In this article we will discuss about:- 1. Origin of Cyclostomes 2. General Characters of Cyclostomes 3. Classification 4. Representative Types 5. Affinities.
Origin of Cyclostomes:
The Cyclostomata (Gr., cyklos = circular + stoma = mouth) are the living agnathans, they are primitive in many respects, but specialised in others. They are a modified and degenerate offshoot of the primitive vertebrate stalk, arose in the Devonian. In the adult state they are parasitic or scavengers on fishes. They have round bodies with laterally compressed or diphycercal tail. They resemble eels superficially.
The suctorial mouth is ventral and round (hence, Cyclostomata). Buccal cavity has a muscular tongue bearing epidermal teeth by which they rasp the flesh of fishes. They are the only living vertebrates without jaws. They have 6-14 pairs of internal gills in different species. Gill-chambers are round pouches (hence, Marsipobranchii). The skin is soft and devoid of scales, paired appendages are absent, though median fins are present and supported by cartilaginous fin rays.
Exoskeleton is lacking. Endoskeleton is cartilaginous with no bones and the vertebral column is primitive. There is a single median nostril, and only one or two semicircular canals are present in each auditory organ. Heart is enclosed in cartilage derived from the hinder visceral arch. They have no spleen. Cyclostomata include lampreys and hagfishes. Its two order are not closely related, because they have evolved from different orders of ostracoderms.
General Characters of Cyclostomes:
1. Body long, rounded and eel-like.
2. Median fins with cartilaginous fin rays, but no paired appendages. Tail diphycercal.
3. Skin soft, smooth, containing unicellular mucous glands but without scales.
4. Trunk and tail muscles segmented into myotomes separated by mycommata.
5. Endoskeleton fibrous and cartilaginous. Notochord persist throughout life. Imperfect neural arches (arcualia) over notochord represent rudimentary vertebrae.
6. Jaws absent (Group Agnatha).
7. Mouth ventral, suctorial and circular. Due to circular mouth, the class name Cyclostomata (Gr., cyclos = circular, stoma = mouth).
8. Nostril is single and median.
9. Digestive system without stomach. Intestine with a fold, typhlosole.
10. Gills 5 to 16 pairs in lateral sac-like pouches of pharynx, hence, another name of class Marsipobranchii. Gill-slits 1 to 16 pairs.
11. Heart 2 chambered with 1 auricle and 1 ventricle, with a conus arteriosus anteriorly. Many aortic arches in gill region. Hepatic portal system present. Blood with leucocytes and nucleated circular erythrocytes. Body temperature variable (poikilothermal).
12. Two metanephric kidneys with ducts to urinogenital papilla.
13. Dorsal nerve cord with differentiated brain 8 to 10 pairs of cranial nerves.
14. Single median olfactory sac. Auditory organ with 1 or 2 semicircular canals.
15. Sexes separate or united. Gonad single, large, without gonoduct.
16. Fertilisation external. Development director with a prolonged larval stage.
Classification of Cyclostomes:
About 50 species of cyclostomes are recognised. They belong to two major divisions (Petromyzontiformes and Myxiniformes). They are termed variously as subclasses, orders or families. Because they possess a round jawless mouth, they are combined in the class Cyclostomata.
The similarity of these two groups is probably the result of convergent evolution. However, they show important and basic morphological differences which can be attributed to their long phylogenetic separation and different habits and habitats.
Order 1: Petromyzontiformes (Gr., petros = stone; myzon = suck):
Members of this order are called lampreys or lamper eels or lamperns or sand pride, etc.
1. Mouth ventral, suctorial with rasping tongue beset with many horny teeth.
2. Nostril dorsal. Nasohypophyseal sac terminates posteriorly in a blind sac, i.e., it does not communicate with the pharynx.
3. 7 pairs of gill-pouches and gill-slits which open into a separate respiratory pharynx below the oesophagus.
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.
Lampreys. Over 30 species. Petromyzon, Lampetra, Ichthyomyzon.
Order 2: Myxiniformes (Gr., myxa = slime; oidea = type of):
Representatives of order are called hagfishes. They are exclusively marine.
1. Mouth terminal and surrounded by 8 small tentades. Teeth few. No buccal funnel.
2. Nostril terminal. Nasohypophyseal sac opens posteriorly in the pharynx.
3. Gill-pouches and gill-slits 6 to 14 pairs.
4. Dorsal fin feeble or absent.
5. Branchial basket poorly developed.
6. Dorsal and ventral roots of spinal newes united.
7. Ear with only 1 semicircular canal.
8. Eggs few, large. Development dark.
9. Hagfishes are all marine species.
Hagfishes. About 15 species Myxine, Eptatretus (= Bdellostoma), Paramyxine.
Representative Types of Cyclostomata:
Members belonging to the order Myxiniformes are commonly known as hagfishes. They are exclusively marine. Myxine (Fig. 12.1) is found buried in the sea bottom. Myxine has a wide distribution along sea coasts of both Atlantic and pacific Oceans, occurring in the waters of Northern Europe, North Atlantic, America, Chili, Africa and Japan, etc. Body is eel-like, measuring about 2 feet (50-60 cm) 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. Branchial 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 vestigial or degenerated due to bottom dwelling habit.
