SPECIALIZED AND DEGENERATED CHARACTERS OF CYCLOSTOMATES
- Body elongated, eel-like.
- Median fins with cartilaginous fin rays, but no paired appendages. Tail diphycercal.
- Skin soft, smooth, containing unicellular mucous glands but no scales. 4. Trunk and tail muscles segmented into myotomes separated by myocommata.
- Endoskeleton fibrous and cartilaginous. Notochord persists throughout life.
- Imperfect neural arches (arcualia) over notochord represent rudimentary vertebrae.
- Jaws absent (group Agnatha).
- Mouth ventral, suctorial and circular, hence the class name Cyclostomata (Gr. cyklos, circular + stoma mouth).
- Digestive system lacks a stomach. Intestine with a fold, typhlosole.
- Gills 5 to 16 pairs in lateral sac-like pouches of pharynx, hence another name of class, Marsipobranch Gill-slits 1 to 16 pairs.
- 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).
- Two mesonephric kidneys with ducts to urinogenital papilla.
Specialized characters of cyclostomes
Adult cyclostomes are too specialized or too degenciative in many respect. It is probable that many adult characteristics are adaptations for parasitic mode of feeding. Some of their specialized features are
- Suctorial mouth and buccal funnel with armature of horny spikes in lampreys for attachment to host body.
- Powerful, muscular tongue, heavily armed with sharp horny teeth serves as a rasping organ while feeding.
- Production of anticoagulants in saliva to feed on blood and body fluids of prey.
- Peculiar sac-like gill pouches located far behind head.
- Posterior position of gill openings, probably an adaptation to burrowing.
- Complete separation of ventral sac-like respiratory pharynx from dorsal oesophagus.
- Respiratory water entering gill pouches as well as leaving them through external gill openings and not through mouth which mostly remains attached to rocks or fishes for feeding.
- Large mucous glands secreting enormous quantities of mucus in hagfishes.
- Dorsal position- of single nostril high on head in lampreys.
Degenerate characters of cyclostomes
The degenerate characters of cyclostomes include :-
- Simple, cylindrical eel-like body form compared to broad fish-like shape of ostracoderms.
- Lack of bony armour or exoskeleton.
- Lack of bony endoskeleton which is cartilaginous.
- Absence of paired fins and girdles.
- Vestigial eyes covered by thick skin and muscle in hagfishes
Advanced or vertebrate Characters Of Cyclostomes
Cyclostomes are undoubtedly vertebrates as they have many advanced though simple features similar to those of fishes and higher vertebrates. These are:
- Formation of a distinct head bearing paired eyes and internal ears.
- Differentiated brain like embryonic vertebrates with several pairs of cranial nerves.
- Cranium for housing brain.
- Beginning of segmental vertebrae.
- Stratified or multilayered epidermis.
- Dorsal root ganglia on spinal nerves.
- Sympathetic nervous system.
- Lateral line organs.
- Grills primarily used for respiration and not for food collection as in Branchiostoma.
- Water enters not into pharynx by muscular activity and by ciliary activity as in Branchiostoma.
- E-shaped myotomes as in fishes.
- Presence of liver, gall bladder, bile duct, pineal and parietal eyes, pancreatic cells in midgut wall and thyroid and pituitary glands.
- Well developed circulatory system with muscular, a contractile heart.
- Blood with erythrocytes and leucocytes.
- Hepatic portal system.
- Lymphatic system
- Mesonephric kidneys.
The chordate characteristic of cyclostomes are clear-cut. They represent the most primitive members among living vertebrates. The similarity of several features of ammocoete larva of lamprey with Cephalochordata (Branchiostoma) indicates primitive relationship. But the adult cyclostomes have certain specialized as well as degenerate features as adaptations to a parasitic habit. Thus, the affinities of cyclostomes may be best described by discussing their (i) primitive, (ii) advanced, (i) specialized and (iv) degenerate characters.