506 



CYCLOSTOMATA. 



Contrast between Hag and Lamprey. 



HAG (Myxinc). 



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 IMellostoma. 



LAMPREY (Petroinyzo>i). 



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. 



Palseospondylus gunni. Under this title Dr. Traquair 

 has recently described a remarkable fossil form from the 

 Old Red Sandstone of Caithness. He speaks of it as a 

 " strange relic of early vertebrate life." 



It is a dainty little creature, somewhat tadpole-like at 

 first sight, usually under an inch in length. The following 

 characters point strongly to its affinities with Cyclostomata :- 



(1) "The skull is apparently formed of calcified cartilage, and 

 devoid of discrete ossifications." An anterior part is comparable to 

 the trabecular and palatal region of a lamprey's skull ; a posterior part 

 is comparable to the parachordal region and auditory capsules. 



(2) " There is a median opening or ring, surrounded with cirri, and 

 presumably nasal, in the front of the head " (;/. , Fig. 242). 



(3) " There are neither jaws nor limbs." 



