MARINE FOOD FISHES III 
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. 
Larve 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. 
Larve of marine fishes, like the trout alevins and the 
roach larve, 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 larve 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, 
