Agnaths

Members of the Subphylum Vertebrata differ from the urochordates and cephalochordates in having the notochord replaced by a vertebral column composed of bone and/or cartilage. The vertebral column, along with the cranium, limb girdles, and limbs, make up the endoskeleton. This internal skeleton is an adaptation for efficient locomotion, as was the notochord. As you work through the vertebrate classes, enter the distinguishing characteristics of each in the results section table.

Agnaths are primitive fishes with a fibrous skeleton and an eel-like body. They lack a jaw as well as the scales and paired fins we usually associate with fish. Many species are parasitic (they attach to the outer surface of a fish with their sucker-like mouth). Rasping teeth (arranged in a whorl) cut into the host. The lamprey then sucks blood from the wound (a fish hickey?). When it is finished its bloodmeal, the fish is released. The injured fish usually dies from blood loss or infection. Although partially under control, the marine lamprey (Petromyzon marinus) has had a devastating effect on the fishing industry in the Great Lakes region. Note the single median nostril at the anterior end (between the eyes) and the seven pairs of gill apertures (making it look somewhat like an old Buick). While the lamprey may be either marine or freshwater, hagfish are exclusively marine. Unlike the lamprey, they are scavengers and are never parasitic. A hagfish's eyes are degenerate and its mouth is surrounded by eight tentacles. There may be five to 15 pairs of gill apertures, depending on the species. While we have relied on a simple classification of vertebrates, more advanced textbooks divide fish into two broad taxa: Superclass Agnatha (jawless fishes) and Superclass Gnathostomata (jawed fishes). Under this arrangement, lampreys are in the Class Petromyzontes while hagfish belong to Class Myxini. The Superclass Gnathostomata is divided into two classes: Chondrichthyes (cartilaginous fish) and Osteichthyes (bony fish).

Figure 1. Ammocoete larva (Lamprey). Ammocoete larvae share many characteristics with their invertebrate ancestors. Compare the structure of this larva with that of the lancelet.

Anterior region showing the external features of the ammocoete. Note: oral hood, buccal cavity, velar fold, brain, gill pouches, pharynx, myomeres, spinal cord, notochord.