This young jellyfish, a coelenterate, is 

 only a half inch wide. As it matures, the 

 space between its tentacles fills up and the 

 adult bell-shaped creature takes form. 



The sea pen, another coelenterate, Is a 

 collection of polyps arranged in featherlike 

 fashion on a stem rooted in the ooze. 

 When brought to the surface, some 

 glow with a pale blue light. 



reproduction, and a third group forms the stinging tentacles. The 

 original larva forms the bladder that floats on the surface and serves 

 as a carrier of the stinging, feeding, and reproduction polyps hang- 

 ing from its underside. 



Whatever their diversity in appearance, all the coelenterates have 

 certain things in common. Each is a bag with a hollow stomach, 

 the wall of which is two-layered with a jelly layer between. And no 

 matter what their size or make-up, they can be easily identified by 

 an outstanding feature they all have in common: Their tentacles 

 are armed with stinging cells. These animals sting like nettles, and 

 their stings - Uke those of the land plant - work on the principle 

 of the hypodermic syringe. 



The sponges and the coelenterates form two large groups of 

 lower invertebrates that inhabit the seas. (The biologist's term for 

 any such branch of the animal kingdom is phylum^ A third and 

 somewhat more advanced group includes the familiar starfish (or 

 sea star), the less familiar sea urchin, and the almost unknown (to 

 the layman) brittle star, sea cucumber, and sea Uly. These animals 

 have not only a stomach but also an intestine, and the waste from 

 the food that enters their mouths leaves by a second, specialized 

 opening. 



Another advance in the structure of this group of animals is 

 their three-layered body wall. The third layer marks a big step 

 forward from the two-layered sea anemone. The outer layer still 

 forms the skin and the inner layer Unes the digestive tube; but the 

 middle layer gives rise to a system of muscles. As a result, the 

 starfish can move about at will, and it can use its arms to exert a 

 great enough pull to force open a mussel or clam. Although this 

 group — Hke the coelenterates — is without a brain, each animal in 

 the group has a ring of nerve cells around its mouth and a nerve 

 cord running through each arm. 



The starfish, sea urchin, and others related to them are grouped 

 under the phylum Echinodermata, or spiny skins. Again the name 

 is something less than accurate since many of the "spiny skins" are 



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