MARINE ZOOLOGY 769 



In general, marine animals are adapted to one of three habits of 

 life : dwelling on the bottom, swimming at various levels, or simply 

 floating at the surface. There are three or four forms of marine 

 animals according to the position they take in the water. (1) 

 Benthos include all nonswimming bottorm-dwelling forms, and they 

 may be vagrant or sessile (stationary). Hydroid and crinoid types 

 exemplify the sessile form and urchin or starfish the vagrant. The 

 benthos may extend from the shore line to the deep sea. (2) Nekton 

 is the name of the entire group of swimming animals that are able 

 to maintain themselves in the water and do not rest on the bottom. 

 The fishes, whales, and porpoises are typical examples. (3) Plankton 

 is the collective name for plants and animals that float in the water. 

 Many of the plankton animals are nearly transparent, and the 

 smaller ones are surprisingly abundant. Because of their trans- 

 parency and size, the casual observer seldom sees this group of 

 animals. The nature of the tissue of the bodies of these animals 

 is largely of gelatinous material and the shape is either that of an 

 umbrella or of a mass with projecting processes. The composition 

 of the tissue is such that it is bulky without great weight. In fact, 

 it is largely water and has a specific gravity slightly greater than 

 water. These adaptations make it possible to float with an easy 

 distribution of weight and very little if any effort on the part of 

 the animal. A large number of Protozoa, jellyfishes, and Entomo- 

 straca (microscopic Crustacea) are typical plankton. (4) Pelagic 

 animals are all of the surface-living forms away from the shore in 

 the open sea. 



Another expression of the distribution of marine animals is by the 

 following zones. (1) Littoral zone, which is the shallow water along 

 the shore. Here the fluctuations of wave action, temperature, and 

 depth (because of tides) are extreme, and the light intensity is at 

 its maximum. Such aaiimals as certain coelenterates, worms, Crus- 

 tacea, echinoderms, and molluscs are common inhabitants of this 

 zone. It requires a rather sturdy type of body and hardy proto- 

 plasm to Avithstand the hardships of the shore and near-shore. Some 

 clams and oysters, also corals, sea urchins, and starfishes can live 

 out of the water for several hours. The majority of corals live 

 where they can depend on wave action to carry floating objects to 



