VARIABILITY IN THE PRESENCE OF ELASTIC FIBER-LIKE STRUCTURES IN THE VENTRAL AORTA OF AGNATHANS (HAGFISHES AND LAMPREYS)

This paper investigates whether elastic fibre-like structures are pres ent in the ventral aorta of hagfishes and lampreys. Fibres, which are morphologically similar to the elastic fibres of gnathostomatous (jawe d) vertebrates, are shown to be present in the ventral aorta of the ha gfish Paramyxine atami, and also, but to a lesser extent, the dorsal a orta of this species and the ventral aorta of another hagfish, Eptatre tus stouti. The 'elastic' tissue formed irregular sheet-like aggregate s, comprising well-defined amorphous material surrounded by tubular mi crofibrils, whose diameters ranged from 10 to 15 nm. While this tissue was most abundant in the same region of the aortae as that occupied b y the elastica interna in the blood vessels of gnathostomes, it was al so found deeper in the wall of these blood vessels. Although tubular m icrofibrils were found in the ventral aorta of the hagfish Myxine glut inosa and the lamprey Geotria australis, these were never associated w ith well-defined, amorphous material. This parallels the results of pr evious studies on Myxine glutinosa and another species of lamprey, Pet romyzon marinus. Thus, the elastic fibre-like tissues found in the ven tral aortae of P. atami and E. stouti provide the first examples of su ch structures in this region in agnathan vertebrates.