446 



GENERAL ZOOLOGY 



the mouth which, with the rasping lingual teeth on the end of 

 the tongue, is used in boring into the bodies of fishes. There 

 are from 6 to 14 pairs of gills, which in some have but a single 

 orifice on each side of the body. The hypophysis connects 

 with the nasal sac in front and with the pharynx behind, 

 forming a channel through which water enters and passes out 

 through the gill slits behind. All are marine. 



Examples: Myxine glutinosa, a North Atlantic species, 

 reaching a length of 2 ft., has six pairs of gills with a common 



Fig. 267. — A, Petromyzon marinus. B, median section of anterior end of body. 

 b, brain; be, buccal cavity; dm, dorsal musculature; g, internal openings of gill 

 pouches; h, hypophysis; I, lingual cartilage; n, nasal opening; nc, notochord; o, 

 olfactory sac; oe., esophagus; of, oral funnel; pm, protractor muscle of tongue; 

 rm, retractor muscle of tongue; rt, respiratory tract; sc, spinal cord; t, tongue. 



aperture on each side of the body. It is thought to be pro- 

 tandrous hermaphroditic (male and female alternately). The 

 pronephros of the embryo is retained in the adult. Bdellostoma 

 stouti, of the California coast, has 6 to 14 pairs of gills, each with 

 a separate opening to the outside. A peculiar fact is that the 

 number of gills may differ on the two sides of the body of the 

 same individual. 

 Order 2. Hyperoartia. Lampreys. The nasal sac opens 

 posteriorly into the hypophysis, but the latter does not com- 

 municate with the pharynx. The mouth is at the apex of a 

 cornified buccal funnel, armed with sharp cuticular teeth 



