified into class Ostracoderm (currently extinct) and class Cyclostomata.

                                    Fig: Classification of Agnatha

Class Ostracoderm

This class includes the earliest known vertebrates which are extinct. These appeared in the Ordovician period. Members had well-developed dermal scales due to which they were called armoured fishes.

                               Fig: Ostracoderm

Class Cyclostomata

The word ‘Cyclostomata’ has been derived from two words ‘cyklos’ meaning circular and ‘stome’ meaning mouth. Thus the class Cyclostomata includes circular-mouthed fishes. Examples include Petromyzon (Lamprey) and Myxine (Hagfish).

General characteristics

The following are the common characteristics of Cyclostomes.

Habitat

They are adapted to thrive in marine environments. For spawning, they move to freshwater.

External features

They have an elongated eel-like and smooth body without scales. They possess a compressed tail.

Lifestyle

They are ectoparasites on some fishes, i.e., they derive nutrition from the host (mainly fishes) while they live on the surface of the host body.

                   Fig: Ectoparasitic nature of Petromyzon (Lamprey)

Mouth

They have a circular mouth without jaws which has the ability to stick to the host body and suck blood from them.

          Fig: Circular sucking mouth of animals belonging to Cyclostomata

Absence of paired fins

Animals belonging to Cyclostomata lack paired fins. Associated girdles are also absent. But median fins like (the fins in the median plane) the dorsal fin, anal fin and caudal fins are present.

Presence of gill slits

Cyclostomes have 6 - 15 pairs of gill slits. They are placed in a sac like pouches.

                                            Fig: Gill slits of Petromyzon

Exoskeleton

They possess a cartilaginous cranium. They have a notochord present in their entire life. Small, crooked neural arches over the notochord serve as a representation for the vertebral column.

                                Fig: Cartilaginous skeleton

Organ systems of Cyclostomata

The following are the major organ systems in Cyclostomata.

Sense organs

Lateral line sense organs are present to detect the waves and current in water. There is only one olfactory sac present which helps in detecting the sense of smell. Membranous labyrinths are the only structures present in the ear.

Circulatory system

They have a two-chambered heart which includes one auricle and one ventricle. They have a closed circulatory system i.e., transport of blood occurs through blood vessels and capillaries only.

Mode of reproduction

Cyclostomes are dioecious in nature, i.e. they have separate sexes with the exception of some hagfishes which are hermaphrodites. In them both male and female sex organ are present although male sex organs mature prior to female sex organs (protandry) .

Although well-adapted to marine environments, cyclostomes move to freshwater for spawning (release or deposition of eggs). They show anadromous condition. Adult cyclostomes die after spawning. As soon as the process of metamorphosis is concluded, the larval forms return back to the ocean.

Type of development

They show indirect development, that means it occurs through intervening larval stages which is morphologically different from the adult. The larval stage is called ammocoete.

                                                     Fig: Life cycle of lamprey

Petromyzon marinus

It is often referred to as lamprey. Native to the Northern Hemisphere, it is a parasitic lamprey. Petro, which means ‘stone,’ and myzon, which means ‘sucking,’ are the roots of the genus name Petromyzon, while marinus is Latin for ‘of the sea.’ Although it is marine, it migrates to rivers to breed. It clings to a huge fish through its suctorial mouth as a parasite it draws blood from the fish.

                                                          Fig: Lamprey

Myxine

It is commonly known as hagfish. The mouth of the Atlantic hagfish is shaped like a star and it lacks jaws and eyes. Each side of the eel-like body has a single gill slit. It can release a slimy mucus through its pores. Hagfish can tangle themselves up because of their extremely flexible bodies. The hagfish's knots aid them to eat more effectively because they lack opposable jaws, pull possible prey from burrows, and clean mucus from their bodies. They can escape from small areas.

                                     Fig: Hagfish

Classification of division Gnathostomata

Gnathostomata is the division that includes animals which have jaws. Gnathostomata is further classified as two superclasses which includes the Pisces and Tetrapoda. Pisces are those organisms which bear fins and Tetrapoda include those organisms which bear limbs.

                            Fig: Classification of subphylum Vertebrata

Practice Problems

1. Identify the correct statement about the division Agnatha.

I) Agnatha lacks jaws, exoskeleton and paired appendages.
II) Class Cyclostomata is extinct.
III) Members of Ostracoderm have well-developed dermal scales.
IV) Cyclostomata includes circular-mouthed fishes.

1. I, II, III, IV
2. I, III, IV
3. I and II
4. I, II, III
    

Answer: Division Agnatha includes the most primitive vertebrates. Along with the jaws they also lack exoskeleton and paired appendages. They are further classified into class Ostracoderm (currently extinct) and class Cyclostomata. The class Ostracoderm includes the earliest known vertebrates which are extinct. These appeared in the Ordovician period. Members had well-developed dermal scales due to which they were called armoured fishes. Class Cyclostomata includes circular-mouthed fishes. Hence the correct option is b.

2. Match the following:

1. i - A, B, ii - C, D
2. i - B, D, ii - A, D
3. i - B, C, ii - A, D
4. i - C, D, ii - A, B
    

Answer: Vertebrates are triploblastic and possess a well-developed organ system. Vertebrates are further classified into two divisions and they are Agnatha and Gnathostomata. Agnatha has only one class called Cyclostomata. Gnathostomata is divided into two super classes called Pisces and Tetrapoda. Division Agnatha includes the most primitive vertebrates. Along with the jaws they also lack exoskeleton and paired appendages. Class Cyclostomata includes circular-mouthed fishes. Examples include Petromyzon (Lamprey) and Myxine (Hagfish). Hence the correct option is c.

3. Explain the characters of Petromyzon?

Answer: Petromyzon marinus is often referred to as lamprey. Native to the Northern Hemisphere, it is a parasitic lamprey. Petro, which means ‘stone,’ and myzon, which means ‘sucking,’ are the roots of the genus name Petromyzon, while marinus is a Latin word meaning ‘of the sea.’ Although it is marine, it migrates to rivers to breed. It clings to a huge fish through its suctorial mouth and as a parasite draws blood from the fish.

4. How is the process of reproduction in Cyclostomata?

Answer: Although well-adapted to marine environments, cyclostomes move to freshwater for spawning (release or deposition of eggs). Adult cyclostomes die after spawning. As soon as the process of metamorphosis is concluded, the larval forms return back to the ocean.

FAQs

1. Which is the living fossil of class Cyclostomata?
Answer: The most prehistoric group of vertebrates is represented by lampreys, which are members of the class Cyclostomata. They have been around for more than 360 million years. Presence of the numerous evolutionary traits have preserved them and hence they are known as living fossils.

2. What happens to the animals of class Cyclostomata during spawning?
Answer: Cyclostomes are marine vertebrates, but during the spawning process, they migrate to freshwater. Spawning is the process of laying eggs in order to produce new life. They perish a few days after spawning. After undergoing metamorphosis, the larvae that are still alive return to the ocean. It is called the anadromous migration.

3. What is Anaspida?
Answer: The Silurian and Devonian eras saw the extinction of the Anaspida, a primordial order of jawless vertebrates. According to conventional wisdom, these are lampreys' ancestors. Small marine agnathans called anaspids have strikingly high hypocercal tails but lack a strong bony shield and paired fins. They first appeared in the Early Silurian and thrived until the Late Devonian extinction, during which most species, aside from lampreys, went extinct as a result of the upheaval in the environment.

4. Agnatha is considered as endothermic or ectothermic?
Answer: Agnatha is a group of fish-like organisms, they are ectothermic or poikilotherms. An ectothermic animal is cold-blooded and they cannot generate internal heat.