NOTES ON AGNATHA

Division Agnatha (Greek: a — without; gnathos — jaw) comprises the jawless fishes, which evolved in the Ordovician period. These most primitive vertebrates dominated the realm of fishes for some tens of millions of years and then they disappeared almost completely. Very little is known about the ancestral agnathans. Fossil remains of only one group, the Class Ostracodermi, are now available. A limited number of species survive as evolutionary relicts.

Salient Features

·         The salient features of Agnatha are:

·         Jaws, teeth, paired appendages and genital ducts are absent

·         The roof of cranium is membranous

·         Vertebral column consists of persistent notochord. Neural tube is fibrous.

·         Nostril is single and median.

·         Number of gill slits ranges from 7-14 pairs.

·         A hypophysial sac is present

·         Internal ear has one or two semicircular canals

·         Kidneys are elongated. The archinephric ducts are also long.

·         Pineal apparatusis well developed.

·         Larval stage is microphagous and possesses endostyle.

Division Agnatha is divided into two classes – Cyclostomata (comprising hagfishes and Iampreys) and Ostracodermi (comprising theextinct forms)(In the other system of classification, Superclass Agnatha into three classes - Myxini comprising the hagfishes, Cephalaspidomorphi including the lampreys and a class of extinct forms, the Ostracodermi).