THE DISTRIBUTION OF ANIMALS 447 



In considering the distribution of animals in the ocean we 

 may take up first the pelagic fauna, which consists of all animals 

 which swim or float in the surface waters. The animals which 

 can swim freely, such as fishes, dolphins, and whales, arc some- 

 times spoken of collectively as the nekton of the fauna, while 

 such as are more or less at the mercy of the waves and currents, 

 as the jellyfishes, for example, constitute the plankton. M 

 of the animals of the plankton are found at the surface only 

 when the waters are quiet; when the surface is ruffled they 

 sink to less disturbed regions ; some rise during the night only, 

 others only in the morning or evening. The depth to which 

 the pelagic fauna sinks to escape surface disturbances is a 

 matter of some conjecture, but it is probably not over 300 

 meters. The animals constituting the marine plankton are for 

 the most part highly transparent organisms, often very deli- 

 cately tinted with various colors; at night they are phosphores- 

 cent to a marked degree. Here are found representatives of 

 practically all the marine Protozoa; medusae, siphonophores, and 

 ctenophores are abundant. Many larvae of the Echinodermata, 

 Vermes, Crustacea, Mollusca, and Enteropneusta, are more or 

 less widely distributed. A few of the Annelida are pelagic, and 

 many of the smaller Crustacea, while the Mollusca are repre- 

 sented chiefly by the Pteropoda and Heteropoda. The unat- 

 tached Tunicata are abundant, especially in tropical waters. Of 

 the vertebrates many fishes are pelagic, and also the Cetacea, — 

 whales, dolphins, porpoises, etc. The pelagic fauna as a whole 

 occupies apparently a relatively shallow zone or stratum of 

 water, probably not over four hundred meters in depth ; as we 

 approach the dry land it becomes continuous with the shore 

 fauna, or littoral fauna, as it is technically called. 



The littoral region is the richest in plant and animal life of 

 any portion of the ocean. Here live many Protozoa, many 

 sponges, nearly all the marine Cnidaria, and nearly all the 

 Echinodermata; Turbellaria are found, the majority ol the 

 Xemertina, the Molluscoidea, the Polychaeta, and the Gephyrea. 

 Nearly all the marine Crustacea are represented here, as well 

 as some insects and many of the marine Mollusca. Balano- 

 glossus, the Tunicata, Amphioxus, and many fishes likewise 

 belong to the littoral fauna, which includes also some ol the 



