Cyclostomata in Vertebrate Classification

They are parasitic, in their adult stage, usually feeding on fish. They are morphologically similar to eels. They are considered to be the only living vertebrate without true jaws and are thus called Agnatha. Cyclostomata is made of hagfish and lampreys.

An introduction to Cyclostomata

Cyclostomata is the modified and degenerate offshoot of the primitive vertebrate stalk. Because of their round mouth, they are named Cyclostomata. They're parasitic, normally feeding on fish during their adult phase. Morphologically, they resemble eels. They are only vertebrates without genuine jaws, subsequently referred to as Agnatha. Cyclostomata consists of hagfishes and lampreys.

Origin of Cyclostomes

The Cyclostomata are the living agnathans (Gr., cyklos = circular + stoma = mouth), they are primitive in many respects but specialized in others. The suctorial mouth is round and ventral (thus, Cyclostomata). The buccal cavity has a robust tongue with epidermal teeth from which they rasp fish flesh. They are the only living, jawless vertebrates. They have 6-14 pairs of inner Gills of different species. Gill-chambers are round bags (so, Marsipobranchii). The skin is soft and devoid of scales, paired appendages are absent, though cartilaginous fin rays support the median fins.

General Characters of Cyclostomes

1. Long, rounded body, and like an eel.

2. Median fins with cartilaginous fine rays, but no appendages paired. Diphycercal tail.

3. Soft, smooth skin, which contains unicellular mucous glands but without scales.

4. Muscles of the trunk and tail segmented into myotomes, separated by commas.

5. Fibrous and cartilaginous Endoskeletons. Notochord lasts for life. Imperfect arches of the neural (aralia) above notochord represent rudimentary vertebrae.

6. Jaws absent (Agnatha Group).

7. Mouth ventral, oval, and suctorial. The class name Cyclostomata (Gr., cyclos = circular, stoma = mouth) is given due to circular mouth.8. Nostril is single and median.

8. No stomach digestive system. Intestine, typhlosole, with the fold.

9. Gills 5 to 16 pairs in lateral sac-like pharynx pouches, thus another class name Marsipobranchii. Gill-slits the pairs 1 to 16.

10. Heart 2 chambered with 1 auricle and 1 ventricle, anteriorly having a conus arteriosus. Lots of aortic Gill arches in the region. The present portal system in hepatic form. Leucocyte blood, and circular nucleated erythrocytes. Varying body temperature (poikilothermal).

11. Two metanephric kidneys of urinogenital papillar ducts.

12. Dorsal nerve cord with 8 to 10 pairs of cranial nerves differentiated from the brain.

13. Individual median olfactory sac. 1 or 2 semicircular canals auditory organ.

14. Spare or combined sexes. Single Gonad, large, without conduct.

15. External Fertilization. Director of growth, with an extended larval period.

Possibly the resemblance of those two classes is the product of convergent evolution. They do, however, show significant and basic morphological differences that can be attributed to their long phylogenetic separation and different habitats and habits.

Order 1: Petromyzontiformes (Gr., petros = stone; myzon = suck)

Members of this order are called lamps or lamp eels, or lamps, or pride in the sand, etc.

1. Mouth ventral, suctorial with lots of horny teeth beset with the rasping tongue.

2. Dorsal nostril. Nasohypophyseal sac ends up in a blind sac afterward, i.e. it does not communicate with the pharynx.

3. 7 pairs of Gill-pouches and Gill-slits that open under the esophagus into a separate respiratory pharynx.

4. Well developed Dorsal Fin.

5. Full branched basket.

6. Spinal nerve roots remain separate from dorsal and ventral roots.

7. Ear with 2 canals in half circle.8. Numerous, small eggs. Indirect development, with long larval stage and metamorphosis.

8. Types both of marine and of freshwater.

Examples:

The torches. Thirty species over. Petromyzon, Ichthyomyzon, Lampetra.

Order 2: Myxiniform (Gr., myxa = slime; oidea = slime type)

Order representatives are termed hagfishes. They are marine only.

1. Mouth terminal and encircled by eight small tentacles. Weak teeth. No funnel buccal.

2. Terminal Nostril. In the pharynx Nasohypophyseal, sac opens later.

3. 6 to 14 pairs of Gill-pouches and Gill-slits.

4. Feeble or absent dorsal fin.

5. Pretty poorly developed branch basket.

6. Spinal news Dorsal and ventral roots joined.

7. Ear with just 1 half-circular canal.

8. Tiny eggs, huge. Dark development.

9. All the marine animals are hagfishes.

Examples:

Hagfishes. About 15 species Myxine, Eptatretus (= Bdellostoma), Paramyxine.

