434 ' CHORD AT A. 



eyes are rudimentary, and the gill-pouches open directly 

 into the oesophagus. 



Order II. — Myxinoidei. 

 The Hag-fish {Myxine) has been described. The other 

 genus is the large Bdellosto^na which has separate external 

 branchial openings. Both are marine. 



Fig. 309. — The River- Lamprey {Petromyzon 

 Jiuviatilis) x %. 



Note the single median iin and tail, the sucker and the ventro- 

 lateral row of branchial openings behind the head. 



A smair fossil from the Devonian {Palceospondylus) has 

 some claims to be regarded as a fossil Cyclostome. 



Class II. — Pisces. 



The fishes are much more heterogenous in structure than 

 the 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 



