58 PELAGIC ZONE. 



Protozoa and the Ccelenterata^ 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 larvae. Nearly all 

 the important larval types are pelagic, such as the different 

 kinds of echinoderm, coelenterate, crustacean and annelid 

 larvae. The blastula, planula, gastrula, trochophore, bi- 

 pinnaria, pluteus, and nauplius are all typical of this zone. 

 All these perform an ontogenetic 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 algae 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). 



