442 FISHES. 



by distinct shoals of herrings. When about to spawn, the 

 herring corne 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 optimum temperature for spawning appears 

 to be 55° F. The incubation of the eggs takes from 6-8 

 days at a temperature of 53° F. to 40 days at 38° F. 



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 con- 

 tinuous 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 relationships of Fishes. 



Balfour regarded the Elasmobranchs as nearest the ancestral stock ; 

 while from hypothetical Proto-Ganoids he derived on the one hand the 

 Dipnoi, on the other hand the Ganoids, and thence the Teleosteans. 



But it must be noted that the Dipnoi are markedly separated in many 

 ways from living Ganoids. To emphasise this I have placed them next 

 Elasmobranchs. Moreover, the extinct Ganoids form a very large and 

 diverse series, which cannot be fairly appreciated by a study of the few 

 survivors. 



Gunther distinguishes the sub-class Teleostei from the sub-class 

 Pateichthyes, including under the latter the Chondropterygii (Elasmo- 

 branchs and Holocephala) and the Ganoidei (along with which Dipnoi 

 are ranked). As two other sub-classes of Fishes, he recognises the 

 Cyclostomata and the Leptocardii (Amphioxus). 



Beard proposes the following classification of Ichthyopsida, insisting 

 especially on the separateness of Dipnoi from Ganoids, and on their 

 nearness to Amphibians : — 



( Marsipobranchii (Cyclostomata). 

 GanodichthyidEe. X Ganoidei. 

 ( Teleqstei. 



Selachodichthyidse. \ |^'^*'lv ,, . , j Dipnoi. 



i Pneumichthyid^e. | A^hibia. 



