434 CHORDATA. 
eyes are rudimentary, and the gill-pouches open directly 
into the cesophagus. 
OrvER II.—Myxinoider. 
The Hag-fish (AZyr’ne) has been described. The other 
genus is the large Bde/ostoma which has separate external 
branchial openings. Both are marine. 
> 
Fig. 309.—THE River-LAMPREY (Letromyzon 
fluviatilis) x %. 
— 
Note the single median fin and tail, the sucker and the ventro- 
lateral row of branchial openings behind the head. 
A small fossil from the Devonian (Paleospondylus) has 
some claims to be regarded as a fossil Cyclostome. 
Cxiass II.—Pisces. 
The fishes are much more heterogenous in structure than 
thé last class. They have paired fins and median fins sup- 
ported on fin-rays. The median fin may be perfectly con- 
tinuous, with dorsal, ventral and caudal portions, as in the 
