REPRODUCTIVE SYSTEM. 



473 



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 Ammoccetes, and though a S trass- 

 burg 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 semicircular, 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., Mcrdacia and Geotria, from the 

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

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

 (Silurian) strata have been interpreted as teeth of lampreys or hags. 



HAG (Myxine). 



CYCLOSTOMATA. 



LAMPREY (Petromyzon}, 



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 

 han in the lamprey. 



Eyes hidden and rudimentary. 



Ear with one semicircular 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 intes- 

 tine. 



Ova large and oval, with attach- 

 ing threads. 



(Development unknown). 



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 

 gill-pouches. 



Eyes hidden and retarded in the 

 larva, exposed and complete in adult. 

 Ear with two semicircular canals. 

 Nasal sac ends blindly. 



Seven pairs of gill pouches, open- 

 ing directly to the exterior, less 

 directly into the adult gullet. 



A slight spiral fold in the intestine. 



Ova small and spherical. 

 Development with metamorphosis. 



