ing suits can take us, coelenterates are very much at 

 home, even in the deepest trenches of the ocean floor. 

 There they have been reached only by the instru- 

 ments and dredges, and at lesser depths also by the 

 cameras, of specially equipped oceanographic ex- 

 peditions. The matchless English Challenger expedi- 

 tion (1872-1876) and the superbly equipped Dan- 

 ish Galathea expedition (1950-1952) dredged 

 many lovely and bizarre coelenterates that were still 

 alive and often still able to luminesce even after 

 rough upward trips from fifteen thousand feet or 

 more. 



All of the attached and cylindrical coelenterates, 

 whether they be large fleshy anemones or minute and 

 glassily transparent members of a hydroid or coral 

 colony, are called polyps, from the French word 

 poiiipe, for octopus. The term goes back to the Greek 

 for "many-footed," and refers to the agile tentacles 

 that capture and pass food to the mouth, but in a few 

 species can be used for moving about. In the jelly- 

 fishes the tentacles have been pushed out, by a 

 spreading of the body, to the rim of the umbrella. 

 Where the handle of an umbrella would be, there 

 hangs a tube with the mouth at its tip, directed down- 

 ward, in contrast with the usually upright polyps. In 

 either group, polyps or jellyfishes, some members 

 may take up the opposite stance, and this is not sup- 

 prising, for when we come to examine them closely 

 we see that the two kinds are built on the same basic 

 plan and that both fixed polyp and free-swimming 

 jellyfish types may occur as stages in the life history 

 of a single species. In both, the body is a sac with 

 only one opening, which doubles as entrance for 

 food and as exit for indigestible residues and body 

 wastes. The main body cavity ("coel") is the in- 

 testine ("enteron"), and the saclike digestive cav- 

 ity, or coelenteron, lends its name to the animals de- 

 scribed in this chapter. 



The digestive lining secretes juices that break 

 down the food into a thick broth and the fluid food 

 then circulates about the whole animal, or through 

 branches to other members of a colony. Some cells 

 lining the cavity engulf the small particles protozoan- 

 fashion, and complete the digestion of what appears 

 to be in most cases wholly animal food. 



The phylum Coelenterata at one time included 

 the sponges and the comb jellies. Then it was real- 

 ized that the main cavity of sponges is a water pas- 

 sage, not a digestive cavity, and the sponges were 

 removed. The coelenterates and the comb jellies still 

 share the same phylum in many books, for they 

 have the coelenteron in common. But they differ in 

 several important ways, notably in the absence in 

 comb jellies of the microscopic thread capsules, or 

 nematocysts, which are characteristic of coelenter- 

 ates and which they use to sting and to hold prey. 

 More will be said of these later. In dividing the coe- 



lenterates from the comb jellies it would be most log- 

 ical to discard the old phylum name and to call the 

 group the phylum Cnidaria {ciiidos = "thread") to 

 indicate the basis for distinction from the comb jel- 

 lies, which (with one possible exception) have no 

 thread capsules. Some leading students of coelenter- 

 ates have already done this, but the name "coelen- 

 terate" is so well established and so widely used 

 that it has seemed best not to change it here. 



The outer surface of the coelenterate body is a 

 protective epithelium, only one cell layer thick, so 

 that the most fragile coelenterate bodies consist only 

 of two microscopic layers of cells held together by a 

 secretion of nonliving jelly. Jellyfishes acquire bulk 

 and buoyancy by a tremendous increase in the 

 amount of secreted jelly, and in the more advanced 

 (scyphozoan) jellyfishes the jelly is invaded by cells 

 and strengthening fibers. Even more cellular ele- 

 ments take over the gelatinous layer of sea anem- 

 ones. Nevertheless, the extraordinary diversity in 

 external form that we see in coelenterates consists 

 only of superficial variations on one simple structural 

 theme. 



The phrase "spineless as a jellyfish" is meant to 

 epitomize the flabby invertebrate way of life, and 

 the animal it describes has little resemblance to the 

 firm, muscular, and speedy fish. Zoologists prefer the 

 name "medusa" for the jellyfish type. It was sug- 

 gested by a fancied resemblance to the snaky tresses 

 of the Gorgon Medusa, the mythological maiden 

 whose hair was turned into serpents that petrified 

 anyone who looked on them. Small animals are in- 

 deed paralyzed when they approach or are ap- 

 proached by coelenterates, for the heavy armature 

 of stinging thread capsules, especially on the ten- 

 tacles, makes them highly deserving of their rep- 

 utation as "the stinging nettles of the sea." The oval 

 capsules contain coiled hollow threads that can be 

 discharged when properly stimulated. There are 

 many kinds of such capsules in the group as a whole, 

 and usually more than one kind in a species. Some 

 are adhesive and used to attach the tentacles in cer- 

 tain modes of locomotion; others adhere to prey; 

 still others wind like tiny lassos around the bristles 

 of small animals and hold them fast. The largest and 

 most important kind has a thread that penetrates 

 small prey and injects a paralyzing poison. The dis- 

 charged threads of the common anemones of tem- 

 perate seashores have little effect on the relatively 

 big, horny hands of human beings. At the most, one 

 senses a sticky feeling as the tentacles adhere to a 

 probing finger. Likewise many of the commonest 

 jellyfishes of temperate seas either are quite harm- 

 less or only slightly annoy swimmers by producing 

 strong prickling sensations. This is no comfort to 

 those who tangle with cyaneas in Atlantic waters or 

 with certain tropical jellyfishes for they are lucky to 



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