Creatures of the Sea 



Among the wide variety of coelenterates are 

 solitary cup polyps (shown above, twice 

 life size) . In aggregates of millions they 

 help form coral reefs lii<e this one on the 

 Barrier Reef. Each animal lives attached and 

 independently captures food with its tentacles. 



One of the biologist's first tasks is to put the two million or so 

 living species into some sort of order. In short, he must classify 

 them. Although our immediate concern is to review life in the sea 

 and to determine how it is linked with our own destinies, no such 

 discussion is possible without some notion of how marine life is 

 classified. 



First, all living things are either plant or animal. Where sea 

 plants are concerned this presents no problems. All true sea plants 

 belong to one group, the Algae, and they are of two main kinds : 

 seaweeds and single-celled plants. But more about plants later. 

 Animals are more varied and more complicated. As we have seen, 

 they can be divided into vertebrates and invertebrates; and the 

 invertebrates can be divided quite arbitrarily into higher inverte- 

 brates and lower invertebrates. Most of the many kinds of lower 

 invertebrates are sea creatures. Let us look at one branch of these 

 animals, the so-called Coelenterata. 



This name derives from two Greek words: koilos (hollow) and 

 enteron (intestine). The Coelenterata, then, are hollow-stomached 

 animals. But can there be a stomach that is not hollow? What is 

 meant is that the coelenterates are animals that are all stomach 

 except for a few accessories. The sea anemone is a good example. 

 It is nothing but a bag with an opening (the mouth) surrounded by 

 a ring of tentacles. The wall of this bag is made up of two layers of 

 cells — an inner or digestive layer and an outer or limiting layer. 

 The outer layer is, so to speak, the skin. Between the inner and 

 outer layers is a layer of jellylike substance. There is also a network 

 of nerve cells, and apart from this network the sea anemone has no 

 further nervous structure, neither brain nor sense organs. Its ten- 

 tacles are sensitive to flavors, though; when they detect the presence 

 of food they grab the substance and cram it into the mouth, and 

 the undigested remains are later ejected through the mouth. 



Life, then, appears to be a simple affair for the sea anemone, but 



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