Superclass Agnatha: Classification, Characteristics, Examples microbiologystudy

The term “Agnatha” is derived from Greek, where “a” means without, and “gnathos” means jaw.  This Superclass of Vertebrae lacks a true jaw and paired fins, distinguishing them from the other Superclass known to possess a jaw (Gnathostomata). Agnatha can be categorized into Class Ostracodermi and Cyclostomata.

Class Ostracodermi

1. Ostracoderms were the earliest record of vertebrates appearing on fossils in the rocks of the late Paleozoic era, approximately 500 million years ago.
2. They are considered the ancestors of all vertebrates present.
3. They were known to inhabit marine as well as freshwater ecosystems. 
4. These organisms with fishlife bodies with dorso-ventrally flattened bodies exhibited no pelvic or pectoral fins but only median fins.
5. Ostracoderms were known for possessing bony dermal plates for a defensive property. 
6. They had a complex sensory system, including large lateral eyes, a pineal eye, and a single nostril, with limited internal anatomical information known.
7. They are notably known to be lack of jaws. Instead, they had small, ventrally located mouths with sluggish bottom-dwelling filter-feeding capability.
8. They had an internal ear with two semicircular canals that maintained balance and orientation in the water.
9. The presence of the lateral line system may have helped them in detecting water currents and movement in the environment. 
10. Furthermore, Ostracoderms can be divided into two sub-classes and five orders, as follows:

Subclass I: Monorhina

Order 1: Euphanerida (Example: Jamoytius kerwoodi)

Order 2:Anaspida (Example: Lasanius, Birkenia)

Order 3: Osteostraci (Example: Cephalaspis)

Subclass II: Diplorhina

Order 1: Heterostraci (Example: Drepanaspis)

Order 2: Coelolepida (Example: Coelolepis)

Class Cyclostomata 

Cyclostomata is a class of jawless fish under Agnatha that includes two Orders: Petromyzontiformes and Myxiniformes. The name “Cyclostomata” is derived from Greek, where “Cyclo” means round and “stomata” means mouth. These organisms are known to possess circular, sucker-like mouths. Here an elaborated characteristics of the class Cyclostomata and its Orders

1. Body form – Cyclostomes are known for their elongated, eel-like appearance. They have a streamlined body capable of moving through water and attaching to substrates.
2. Fins and Appendages – They possess median fins supported by cartilaginous fin rays; however, they lack pectoral and pelvic fins. Their tail is diphycercal (vertebral column extending straight to tail fin, creating a symmetrical, round appearance).
3. Skin and Glandular Features – They have soft skin without scale and secrete a thick layer of mucus, playing role in protection. 
4. Musculature – Musculature is essential for undulating movements during swimming. Cyclostomes have their trunk and tail organized into myotomes, and the segmentation is separated by blocks of muscles called myocommata. 
5. Endoskeleton – They possess a fibrous and cartilaginous skeleton with notochord persisting throughout life for structural support.  A rudimentary structure called arcualia is present, partially protecting the nerve cord.
6. Lack of Jaw – As a part of Agnatha, Cyclostomes do not have a jaw, and their mouth is adapted to possess a suctorial feeding mechanism.
7. Digestive System –  Cycloctomes lack a stomach, and their digestive system is simple, where the intestine consists of a spiral fold called typhlosole, assisting increased digestion and absorption.
8. Respiratory System – Cyclostomes possess 5 to 16 pairs of gills located in sac-like pouches along the later sides of the pharynx. This feature is termed as Marsipobranchii (pouch gill). Furthermore, they also possess multiple gill slits ranging from 1 to 16 pairs that depend on species.
9. Circulatory System – The heart is simple with two chambers. One is an atrium, and the other is a ventricle. They lack a renal portal system, and cyclostomes are poikilothermic (cold-blooded).
10. Excretory System – Cyclostomes have two mesonephric kidneys for filtering waste from the body. A structure, the Urinogenital papilla, is responsible for both excretion and reproduction.
11. Nervous System –  Cyclostomes have 8 to 10 pairs of cranial nerves for sensory and motor functions. Furthermore, the dorsal nerve cord is differentiated into a simple brain.
12. Sensory Organ – They possess a single median olfactory sac and a median nostril for detecting chemical signals. Furthermore, 1 or 2 semicircular canals in the internal ear are for balance and spatial orientation.
13. Reproductive System – Cyclostomes have a single, large gonad that produces either eggs or sperm. There is no product. The gametes are released into the body cavity through the urinogenital papilla. Hence, fertilization is external.
14. Development – Cyclostomes possess a larval stage where the larva undergoes metamorphosis to become adult. 

References

1. Brittanica, T.E. (2024). Agnatha: Evolution and classification. In Britannica. Available at: https://www.britannica.com/animal/agnathan/Evolution-and-classification.
2. Hardisty, M.W. (1982). Lampreys: Life Without Jaws. Springer-Verlag.
3. Kardong, K.V. (2018).Vertebrates: Comparative Anatomy, Function, Evolution (8th ed.). McGraw–Hill Education.
4. Kotpal, R.L. (2016). Modern Textbook of Zoology: Vertebrates | Animal Diversity – II. Rastogi Publications (pp. 111-131)
5. OpenStax, R. (2024). Fishes. In General Biology (2nd ed.). OpenStax. Available at https://openstax.org/details/books/biology-2e/
   Rastogi, V. B. (2024). Textbook of Zoology: Vertebrates, 8/E, Volume 2 (8th ed.). MedTech Science Press.