22 Vertebrata. 



Fishes are oviparous, that is, their .young are 

 produced from eggs, and for the most part they are 

 enormously prolific. The egg-organ of the cod some- 

 times contains over a million eggs, and some other 

 fishes are equally fruitful ; the eggs are of small size, 

 and contain very little food yolk. The majority of 

 fishes are marine ; those found in fresh water are, as 

 a rule, simpler in organisation and retain many of the 

 embryonic characters of the class. About 13,000 

 different species of fish are known, and they are 

 divided into five orders. 



CHAPTER IV. 



ORDER I, LAMPREYS j ORDER 2, SHARKS. 



14. Order 1, Marsipobranchii (Lampreys). 

 This, the most lowly organised order, consists of 

 wormlike, limbless, scaleless fishes with no lower 

 jaw, a circular suctorial mouth, a persistent noto- 

 chord, and gills in the form of lateral pouches. They 

 are also remarkable among fishes for having circular 

 blood corpuscles. 



The most familiar examples are the little fresh- 

 water lamprey, the large sea lamprey, and that curious 

 parasite the glutinous hag, which, by means of its 

 large, jagged tooth, bores its way into the body of the 

 cod, ling, or other large fish, and lives therein, feeding 

 on the juices of its prey. In all these there is but 



