Division Agnathostomata: Class Cyclostomata

Division Agnathostomata includes the most primitive living vertebrates — animals without jaws. Among living vertebrates, it contains only one class, Cyclostomata (Greek cyclos = circular, stoma = mouth), which includes animals such as Petromyzon (lamprey) and Myxine (hagfish).

Cyclostomes are jawless, eel-like animals with a soft, smooth skin containing unicellular mucus glands but no scales. Median fins are present, but paired fins (like those of true fishes) are absent. They are ectoparasites, attaching to other fish using a sucking, circular mouth. The skeleton — both cranium and vertebral column — is made entirely of cartilage.

The digestive system lacks a stomach. Respiration occurs through 6 to 15 pairs of gill slits, which lack an operculum (gill cover). The heart is two-chambered, with a single auricle and a single ventricle. A single, large gonad is present, without a separate gonoduct to carry gametes out. Fertilisation is external. …