526 CYCLOSTOMATA. 
fashion. Thus the two main ways in which a body cavity arises— 
(a) from ccelom pouches of the archenteron, (4) from a splitting of solid 
mesoblast rudiments—are here combined. : 
Metamorphosis of Lampreys.—The larve live wallowing in 
the sand or mud of streams, and feed on minute animals. Those of 2. 
planeri are so unlike the adults that they were once referred to a dis- 
tinct genus Avmocetes, 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. Inthe 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 Ammocetes, 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 
Hac (Myxine). 
Lamprey (Petvomyzon). 
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 unknown 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 
blastic. 
Development with metamorphosis. 
spherical; holo- 