I. Primitive Characters of Cyclostomes

(A) Characters Resembling those of Amphioxus

1. Jaws, exoskeleton, and paired fins are absent.

2. Notochord continuous (but with an added sheath).

3. Segmental musculature (myotomy) but with little change from head to tail.

4. Food tract ciliated straight and without extensive regional specialization.

5. Gill-slits are relatively large in number.

6. Endostyle in the larva of lamprey.

7. Gonads and no products.

Besides these, the ammocoete larva of lampreys resembles Amphioxus as follows:

1. Body similar to a fish.

2. Oral hood front to mouth.

3. The dorsal and caudal fins are continuous.

4. Digestive tract hacked.

5. The habit of feeding filters and

6. Endostyle functions in nutrition.

(B) Characters More Primitive Than In Fishes (Differences From Fishes)

1. No biting jaws, hair, true teeth, actual fin rays, girdles, ribs, stomach, spleen, and products.

2. The median dorsal fin is continuous.

3. Caudal fin diphycerclic.

4. Median single nostrils, instead of paired.

5. Incomplete or badly formed cranium.

6. No vertebrae, or vertebrae with weak growth.

7. Pancreas which is rudimentary.

8. No spinal valve in the intestine, or even a poorly formed spiral valve.9. Brain relatively small or generalised.

9. Ninth and tenth cranial nerves which are not in the cranium. Lack of medullated nerves.

10. Rather weak and poorly developed Sympathetic Nervous System.

11. Heart a rather loosely twisted S-shaped tube, with no arteriosus CONUS.

12. Poorly developed lateral organs and in isolated pits.

13. A very broad hypophysial duct, exposed to the outside and not attached to the pituitary gland.

Characteristics 

• It may be marine or clean water in the habitat.

• The frame is elongated and Eel like. It also consists of a trunk and a tail. Trunk and tail muscle tissue are segmented into myotomes separated by way of myocommata.

• Paired fins are absent. Median fins are supported by using cartilaginous fin rays.

• The pores and skin are smooth, tender and slimy. Scales are absent.

• The pharyngeal Gill consists of 6-15 pairs of Gills.

• The mouth is ventral, suctorial and round. The mouth is surrounded by tentacles.

• The digestive system lacks a belly. The oesophagus opens directly into the intestine.

• Excretion takes place by using two mesonephric kidneys.

• The heart has two chambers with one auricle and one ventricle. Blood includes R.B.C, leukocytes and poikilotherms. 

• A median nasal chamber is found in a single nostril.

• Sexual reproduction, Fertilization is external.

Order : Petromyzontiformes

Lampreys, along with hagfishes, are the best present day-day representatives of the jawless fishes. InEel-like fishes have a cartilaginous skeleton and shortage of paired fins. Often termed ‘cyclostomes’ due to their spherical mouths, lampreys and hagfishes are finely acknowledged for their unsavory feeding habits. Parasitic lampreys are suctorial feeders, the use of an oral disk (sucker) to latch onto fish and constantly ingest the blood and/or flesh in their prey.

Order: Myxini Form

Mucus production is the result of these holocrine slime glands as well as merocrine exudates from the epidermis itself. The slime glands comprise both mucous cells and thread cells, the latter being a completely unique trait in hagfishes that could make the slime stronger. Each slime gland is surrounded by connective tissue and striated muscle fibres that help exude the slime upon stimulation. The mucus contents are a protein plus and carbohydrate that binds to water and expands to form a loose jelly type.

Hagfishes commonly produce slime in reaction to being disturbed. Mucus undoubtedly serves a few anti-predator features, perhaps by making the fish too slippery to address or by clogging the Gills of a capable predator and threatening it with suffocation. 

They own 4 rudimentary hearts: a primary, three-chambered branchial or systemic heart posterior to the Gills, and three auxiliary, single-chambered hearts placed simply behind the mouth (the paired cardinal coronary heart), at midbody (the portal heart), and on the give up of the tail (the paired caudal coronary heart).

Hagfishes lack a true belly, having rather a gut that starts on the pharynx and ends at the anus, with an anterior muscular subdivision that stops water inflow.

Physical Characteristics

All hagfish species have a cartilaginous skeleton without vertebrae, true fin rays, paired fins, or scales.  The mouth is an oval slit, with four fleshy barbels, and robust teeth at the tongue. The single nose is surrounded through some other four sensory barbels that permit the hagfish to accurately scent food. Hagfishes have six to ten pairs of inner Gill pouches.

Hagfishes have approximately 150 to 200 slime glands alongside the side of the body. In case if a hagfish is attacked or handled, it will excrete small quantities of slime. The slime is used as a protection mechanism.