32 STRUCTURE OF THE VERTEBRATES 



tube. The naso-pituitary pouch ends blindly under the brain. 

 There are two semicircular canals in the inner ear. The eyes 

 are better developed than those of the following order. 



Myxinoidei, the hag-fishes, are ecto-parasitic and a serious 

 menace to fish. They are limited to the Pacific Ocean. This 

 order has a reduced branchial basket and fourteen gill open- 

 ings. These may open directly to the outside, or unite as a 

 single ventral outlet. The oral hood of some forms is surrounded 

 by fleshy barbels. The eyes are degenerate, and there is a single 

 semicircular canal in the ear. 



Ostracodermi. These are fossil forms which have clear-cut 

 cyclostome affinities (Stensio, 1927 and 1931). They lack jaws 

 and teeth, and have no paired appendages. Sections of the ani- 



A. Thelodus B. Birkema 



Fig. 8. Ostracoderms ; extinct cyclostomes with dermal plates. Birkenia 



is a free-swimming type with a few dorsal spines. Thelodus has numerous 



dermal denticles and is specialized for bottom living. 



mals show the following agreement with the living cyclostomes: 

 (1) round gill openings; (2) the structure of the brain and 

 spinal nerves; (3) the arrangement of the cranial circulatory 

 system; (4) two semicircular canals in the inner ear; (5) a single 

 dorsal nostril; and (6) an unconstricted notochord. The ap- 

 parently well developed eyes, the round mouth, and the lack 

 of jaws can be determined from a study of the external anatomy. 

 The ostracoderms differ from the living cyclostomes in hav- 

 ing dermal denticles, scales, or plates covering the body. The 

 more primitive genera had small bony denticles without a 

 broad base; the more specialized groups developed a broad base 

 to the denticle, and the most specialized forms had plates cov- 

 ering the body. 



Development of Petromyzon. The egg is approximately a 

 millimeter in diameter, and the early stages of development 



