Agnatha: General Characters, Classification and Types

Agnatha: General Characters, Classification and Types

GENERAL CHARACTERS:

1. Agnatha are earliest known vertebrates and are character-ized by the absence of jaws.

2. Teeth, paired appendages and exoskeleton are absent.

3. Skull has a membranous roof.itt 

4. Single median nasal opening.

5. Vertebral column consists of a persistent notochord with a fibrous neural tube. 

6.Presence of large number gill-slits, from 7-14 pairs.

7. Absence of conus in the heart. lying.

8. A persistent hypophysial sac.

9. One or two semicircular ducts in the ear.

10. Long kidneys and long archinephric ducts. 

11. Genital ducts absent.

12. Pineal apparatus fairly well-developed.

13. Larval stage is microphagus and has an endostyle like protochordates. 

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Subphylum Agnatha is divided into two classes:

1. Ostracodermi

2. Cyclostomata.

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Class 1. Ostracodermi

1. Fossil jawless Agnatha of fresh water. 

2. They had fish-like bodies with heavy head armour.

3. They had heavy bony dermal plates in the skin. 

4. Some of the forms had one pair of fins behind the head.

5. They had single nostril on the top of the head.

6. There was a slit-like mouth at the extreme front end of the head. It was used for scooping decaying matter from the floor of the lake.

7. The paired eyes were situated on the top of the head. Median pineal eye was also present. 

8. The gill-slits were round and all have similar gill-pouches.

9. The endoskeleton was moderately ossified.

10. Two semi-circular canals were mostly present in the ear. Example: Cephalaspis.

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Class 2. Cyclostomata

1. Body is long, rounded and eel-like.

2. Skin is soft, smooth and without exoskeleton.

3. Mouth is suctorial devoid of functional jaws.

4. Nostril is single and median. 

5. Paired fins or lateral appendages are absent.

6. Skeleton is cartilaginous. 

7. Notochord is persistent.

8. Heart is two chambered and aortic arches are many. 

9. Single gonad without duct.

10. Development is direct or indirect. 

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Order 1. Petromyzontia

1. Dorsal fin well developed and branchial basket complete. 

2. The naso-hypophysial sac terminates posteriorly in a blind sac, i.e., it does communicate with the mouth.

3. Mouth suctorial with rasping tongue. 

4. Seven pairs of gill-slits.

5. The gills open into a respiratory tube below the oesophagus.

6. Development is indirect.

Example: Petromyzon.

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Order 2. Myxinoidea 

1. Dorsal fin absent.

2. Branchial basket is reduced.

3. The naso-hypophysial sac opens posteriorly in the mouth.

4. Mouth is terminal and surrounded by 6 small tentacles, 5. Gill-alits 6-14 pairs.

6. Development is direct.

Examples: 

1. Myxine. 2. Bdellostoma.

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TYPES:

1. Cephalaspis: 

Classification:

Phylum: Chordata
Group: Craniata
Subphylum: Agnatha
Class: Cyclostomata
Order: Myxinoidea
Genus: Cephalaspis

Cephalaspis is a bottom dweller and in- habits the shallow waters. 

It excavates through mud and sucks in minute food particles that enter through the mouth. 

It has a mall fish-like body. The body is divisible into head, trunk and Tail.

The head is dorsoventrally flattened and remains covered by a stout bony carapace. 

The trunk is protected by bony plates which are oriented vertically. 

A median dorsal fin is present in the posterior part of the trunk. 

The tail is heterocercal and is provi- ded with small scales.

 The head shield is formed of a single bony piece which is projected posteriorly in the form of two lateral horns, one on each side of the head. 

There are two lobed pectoral fins attached to the lateral A pair of closely placed eyes is present on the top of the head. 

A single median nostril is situa ted immediately in front of the eyes. 

A small pineal opening is placed in between the eyes. 

Two prominent dorso-lateral depres. sions called the lateral sensory fields have been observed. 

The ventral surface of the head is protected by a few small fine plates. 

The mouth is a circular opening lying at the anterior end of the ventral surface.

 The mouth is devoid of biting jaws. 

The gill-slits are round openings and are arranged in two semi-circles on either side of the mouth. 

There are about ten pairs of gill-slits. 

Internal anatomy of Cephalaspis is not completely known.

              Fig. Cephalaspis

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2. Myxine: 

Classification:

Phylum: Chordata
Group: Craniata
Subphylum: Agnatha
Class: Cyclostomata
Order: Myxinoidea
Genus: Myxin

Myxine is commonly known as hagfish. 

It is found buried in the sea bottom. 

Myzine has a wide distribution along sea coasts of both Atlantic and Pacific Oceans, occurring in the war of Northern Europe, North Atlantic, America, Chili and Japan, etc. 

Body is eel-like, measuring about 2 feet in length and differentiated into head, trunk and tail. 

The surface of the body is soft and smooth without scales. The mouth is terminal and surrounded by soft lips.

 Buccal funnel and jaws are absent. Bran- chial basket is also reduced. Lateral to the mouth are four pairs of short tentacles supported by skeletal rods. 

Nostril is single, lies very close to the mouth and opens terminally.

 Single pineal eye is visible on the top of the head. 

Paired eyes are vestigeal or de- generated due to bottom dwelling habit] Six pairs of gills which do not open separately to the outside but open by a single exter- nal gill opening. 

 Single median fin runs from about the middle of the ventral surface extending around the tail region. 

Large mucous glands are present opening by mucous pores.

  Herma-phroditio with single ovotestis, anterior part being ovary and posterior testis. (These animals are parasitic or quasi-parasitic because they are sometimes found within the bodies of their prey, which are fishes of various types.)

 Nocturnal feeders. 

  During the day time they live buried in the sea bottom mud at depths of over 2,000 feet.

             Fig. Myxine

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3. Bdellostoma: 

Classification:

Phylum: Chordata
Group: Craniata
Subphylum: Agnatha
Class: Cyclostomata
Order: Myxinoidea
Genus: Bdellostoma

Bdellostoma is also commonly known as hagfish. 

It is found buried in the bottom mud of sea. 

It occurs off the Pacific coasts of both North and South America, South Africa and New Zealand. 

The long eel-like body has a soft integu. ment without scales. The mouth is terminal surrounded by soft lips.

 Buccal funnel and jaws are absent. Four pairs of short tentacles supported by skeletal rods are present on the lateral sides of the mouth.

 The single nostril lies very close to the mouth and opens terminally. 

Single pineal eye is present on the top of the head. Paired eyes are vestigial or degenerated due to the bottom dwelling habit. 

 The gill openings are 6-14 in number which all open independently by round pores. 

 The median fin is confined to the caudal region. 

Large slime or mucous glands are present open- ing by mucous pores.

 Hermaphroditic Single ovotestis, the ante- rior part being ovary and the posterior testis. 

It is parasitic or quasi-parasitic. Nocturnal feeders. 

  During the day time they live buried in the sea bottom mud.

          Fig.  Bdellostoma

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