526 



CYCLOSTOMATA. 



fashion. Thus the two main ways in which a body cavity arises 

 (a) from ccelom pouches of the archenteron, (b] from a splitting of solid 

 mesoblast rudiments are here combined. 



Metamorphosis of Lampreys. 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 dis- 

 tinct genus Ammoccetes, and though a Strasburg fisherman, Baldner, is 

 said to have discovered their true nature about two hundred 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 some- 

 times 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. 



Contrast between Hag and Lamprey 



HAG (Myxtne). 



LAMPREY (Petromyzori). 



Exclusively marine. 



The fin is confined to the tail. 



Numerous large glands in the com- 

 plex, slimy skin. 



Mouth with barbules, no lips, few 

 teeth. 



Skull without any roof. 



Skeletal system less developed than 

 in the lamprey. Only a hint of a 

 branchial basket. 



Cerebrum and cerebellum rudiment- 

 ary. 



Eyes hidden and rudimentary. 



Ear with one semicircular canal. 

 Nasal sac opens posteriorly into the 

 mouth cavity. 



Six pairs of gill-pouches, opening 

 directly into the gullet, less directly to 

 the exterior. 



Longitudinal ridges in the intestine. 



No urogenital sinus; one genital 

 pore. 



Ova large and oval, with attaching 

 threads ; meroblastic in Bdellostoma. 



Development unjcnown in Myxine ; 

 direct in Bdellostoma. 



In rivers and seas. 



Two unpaired dorsal fins. 



Sensory structures in the complex, 

 slimy, pigmented skin. 



No barbules (except in the larva), 

 but lips, and many teeth. 



Skull very imperfectly roofed. 



Hints of vertebral arches. 



Cartilaginous basket - work around 

 gill-pouches. 



All the usual parts of the brain are 

 distinct. 



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, opening 

 directly to the exterior, less directly 

 into the adult gullet. 



A slight spiral fold in the intestine. 



A urogenital sinus, and two genital 

 pores. 



Ova small and spherical ; holo- 

 blastic. 

 Development with metamorphosis. 



