20 The Under-Water World 



tolerable equilibrium by means of spines 

 and other prominencies which serve to 

 counterpoise their disproportionate heads 

 and tails. Hundreds of these baby crusta- 

 ceans have been found in a single cubic 

 centimetre of sea-water. Some of the 

 most arresting examples of dissimilarity 

 between the larvae and parental forms 

 have been brought to light amongst 

 the great tribe of fishes. The fishes 

 have naturally a large claim upon our 

 attention by reason of their economic 

 importance, and much work is being 

 done in this form of scientific detective 

 work by fishery commissions in all parts 

 of the world. The earliest known fishes 

 were toothless, and provided with mere 

 " key-hole " mouths, fitted for the pur- 

 pose of obtaining food by suction. In 

 the young of many fishes that, when 

 adult, are provided with powerful jaws 

 of enormous gape, the mouth is but 

 feebly developed, and the tiny creatures 



