94 UNDERWATER GUIDE TO MARINE LIFE 



fibers, but it does allow coordinated muscular action so that these animals can 

 perform simple coordinated movement, such as contraction of the animal as a 

 whole or movement of tentacles. 



Basically, there are two types of body form in this group, the sessile polyp 

 and the free-swimming medusa. Actually, these two are merelv two forms of 

 the same thing, the polyp being long of body with the mouth up and the medusa 

 being short and turned over with the mouth down (fig. 26). The primitive 

 species have both of these forms present in an alternation of generations, a name 

 given for the extraordinary breeding method in which the polyp reproduces 

 asexually by budding to produce medusas and the male and female medusas 

 reproduce sexually by union of egg and sperm to produce more polyps. The 

 whole phylum is divided into classes on the basis of which one of these 

 generations, the polyp or the medusa stage, has become dominant. Because 

 medusas are active and free-swimming, they show greater responsiveness to 

 environment than do polyps. 



All of the species of coelenterates are united by the possession of two charac- 

 teristic features. First, they are radially symmetrical; in other words, the top of 

 the body differs from the bottom, but all the sides are the same. Second, most 

 have unique stinging structures called "nematocysts" (_fig. 26), which, in 

 response to a touch on a trigger (cnidocil), are able to shoot out a piercing 

 barbed thread, injecting poison into the object with which it comes into 

 contact. The poisonous thread acts like a fish line, holding the prey to the 

 tentacle. One nematocyst by itself would not be very potent in either poisoning 

 or holding prey, but as the prey struggles, it becomes more and more enmeshed 

 in nematocysts and is subdued shortly. Some nematocysts are large and powerful 

 enough to cause serious poisoning to man, particularly those of the Portuguese 

 man-of-war and the lion's-mane jellvfish. These cells are the organs of offense 

 and defense of coelenterates. 



Coelenterates are found in great numbers all over the world in shallow to 

 deep waters. Some eat fishes, some feed by filtering plankton, and some have 

 food manufactured for them by other organisms living in them (symbiotic 

 zooxanthellae or zoochlorellae). Some of them are verv striking because of the 

 beauty of their skeletons, as in corals, or the beauty of the iridescent blues, 

 yellows, and reds of their bodies, as in anemones and jellyfishes. As unattractive 

 as jellyfishes may be lying dead on a beach, in the water they are very beautiful 

 as they move by gentle, rhythmic pulsation of their translucent bells, their 

 tentacles streaming gracefully behind. But the diver should not be too carried 

 away by the beauties he finds in this phylum. While most are harmless, many 

 can produce painful stings. 



Hydrozoans: Class Hydrozoa — Figure 27 



This is a group of great variation, having species that show alternation of 

 generations between polyp and medusa stages and those that are exclusively 

 polyps or medusas with no alternation of generations. There are twenty-seven 

 hundred species of hydrozoans in all waters of the world. 



The hydroids are the most typical of the hydrozoans, showing typical 

 alternation of generations. Most of them arc small, colonial, plantlike, branching, 

 and have a skeleton of a stiff, horny material (though some are without 



