Cyclostomata

What are Cyclostomata?

The class Cyclostomata represents the jawless fish. It consists of two basic divisions. These are hagfishes (Myxini) and lampreys (Petromyzontida). These animals lack jaws and paired fins, which allows their study to be of major importance in understanding vertebrate evolution. Cyclostomes are among the most primitive living vertebrates, which help us to understand vertebrate development at an early stage.

Classification of Cyclostomata

Cyclostomata is a superclass under the subphylum Vertebrata and includes two main living classes, Petromyzontida (lampreys) and Myxini (hagfishes). These are grouped due to their shared jawless condition and similar morphology. The taxonomy and classification of cyclostomata are described below-

Cyclostoma Origin & Evolutionary Significance

Cyclostomata are jawless vertebrates that represent one of the most primitive groups of living chordates. They are believed to have evolved from early vertebrate ancestors during the Palaeozoic era. Below are a few points on the origin of Cyclostomata:

• They are ancient vertebrates that appeared 500 million years ago during the early Palaeozoic era.

• They evolved from early vertebrate ancestors and represent one of the most primitive forms of living vertebrates.

• Cyclostomes are part of the Agnatha superclass, which means jawless fish, and they existed even before jawed fish evolved.

• Their skeleton is made of cartilage and a simple body structure suggesting that they branched off early from the early vertebrate line.

• Fossil records of ancient jaw fish help scientists trace the evolution of the vertebral column, nervous system, backbone, and brain.

Subdivisions of Cyclostomata

The group is mainly divided into two extant orders, Petromyzontiformes (lampreys) and Myxiniformes (hagfishes). It is based on differences in feeding behaviour, reproductive methods, and anatomy. The cyclostomes are subdivided into two major orders.

Myxini (Hagfishes)

• Hagfishes are soft-bodied, jawless animals that produce slimy mucus when disturbed, which helps them escape predators.

• They are scavengers, meaning they feed on the bodies of the dead or dying animals by burrowing into them and eating from the inside.

• Hagfishes live mainly in the marine environment and are rarely seen because they stay deep in the ocean.

• Examples- Paramyxine and Myxine

Petromyzontida (Lampreys)

• Petromyzon are parasitic, meaning they attach themselves to other living fish using their sucker-like mouth and feed on their blood.

• They have a tooth-like mouth to latch onto their host.

• Lampreys are found in both freshwater and marine habitats, and some species even migrate between the two.

• Examples- Lampetra and Petromyzon

Characteristics of Cyclostomata

Cyclostomes are jawless, eel-like vertebrates with a soft, elongated body and a circular, sucker-like mouth. They lack paired fins, jaws, and scales. Some general characteristics of Cyclostomata are discussed below:

• Body Structure: The cyclostomes are characterised by their elongated bodies, eel-like in shape, with no paired fins and no jaws. The mouth is sucker-like and circular.

• Skeleton: They have cartilaginous skeletons, that is, they are made of cartilage rather than bone. They do not have true vertebrae, only a notochord. The notochord is a long, flexible, rod-like structure that tends to help support the animal.

• Sensory Organs: The cyclostomes have rather poorly developed eyes and a lateral line system that detects vibrations and movements in the water. This enables them to steer through the water and to search for food.

• Respiration: The cyclostomes breathe through gills, which are present in pairs of gill slits located on the sides of the head. These gills are used for aquatic respiration and are covered by a soft skin-like structure instead of hard gill covers.

• Circulatory system: They have a closed circulatory system with a two-chambered heart. Blood flows in one direction, unlike higher vertebrates. The red blood cells are usually nucleated. The circulation is simple but efficient for the body's needs.

• Nervous System: Cyclostomes have a simple but well-developed nervous system with a brain and a spinal cord. Their sense organs are adapted to their aquatic and parasitic life cycle, including a single median nostril and eyes that may be poorly developed in some species.

