Agnatha, Cyclostomes vertebrate zoology

Agnatha

Agnatha is jawless vertebrate comprises two classes: cyclostomata and ostracodermi. The first fossils of vertebrates are found in the rocks of Ordovician period in the form of ostracoderms.

Cyclostomata

Phylum: Chordata

Subphylum: Vertebrata

Group: Agnatha

Class: Cyclostomata

Order: i) Petromyzoniformes ii) Myxiniformes

Diagnostic characters

1.       Body is elongated, cylindrical, eel-like.

2.       Skin soft, smooth, containing unicellular mucous glands but no scales.

3.       Endoskeleton is fibrous and cartilaginous, median fin supported by cartilaginous fin rays, no paired appendages.

4.       Notochord persists throughout life. Imperfect neural arches (Arcualia) over notochord represent rudimentary vertebrae.

5.       Mouth ventral, suctorial and circular, hence the class name Cyclostomata.

6.       Gill is supported by brachial basket and Gills 5 to 16 pairs in lateral sac-like pouches of pharynx.

7.       Digestive system lacks a stomach. Intestine with a fold, typhlosole.

8.       Trunk and tail muscles segmented into myotomes separated by myocommata.

9.       Heart 2-chambered with 1 auricle and 1 ventricle, with a conus arteriosus anteriorly. Many aortic arches in gill region. No renal portal system. Hepatic portal system present. Blood with leucocytes and nucleated circular erythrocytes. Body temperature variable (poikilothermous).

10.    Two mesonephric kidneys with ducts to urinogenital papilla.

11.    Dorsal nerve cord with differentiated brain. 8  to 10 pairs of cranial nerves.

12.    Single median olfactory sac and single median nostril. Auditory organ with 1 or 2 semicircular canals.

13.    Lateral line system is well developed.

14.    Pharynx has taste buds.

15.    A pair of eyes with functional or not in marine forms., cover by 6 external muscles.

16.    Sexes separate or united. Gonad single, large, without gonoduct.

17.    Fertilization external. Development direct or with a prolonged larval stage i.e. ammocete larva.

Classification

There is about 50 species of living jawless fishes. They belong to two major orders Petromyzontiformes and Myxiniformes. Basic morphological differences which can be attributed to their long phylogenetic separation and different habits and habitat for classification into orders.

Order 1. Petromyzontiformes

(Gr., petros, stone + myzon, suck)

1.       Mouth ventral, within a suctorial buccal funnel beset with many horny teeth.

2.       Nostril dorsal. Nasohypophyseal sac closed behind, not connected to pharynx.

3.       Gill pouches and gill slits 7 pairs each, opening in a separate respiratory pharynx.

4.       Dorsal fin well developed.

5.       Branchial basket complete.

6.       Dorsal and ventral roots of spinal nerves remain separate.

7.       Ear with 2 semicircular canals.

8.       Eggs numerous, small. Development indirect with a long larval stage and metamorphosis.

9.       Both marine and freshwater forms.

For examples: Lampreys. Over 30 species. Petromyzon, Lampetra, Entospherus, Ichthyomyzon.

Order 2. Myxiniformes

(Gr., myxa, slime + oidea, type of)

They are exclusively marine.

1.       Mouth terminal with 4 pairs of tentacles and few teeth. No buccal funnel

2.       Nostril terminal. Nasohypophyseal duct opens behind into pharynx.

3.       Gill pouches 6 to 15 pairs. Gill slits 1 to 15 pairs.

4.       Dorsal fin feeble or absent.

5.       Branchial basket poorly developed.

6.       Dorsal and ventral roots of spinal nerves united.

7.       Ear with only 1 semicircular duct.

8.       Eggs few, large. Development direct.

9.       Hagfishes are till marine 15 species.

Example : Myxine, Eptatretus (=Bdellostoma), Paramyxine

Petromyzon(lampreys)

Distribution: Lampreys are worldwide in distribution, found in both salt and fresh water. There are 3 species of Petromyzon in northern hemisphere. They are P.marinus(sea lamprey), Lampetra flaviatilis(freshwater lamprey) and L.planeri and inhabits the waters of the Great Lakes between U.S.A. and Canada as well as along the Atlantic coasts of North America, Europe and Africa. (Kotpal)

