Two -Layered Animals — Phylum 

 Coelenterata 



This phylum name means "hollow intestine" and is chosen because 

 members of this group all have a rather large hollow area in the middle 

 of the body which serves as an intestine. They include more than 9,000 

 very interesting species ; yet most of them are marine and the few fresh- 

 water forms are small so, to the average person, this phylum is hardly 

 known. While in swimming during a vacation at the seashore, perhaps 

 a bather's unfortunate contact with a jellyfish represents the extent of 

 familiarity with this group. 



Class — Hydrozoa 



As a type animal we will pick hydra which is not typical of the phy- 

 lum in its habitat since it is a fresh-water form, but does serve very well 

 to illustrate the life processes of the phylum. In nearly all sections of 

 the United States hydra can be found attached to submerged rocks and 

 leaves in ponds, lakes, and streams where the water does not become too 

 foul or get too warm. As usually seen, the main part of the body con- 

 sists of a slender cylinder which is attached temporarily to some solid 

 object at one end, called the base, and bears the mouth, surrounded by 

 from five to seven tentacles at the other end. The mouth is found on a 

 little elevated region between the tentacles called the hypostome, a word 

 which means "below the mouth." The whole body, including the tenta- 

 cles, will probably not be more than a half of an inch long. It is able to 

 move about by sliding slowly on its base or by reaching out and attach- 

 ing the oral end (mouth end) and then pulling the aboral end (away 

 from the mouth end or base) up and so on in the same manner as a 

 measuring worm moves. Hydra has also been seen putting the oral end 

 down and flipping the base over and attaching it and so on like an acro- 

 bat doing handsprings. If you touch one of the tentacles with a small 

 object, you will probably see the hydra jerk itself up into a little ball 

 less than a fourth the length of the extended form. This is an avoiding 

 reaction used to get away from enemies. When you compare this 



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