• Reproduction: Cyclostomes are unisexual, meaning that male and female reproductive parts are separate. Fertilisation is external, and they do not have larval care. The reproductive cycle involves migration to freshwater for spawning, after which the adult usually dies.

Cyclostomata Anatomy & Physiology

The cyclostomes have a mouth, gill slits and nostrils that help in respiration and eyes to help them see. The general anatomy and physiology of Cyclostomata are discussed below:

• Mouth: Circular, sucker-like structure with keratinised teeth (particularly in lampreys).

• Gill Slits: Multiple external openings along the sides of the body used for respiration.

• Nostril: A single median nostril located on the top of the head, used for olfaction.

• Eyes: Simple, lacking the complexity of higher vertebrates.

• Fins (if present): Some species may have caudal fins for stabilisation during swimming.

• Segmented Body: Cyclostomes have elongated, cylindrical bodies that are segmented.

• Setae: Absent in cyclostomes, differentiating them from other segmented animals like annelids.

• Clitellum: Not present in cyclostomes, unlike in annelids such as earthworms.

Cyclostoma Habitat & Ecology

These cyclostomes inhabit a variety of habitats. Hagfishes are mostly deep-sea marine, while lampreys are found in both marine and freshwater systems. Lampreys also make very long migrations from one habitat to another.

Ecological Roles:

• Some of the species, like the hagfish, act as scavengers, devouring dead and decaying remains.

• The impact that lampreys, particularly their parasitic forms, have on fish populations involves sucking their blood and feeding on tissues.

• Lampreys can significantly affect fish populations, both through predation and parasitism.

Cyclostomata NEET MCQs

Q1. Which one of the following pairs of animals comprises 'jawless fishes'?

1. Mackerals and Rohu

2. Lampreys and hag fishes

3. Lampreys and Rohu

4. Lampreys and eels

Correct answer: 2) Lampreys and hag fishes

Explanation:

Jawless fishes are a group of primitive fishes that lack jaws. They are known as Agnatha, and the two main groups within Agnatha are lampreys (order Petromyzontiformes) and hagfishes (order Myxiniformes).

Lampreys and hagfishes both belong to the category of jawless fishes. These animals have evolved unique adaptations to compensate for the absence of jaws. Lampreys are characterized by their sucker-like mouth, which they use to attach to other fish and feed on their blood and body fluids. Hagfishes, on the other hand, are known for their ability to produce copious amounts of slime as a defense mechanism.

Hence, the correct answer is option (2) Lampreys and hagfishes.

Q2. A jawless fish, which lays eggs in fresh water and whose ammocoetes larvae after metamorphosis return to the ocean is :

1. Myxine

2. Neomyxine

3. Petromyzon

4. Eptatretus

Correct answer: 3) Petromyzon

Explanation:

Petromyzon(the lamprey) belongs to the section Agnatha of the sub-phylum-Vertebrata. They have long, greenish-brown, cylindrical bodies with smooth scaleless, slimy skin, jawless mouths, etc. They lay eggs in freshwater but their ammocoete larvae(lower) after metamorphosis return to the ocean.

Hence, the correct answer is option (3) Petromyzon.

Q3. Cyclostomata has an elongated body bearing 6-15 pairs of gill slits which help in

1. Respiration

2. Digestion

3. Excretion

4. All of these

Correct answer: 1) Respiration

Explanation:

Cyclostomata is characterized by jawless fish like lampreys and hagfish with specific anatomical characteristics.
Body Shape: They have a long body that usually carries 6 to 15 pairs of gill slits to respire. The gill slits are important in exchanging gases in aquatic conditions.
The cartilaginous cranium and vertebral column of a cyclostome are composed primarily of cartilage and they maintain a persistent notochord, which gives it support throughout their life cycle. These characteristics point to the unique evolutionary adaptations of Cyclostomata within vertebrates, which differentiates them from other fish groups.

Hence, the correct answer is option (1) Respiration.

Also Read:

• MCQs on Characteristics Features of Cyclostomata

• Reproductive System of Earthworm

• Phylum Porifera

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