Six pairs of gills which do not open separately to the outside but open by a single pair of external gill openings. Single median ventral fin runs from about the middle of the ventral surface extending around the tail region. Large mucus glands are present opening by mucous pores along both the sides of the body and secrete mucus.
Hermaphroditic 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. Hagfishes do not migrate to freshwater to spawn. Development is direct.
2. Eptatretus (= Bdellostoma):
Bdellostoma (Fig. 12.2) 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 smooth integument without scales. It is about one metre in length. 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 to the exterior by round pores. The opening of pharyngocutaneous duct lies behind the last gill-slit of left side.
It opens into the pharynx. The median fin is confined to the caudal region. Large slime or mucous glands are present opening by mucous pores on both sides of the body. Hermaphroditic. Single ovotestis, the anterior 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.
Affinities of Cyclostomes:
Cyclostomes are evidently chordate. They are primitive vertebrates. Their ammocoete larva resembles in most characters with that of Branchiostoma, which shows primitive relationship. Whereas, adult cyclostomes possess specialised as well as degenerate characteristics.
I. Primitive Characters of Cyclostomes:
(A) Characters Resembling those of Amphioxus:
1. Absence of jaws, exoskeleton and paired fins.
2. Continuous notochord (but with an added sheath).
3. Segmental musculature (myotomes) but little modified from head to tail.
4. Ciliated alimentary tract straight and without much regional specialisation.
5. Relatively large numbers of gill-slits.
6. Endostyle in lamprey larva.
7. Gonads without gonoducts.
Besides these, the ammocoete larva of lampreys resembles Amphioxus as follows:
1. Fish-like body.
2. Oral hood anterior to mouth.
3. Continuous dorsal and caudal fins.
4. Ciliated digestive tract.
5. Filter feeding habit and
6. Endostyle functions in feeding.
(B) Characters more Primitive than in Fishes (Differences from Fishes):
1. No biting jaws, Scales, true teeth, true fin rays, girdles, ribs, stomach, spleen and gonoducts.
2. Continuous median dorsal fin.
3. Diphycercal caudal fin.
4. Single median nostril, instead of paired.
5. Cranium incomplete or poorly developed.
6. No vertebrae or poorly developed vertebrae.
7. Rudimentary pancreas.
8. No spinal valve, or only slightly developed spiral valve, in intestine.
9. Brain relatively small or generalised.
10. Ninth and tenth cranial nerves not enclosed in the cranium. Absence of medullated nerves.
11. Sympathetic nervous system very primitive and poorly developed.
12. Heart a rather loosely twisted S-shaped tube without conus arteriosus.
13. Lateral line organs poorly developed and in isolated pits.
14. Hypophysial duct rather large, open to the exterior and not connected with the pituitary body.
(C) Affinities with Ostracoderms:
Cephalaspids and anaspids are fossil agnathans that show similarity to the modem cyclostomes than pteraspids (Neterostraci or Pteraspida). Ostracoderms belonging to Ordovician are the Oldest fossil vertebrates. They were abundant in Silurian period and become extinct in Devonian. Probably they were the forerunners of higher fishes.
The fossil ostracoderms and present cyclostomes are kept in Agnatha due to the following similarities:
1. Presence of a median pineal eye.
2. Presence of velar pump-like lamprey.
3. Endostyle sac-like.
4. Single nasal opening though nasal sacs are paired.
5. Brain is like that of lamprey.
6. Two semicircular canals in the ear.
7. Dorsal and ventral nerve roots separate up to 15.
8. Pairs of branchial pouches surrounded by a branchial basket.
9. Continuous uncostricted notochord.
10. Absence of jaws.
Stensio holds that the pteraspids have given rise to the myxinoids, and the cephalaspids to the lampreys. The Agnatha were the first animals of the chordate type to become large. They feed on detritus on the bottom. The lampreys and hagfishes have been derived from early Agnatha by the evolution of a sucking mouth, perhaps with loss of the bony skeleton and paired limbs.
II. Specialised Characters:
1. Sucking mouth buccal funnel and horny teeth (in lampreys) for attachment.
2. Powerful tongue armed with sharp horny teeth works as a rasping organ.
3. Secretion of anticoagulants in saliva to feed on blood of prey.
4. Sac-like gill-pouches. Located far behind head. It is probably an adaptation to burrowing.
5. Complete separation of lower sac-like respiratory pharynx from upper digestive pharynx.
6. Water entering gill-pouches and also leaving them through external gill openings and not through mouth.
7. In hagfishes, presence of large mucus secreting mucous glands.
8. Dorsal position of nostril on head in lampreys.
9. Large, heavy-yolked egg, with meroblastic cleavage and no larval stage in hags.
III. Degenerate Characters:
1. Simple elongated eel-like body more marked in hags. Whereas ostracoderms body is broad fish-like.
2. Rudimentary paired eyes covered by thick skin in hags.
3. Lack of exoskeleton or bony armour.
4. Absence of paired fins and girdles.
5. Lack of ossification of endoskeleton, it is cartilaginous.
6. Reduced liver and lack of gall-bladder and bile-duct in adult lamprey.