Habits and habitat: They are ectoparasites. Two different phases occur in lifecycle of Petromyzon. The one is larval called ammocete which is a freshwater, sedentary, filter feeding and microphagous creature. The fish-like adult lives in the sea and is parasitic on fishes. It clings to fishes, turtles, etc., with its powerful suctorial mouth and then removes small bits of tissue with its rasping tongue and feed on tissue, causing great threat for fishes. They swim in bottom by undulations of their body. When moving upward in a strong current, it darts suddenly forward and clings to rocks. In autumn, all adult lampreys usually ascend rivers to spawn in the spring, after which they die. While, in the river system, lampreys do not eat anything, but utilize accumulated subcutaneous fat that provides nourishment. They migrate to the rivers during autumn after attaining maturity in winter. However, breeding takes place in spring.

External features

Adult lampreys are elongated eel like. Body is divisible into head, trunk and tail. Head and trunk are cylindrical whereas tail is laterally compressed. Body surface or skin is greenish-brown in color. Upper body surface is usually dark and lower surface is light. It is without exoskeleton, soft and made slimy by secretions from epidermal gland.

Paired fins are absent but there are presence of two unequal median dorsal fins, along with 2^nd dorsal fin there is caudal fin. The fin rays are supported by cartilaginous rods known as fin rays. These rays are fused together at their bases to the membranous sheath that surrounds the notochord and neural tube and help to strengthen the latter.

Head bears ventral large cup like depression called buccal funnel. It is surrounded by marginal membrane with numerous small projections called oral papillae or fimbriae which help in attachment to a fish. In between, the papillae project out longer sensory processes, the cirri. The inside of buccal funnel there is radiating rows of conical yellow, horny, epidermal teeth which have a very definite arrangement. The teeth of upper sides and lower sides of mouth fuse to form large tooth plates called supraoral and infraoral tooth plates respectively. Teeth aren’t homologous with vertebrate teeth. Small circular mouth open at apex of buccal funnel and below and behind there is protruding and rasping tongue.

A large prominent eyes without eyelids lie laterally and are covered by a transparent area of skin. Between the eyes and on the head is a single small mid-dorsal nostril or nasohypophyseal aperture. Behind the nostril, a transparent area of skin indicates the position of the pineal organ. Seven small rounded openings of external gill slits form a longitudinal row on each lateral side of the head, just behind each eye. Numerous sensory pores of lateral line system extend along each lateral side of the body and below the head. On the ventral side, at the junction of trunk and tail, is a slit-like depression, the cloaca. Urinogenital aperture protrudes through cloaca and at its tip urinogenital papilla is present. Just infront of it, lie the small anus within the cloacal depression.

Myxine (Hagfish)

They are exclusively marine. They are found in water of Japan, South Africa, North Atlantic, North Europe. Found buried in mud of sea bottom during day and live in colonies below 600 meters. They feed on living polychaete worm and are scavengers as they feed on dead fish and are internal parasite called borer. They are nocturnal in nature.

Hagfishes are elongated eel like or worm like body without scale, reach a length of 50-60cm and differentiated into head, trunk and tail. Skin is soft and color is pink. Suctorial mouth is terminal with soft wrinkled lips, like those of an old ugly woman or hag, hence the common name hagfish. Eyes are degenerate and covered with a thick skin. Six cartilage supported cirri or tentacles around mouth compensate for the sightless degenerate eyes. The protrusible tongue, bordered by two multitoothed horny plates, serves as a powerful rasping tongue. The single median nostril lies close to the mouth. 6 pairs of gill pouches are located far behind the head legion and their efferent ducts join into a single pair of external gill slits, probably an adaptation to burrowing. Large mucous glands open along the side of body and secrete enormous quantity of slime, hence another common name, the slime eel. A feebly developed mid-ventral fin and a caudal fin are present. Myxine attacks injured or dead fishes and burrows into their body for flesh consumption, hence also called a borer. It is really an internal parasite. It may pose a serious threat to fisheries in some regions. Unlike lampreys, the hagfishes do not migrate to fresh water to spawn. The eggs hatch directly into miniature adults without passing through a larval stage.