MARINE FOOD FISHES m 



big eye, the organ of hearing just above it, and the 

 absence of the mouth, which has not as yet developed. 

 Attached to the body of the fish is the large spherical 

 yolk sac, and between the yolk sac and body is seen the 

 intestine as a straight, simple tube. On the body and 

 primitive median fin are shown numerous black and 

 canary-yellow star-shaped colour cells. 



Larvae at this stage are quite transparent, and if seen 

 in a tumbler of sea-water, their eyes only can be detected. 

 The photograph shown was taken by a combined re- 

 flected and transmitted light, and thus the edges of the 

 delicate structures are lighted up, while the transparent 

 appearance of the fish is still maintained. 



The larva when first hatched floats upside down. 



Larvae of marine fishes, like the trout alevins and the 

 roach larvae, already described, live for a time upon the 

 yolk contained in the sac. At the end of a week or more 

 according to the particular fish under consideration, the 

 yolk is all absorbed. The mouth has by this time 

 become open, and the fish commences to feed on diatoms 

 and the microscopic larvae of minute creatures which 

 swarm in the sea. Diatoms are a low form of plant life, 

 consisting of but a single cell, to which further reference 

 will be made in a subsequent chapter. 



The fish is now in the post-larval stage and is still 

 transparent. Next, the bony skeleton gradually forms ; 

 bony fin rays appear in the continuous median fin, and 

 separate fins are formed; colour cells and light-reflect- 

 ing spicules are developed in the skin, and the fish, 



