58 FELAGIC ZONE. 
Protozoa and the Calenterata, though there are a consider- 
able number of Crustacea and fishes and a few representa- 
tives of the Mollusca and Tunicata. It is very important 
to notice that a great number, if not the majority, of the 
neritic types pass the earlier part of their career in the 
pelagic zone. Many have pelagic eggs, as, for example, 
most fishes, Amphioxus, and a number of Crustacea and 
worms, whilst still more have pelagic larva. Nearly all 
the important larval types are pelagic, such as the different 
kinds of echinoderm, ccelenterate, crustacean and annelid 
larve. The blastula, planula, gastrula, trochophore, bi- 
pinnaria, pluteus and nauplius are all typical of this zone. 
All these perform an onfogenetic migration from shore to 
pelagic water and back again, and the most natural inference 
is that this is a repetition of a past phylogenetic migra- 
tion when the neritic zone was peopled from the open sea. 
Throughout the pelagic zone are countless myriads of 
microscopic algeze which form the chief food-basis of the 
animal life. Hence the food-supply, although of small in- 
dividual dimensions, is inexhaustible, evenly diffused, and 
easy of capture. Upon these organisms feed the multitudes 
of Radiolaria and Foraminifera and swarms of Copepod 
Crustacea. The smaller pelagic animals exhibit a perfect 
translucency, the only means of concealment from foes in a 
region suffused with light. The larger types, of too great a 
bulk for this device (such as dolphins, mackerel, &c.), have 
the dorsal part of the body of a sea-green or dark-bluish tint 
and the ventral part a pearly-white. 
We may note that the majority of pelagic organisms 
have pelagic eggs and have no connection at any time 
of their life with the neritic region. Some of the jelly- 
fishes form a remarkable exception to this rule. 
Pelagic organisms may be divided into two great groups, 
according to their habits, often called the Plankton and 
Nekton. These two rather cumbersome words merely mean 
the floating and swimming forms respectively. 
The Plankton are the lowest and simplest types, and 
either drift passively or sustain themselves actively in the 
water. Many have air-vesicles to render themselves buoyant 
and the majority show axial symmetry (Cydippe and Aurelia 
are examples). 
