“The Agnatha”

“The Agnatha” JawlessVertebrates

Dr. Babita Sharma Zoology Department (UG)

Patna Science College Patna University, Patna 800005

Classification adopted from J. Z. Young 3rd edition.

For B.Sc. Part II

What we already know

Classification: Agnatha (lack jaws)• Superclass: Agnatha (without jaws) = Cyclostomata

• Class 1. Pteraspidomorphi (fin shield) = Diplorhina (double nose)

✴Subclass 1. Heterostraci (different shells)

✴Subclass 2. Thelodonti (nipple teeth)

• Class 2. Cephalaspidimorphi (head shield) = Monorhina (single nose)

✴Order 1. Osteostraci (bone shell)

✴Order 2. Anaspida (no shield)

✴Order 3. Galeaspida (helmet shield)

✴Order 4. Cyclostomata (round mouth)

Order: Cyclostomata

• Class 1. Diplorhina (double nose)

• Class 2. Monorhina (single nose)

✴Order 4. Cyclostomata (round mouth)

❖ Suborder 1: Petromyzontidae (stone sucker) e.g. Petromyzon (Lamprey)

❖Suborder 2: Myxinoidea (Slimy one) e.g. Myxine (Hag fishes)

Agnatha: General Characters• Without Jaws.

• Only living agnathans are Cyclostomates

• Ostracoderms (bone skinned) also show agnathan characteristics.

• Adults are parasites or scavengers. Larvae feed on microscopic materials using endostyle muscular contraction.

• Lampreys are most familiar cyclostomes.

• Ammocoete larva (sand sleeper)

• Microphagous larva but adult has a sucking mouth

General CharactersGr. cyclos=circular, stoma=mouth

• May be marine of fresh water habitat. • Body is elongated and eel like. • Body divisible into trunk and tail.Trunk and tail muscles are segmented into myotomes separated by myocomata.

• Paired fins are absent. Median fins are supported by cartilaginous fin ray.

• Skin is smooth, soft and slimy. Scales are absent. • Notochord persistent throughout life. Vertebrae represented by imperfect neural arches (arcualia) present surrounding notochord.

• Pharyngeal gill-slits are found in which about 6-15 pairs of gills are present.

• Mouth ventral, suctorial, circular and without jaws.

General Characters continued• Stomach absent. Oesophagus opens in intestine. • Two mesonephric kidneys for excretion. • Heart two chambered with one Auricle and one Ventricle. Conus Arteriosus is found anteriorly.

• Hepatic portal system is well developed. Blood contains RBCs and Leucocytes. These are Poikilotherms.

• Nervous system includes a brain and a dorsal nerve chord. 8-10 cranial nerves present.

• Median nasal chamber with single nostril. • Auditory organ with 1-2 semicircular rings. • Reproduction sexual. Fertilization external. • Development direct or Ammocoete larva. • Dioecious animals.

Lampreys

Petromyzon:Lampetra fluviatilis• Common lamprey (UK).

• Eel like adult ~30cm long.

• Parasite. Adult at 2.5 years migrates to fresh water for breeding—> Anadromous.

• Dark dorsally and white ventrally.

• Smooth surface. No scale

• Slime secreting glans cells on skin that cause gastro intestinal poisoning.

• Chromatophores present.

• One pair of eyes and a sucker mouth.

• 7 pair round gill openings, along with eyes and nasal papilla —> Nine eyes

Ammocoetes Larva

Ammocetes larvaThe larval stage of the primitive lamprey, known as an ammocoetes larva, serves to illustrate the concept of ontogeny recapitulating phylogeny by resembling the more primitive hemichordate Amphioxus in morphology and behavior. The ammocoetes displays many characteristics of ancient chordates, and shared embryological traits of the vertebrates including gill slits, a notochord, myomeres (body muscle segments), a dorsal nerve cord, a dorsal fin, and a primitive nervous system with a brain and eyes.

Ammocoete larva is important in the evolution of the petromyzon fish. It undergoes metamorphosis which changes its internal organs leading to their development. Originally described in Europe, Ammocoetes was at first incorrectly believed to be a separate genus of the Cyclostomata (lampreys and hagfish)

Eptatretus cirrhatus New Zealand Hag Fish Hag Fishes

Myxine glutinosa

• Myxine, Bdellostoma, • Polistotrema and Eptatretus.

General characters:★Commonly known as Hag Fishes.

★Highly modified for sucking. Live buried in mud or sand and eat polychaetes, other invertebrates and are also scavengers.

★Eyes functionless. Rudiments sensitive to illumination. Sensory tentacles around mouth. Teeth and sucking apparatus well developed.

★Efficient jaws that bite laterally to tear and fragment the prey. They burrow into bodies of dead or dying fishes. Serious source of loss to fishermen as they attack fishes caught in drift nets. Myxine glutinosa

Gill Pouches in Hag Fishes★Gills modified

into pouches. 6-14 in Bdellostoma and 6 in Myxine opening by tubes into pharynx and exterior. All tubes join and open outside by a single aperture on each side.

Other systems★The circulation is largely an open system of large sinuses and

there are accessory hearts in the portal system, cardinal veins and tails.

★Absence of stomach like Lampreys. Zymogen cells throughout intestine.

★Thyroid gland consists of a long series of sacs formed ny evagination from the floor of pharynx.

★No pineal eyes. One semicircular canal in the ear.

★Blood isotonic with sea water.