154 MARVELS OF FISH LIFE 
which is carried hither and thither by tides and currents, 
and is to be found in every ounce of sea water. To 
this third form of life is applied the term “ plankton,” 
and of this plankton, diatoms constitute one of the most 
a 
important elements. 
The reason why men of exceptional marine biological 
knowledge and ability, such as Henson and Herdman, 
have devoted, and are devoting, so much of their time to 
the study of plankton, is because everything ultimately 
depends upon it, and in order to understand the con- 
ditions most favourable to fish life, it is necessary first 
to have a thorough knowledge of plankton. 
Plankton consists, firstly, of those minute creatures 
which spend their whole existence drifting about in the 
sea, aS for example, the copepod, and the lowest forms 
of animal and vegetable life, such as protozoa, infusoria, 
and diatoms; and, secondly, of what is known as 
“transitory plankton.” 
The latter mainly consists of the larve of various 
marine animals which pass only the early part of 
their existence drifting in the sea, but in adult life 
settle on the bottom, or roam the ocean. 
The spat of the oyster already mentioned, while 
it drifts about, is transitory plankton. Likewise the 
minute larve of crabs, lobsters, mussels, whelks, star- 
fishes, urchins and many others. All these larve are 
entirely different in shape and appearance from the 
ultimate form which they will take in adult life, and most 
of them are ciliated, like the oyster spat. To transitory 
