STRUCTURE OF THE LAMPREY, 



409 



The time between fertilisation and the hatching of the larva, or 

 Ammoccetes^ varies with the temperature, being seventeen days in North 

 Germany, but only eight at Naples. 



The larvae live wallowing in the sand or mud of streams, and feed on 

 minute animals. Those of P, planeri are so unlike the adults that they 

 were once referred to a distinct genus AmmocateSy and though a Strassburg 

 fisherman, Baldner, is said to have discovered their true nature about 

 200 years ago, the fact was overlooked until August Miiller traced the 

 metamorphosis in 1856, In the small lampern the change to the adult 

 state is sometimes postponed until the autumn of the fourth or fifth year, 

 when it completes itself rapidly. Less is known about the metamorphosis 

 of the other species. 



In the Ammoccetes or larva before metamorphosis the head is small ; 

 the dorsal fin is continuous, the upper lip is semi-circular, the lower lip is 

 small and separate ; the mouth is toothless, and not suctorial ; the brain 

 is long and narrow ; the eyes are half-made, and hidden beneath the 

 skin ; the future gullet, as distinguished from the respiratory tube, is not 

 yet developed. 



Lampreys are distributed in the rivers and seas of north and south 

 temperate regions. They are often used as food. Besides Petromyzon 

 there are several related genera, e.g,, Mordacia and Geotria, from the 

 coasts of Chili and Australia, and Ickthyomyzon, from the west coast of 

 N. America. Certain structures called "conodonts" from very ancient 

 (Silurian) strata are interpreted by some as teeth of lampreys or hags. 



Cyclostomata. 



Hag {Myxine). 



Exclusively marine. 



The fin is confined to the tail. 



Numerous large glands in the complex, 

 slimy skin. 



Mouth with barbules, no lips, few 

 teeth. 



Skull without any roof. 

 Skeletal system less developed than in 

 the lamprey. 



Eyes hidden and rudimentary. 



Ear with one semi-circular canal. 

 Nasal sac opens posteriorly into the 

 pharynx. 



Six pairs of gill-pouches, opening directly 

 into the gullet, less directly to the exterior. 



Longitudinal ridges in the intestine. 



Ova large and oval, with attaching 

 threads. 

 (Development unknown). 



Lampkev {Petromyzon). 



In rivers and seas. 

 Two unpaired dorsal fins. 

 Sensory structures in the complex, slimy, 

 pigmented skin. 

 No barbules, but lips, and many teeth. 



Skull very imperfectly roofed. 

 Hints of vertebral arches. 

 Cartilaginous basket-work around gullet. 



Eyes hidden and retarded in the larva, 

 exposed and complete in adult. 

 Ear with two semi-circular canals. 

 Nasal sac ends blindly. 



Seven pairs of gill-pouches, opening 

 directly to the exterior, less directly into 

 the adult gullet. 



A slight spiral fold in the intestine. 



Ova small and circular. 



Development with metamorphosis. 



