570 The Animal Kingdom 



zoic and are at present the dominant group of all waters. These are the 

 abundant, important forms that seem to have taken on all possible 

 shapes, appearances, and habits. 



THE CLASS AGNATHA 



The members of this class are often considered to be the most 

 primitive of all the living vertebrates inasmuch as they do not possess 

 true jaws and their vertebrae are little more than small arches over 

 the notochord. Despite these seeming limitations of structure, the 

 modern Agnatha are a successful group of animals ; in fact, at times 

 they assume a role of great economic importance. 



The hagfish and lamprey eels are the only living members of this 

 class, and the ostracoderms are ancient representatives. 



Characteristics of the Class. — The most characteristic feature of 

 the living members of the class, and the one from which the name is 

 derived, is their possession of a round, jawless oral or buccal funnel 

 which may be used as a sucker. It is surrounded by papillae or ten- 

 tacles, and has the mouth at its apex. In position, this buccal funnel 

 is ventral and anterior. The body of the animal is round and elongate 

 with a compressed tail region. There is a median dorsal fin, but no 

 paired fins. The body is scaleless and is provided with many mucous 

 glands. The poorly developed skull and visceral arches are composed 

 of cartilage, and the notochord persists throughout life. The vertebrae 

 are only small neural arches. The gills number 6 to 14; there is a 

 two-chambered heart and many aortic arches. But a single gonad is 

 present, and it is not provided with ducts. There is a urogenital papilla 

 with an opening through which the eggs or sperm and urinary wastes 

 are discharged to the outside. 



There is a single median nasal sac with endings from the olfactory 

 nerve terminating in it. The species possess paired eyes, a well-de- 

 veloped median pineal eye complete with a lens and retina, and paired 

 organs of equilibrium, each of which contains one or two semicircular 

 canals. 



Divisions of the Class Agnatha. — There are but two orders in 

 this class. They are : 



Order 1. Osteostraci. Fossil forms with expanded head region. Example: 

 the ostracoderms. 



Order 2. Cyclostomata. Elongated, eel-like animals without scales. 



