SPECIAL ICHTHYOLOGY - R. S. Pentyliuk - 2011

1. ORIGIN AND EVOLUTION OF CYCLOSTOMES AND FISHES. SYSTEMATICS OF EXTANT CYCLOSTOMES AND FISHES. SUBPHYLUM CRANIATA. INFRAPHYLUM AGNATHA. CLASS CYCLOSTOMATA

1.3. Subphylum Craniata. Infraphylum Agnatha. Class Cyclostomata

Infraphylum Agnatha

These are primitive, eel-like aquatic animals inhabiting marine and freshwater environments. Their Skeleton lacks Bone tissue, and the notochord persists throughout life. Paired fins and true jaws are absent. The Mouth is round. Unlike those of true Fishes, the pouch-like gills are of endodermal origin. The branchial skeleton is unique, located subcutaneously, superficial to the branchial Arteries and nerves. The Inner ear contains only two semicircular canals instead of the three found in Gnathostomes.

Agnathans are a very ancient group of vertebrates that, in past geological epochs, were numerous and diverse in both external and internal Morphology, with many species encased in bony armors. Only hagfishes (Myxini) and lampreys (Petromyzones) have survived to the present day; most zoologists unite them into a single Class, Cyclostomata. It should be noted, however, that despite their superficial resemblance, hagfishes and lampreys exhibit profound anatomical differences and are therefore frequently classified as separate classes.

Class Cyclostomata

Cyclostomes possess an elongated, scaleless, eel-like body with Skin rich in mucous glands. The mouth is shaped as a suctorial funnel supported by an annular Cartilage, bearing cornified Teeth. They possess dorsal and caudal fins. The notochord persists throughout life, functioning as a support for the nerve cord. The gills are sac-like and of endodermal origin. The branchial skeleton forms an openwork lattice or basket, undivided into separate gill arches. The Circulatory system is closed, featuring a single circuit and a two-chambered Heart filled with venous Blood. The oral funnel leads into the Esophagus, which transitions into a straight intestine devoid of bends or loops, terminating in an anus.

The Gonads are unpaired and lack specialized ducts—Gametes are released directly into the body cavity, then into the urogenital sinus, and subsequently expelled. Unlike hagfishes, lampreys possess a series of cartilages that protect the Brain laterally, forming a branchial basket and small cartilaginous projections (rudimentary vertebrae) along the notochord adjacent to the Spinal Cord. Hagfishes have three additional accessory hearts located in the HEAD, Liver, and caudal regions, which facilitate venous blood return. They inhabit both marine and freshwater bodies. The class Cyclostomata comprises about 40 species divided into two subclasses: Myxini (hagfishes, about 20 species) and Petromyzontida (lampreys, about 20 species).