THE ORDERS OF FISHES. 55 



each sex there is a single external opening behind the anus. 

 The number of mature eggs spawned at one time by the 

 female has been variously estimated at from 10,000 to 

 30,000. 



In British waters there is a spring as well as an autumn spawning 

 probably, however, by distinct shoals of herrings. When about to 

 spawn, the herring come near the coasts into water of from ten to twenty 

 fathoms depth. While the eggs are being shed by the females, the 

 spermatic fluid is passed into the water by the males, and the eggs are 

 thus fertilised before reaching the bottom, where they adhere to stones, 

 zoophytes, and even crustaceans. The hatching of the eggs takes 

 from 8 to 40 days, according to the temperature. 



Development. The young herring on emerging from the egg has the 

 yolk sac attached ; its skeleton is rudimentary ; it has no scales ; the 

 ventral fins are undeveloped ; one continuous fin passes along the back, 

 round the tail to the anus. A month after hatching, the larva is about 

 two-thirds of an inch long, and has absorbed all its yolk. About the 

 third month the scales appear, and though only two inches in length, 

 the form is then that of the adult. Growth continues at the rate of less 

 than half an inch per month, and at the end of eighteen months the 

 herring is sexually mature. 



Closely allied to the herring are the sprat, the shad, and the pilchard. 

 Thames "whitebait" are herring not six months old. 



THE ORDERS OF FISHES. 



(See Table, pp. 518-19). 



Order I. ELASMOBRANCHII Cartilaginous Fishes. 



Synonyms. Selachii. Plagiostomata (with transverse 

 ventral mouth). 



Sharks and skates represent the two distinct types included 

 in this order. They are voracious carnivorous fishes. The 

 scales are " skin teeth." There is no cover over the (5-7) 

 gill apertures ; anterior to these there is often a spiracle, 

 the first gill cleft with a rudimentary gill. The fins are 

 large. The skeleton is mostly cartilaginous. The tail is 

 asymmetrical or heterocercal. The mouth extends trans- 

 versely on the under side of the head. The nostrils are also 

 ventral. A spiral fold extends along the internal wall of the 

 large intestine. Into the terminal chamber (or cloaca) of 

 the gut, the genital and urinary ducts also open. The 

 ventricle of the heart has an anterior auxiliary region a 

 contractile conus arteriosus. The males are provided with 

 copulatory modifications of the hind limb, known as claspers. 



