I] 



THE CATEGORIES OF LIFE 



23 



in the upper levels of the sea-water. Some fishes 

 indeed, are viviparous thus some of the dogfishes, 

 rays, and sharks produce their young alive, and some 

 others lay large eggs which incubate while lying at 

 the sea-bottom in sheltered places. But the majority 



8 



Fig. 2. Plankton: Larvae. 1, Crab; 2, Fish-egg; 3, Sea-urchin; 

 4, Barnacle; 5, Fish; 6, Mussel; 7, Copepod; 8, Worm. All 

 magnified. 



of fishes produce young which live in the plankton as 

 microscopic drifting animals, for a period of their lives. 

 Many of the Crustacea also breed in this way, and their 

 larvae are planktonic ; so are most of the very young 



