524 CYCLOSTOMATA. 



has only the very remotest claim to be called a vertebrate animal ; but since 

 some of the sharks are in a very similar predicament as regards the differ- 

 entiation of their backbones, this deficiency can scarcely be regarded as a bar 

 to the title. The heart of a lamprey is decidedly of a lower type than that 

 of a fish, since it lacks the chamber at the front end technically designated 

 the bulbus arteriosus. The digestive tract is peculiar in being quite simple 

 and straight; and the organs of reproduction discharge their products directly 

 into the general cavity of the body. The function of teeth is discharged by 

 a variable number of horny plates or cusps. 



As a family, the lampreys are distinguished from the hag-fishes by under- 

 going a metamorphosis, and by the nostril terminating behind as a blind sac 

 without perforating the palate. In the adult form rasping 



Lampreys. horny teeth cover the tongue, and teeth of similar structure 

 Family are present above and below the mouth, as well as in the 



Petromyzidw. surrounding adhesive disc, while the aperture of the nostril 

 is situated near the middle of the head, and there are well- 

 developed eyes. The gill-pouches are seven in number, and each opens by a 

 separate aperture on the side of the neck ; but internally those of each side 

 have only a single opening into the pharynx, their ducts uniting to form one 

 tube. On the other hand, the larvte, which were long regarded as distinct 

 creatures, under the name of Ammoccetes, have an undivided median fin, and 

 toothless mouth. All the members of this family lay very small eggs, and 

 the lining membrane of the intestine forms a spiral valve. The true 

 lampreys, or those included in the genus Petromywnn, appear to be four in 

 number, and have a wide range in the Northern Hemisphere, one extending 

 as far south as the west coast of Africa. In this genus the hinder of the two 

 fins on the back is continuous with the tail-fin ; the rasp-like teeth on the 

 tongue are serrated ; and on the upper side of the mouth there is either a 

 transverse horny ridge bearing one two-cusped, or two separate teeth situated 

 close together. The largest form is the sea lamprey (P. marinuni), which is 

 not unfrequently as much as three feet in length, whereas the common river 

 lamprey (P. fluviatile) does not reach a couple of feet. Lampreys live 

 chiefly or entirely on the flesh of fishes, to whose bodies they attach them- 

 selves by their adhesive discs, and then rasp off the flesh with their horny 

 dental organs. All ascend rivers for the purpose of breeding, and the larvae 

 of some kinds remain in fresh water until they develop into the adult form. 

 At the breeding season some lampreys ascend the rivers of the Northern 

 Hemisphere in vast shoals. They deposit their eggs in furrows excavated in 

 the river bottom. Another genus (Mordcicia) is represented by a species 

 found in localities as remote from one another as the coasts of Tasmania and 

 Chili, and differs in that the teeth above the mouth are arranged in two 

 three-cusped groups. In the allied genus Geotria, of which one species is 

 found in South Australia, and the second in Chili, the hinder fin on the back 

 is distinct from the tail-fin. The last genus is the little-known Exomegas, 

 from the Argentine coast. 



In this family the nostril is extended backwards to perforate the palate, 



and its front aperture is placed close to the muzzle. Two pairs of barbels 



decorate the sides of the muzzle, the mouth has no lip-like 



Hag-Fishes. structure, a single tooth occupies the middle of the palate, 



Family and the teeth on the tongue are arranged in a double comb- 



Myxinida. like series. Each gill-pouch opens by a duct of its own into 



the pharynx, and the external apertures of the pouches are 



