PHYLUM CCELENTERATA 



In the phylum Coclenterata there is laid down the fundamentals 

 of body organization which are adhered to in all higher types of 

 animals. In contrast to the sponges the representatives of this 

 phylum fall in the direct line of animal descent. Only two cell 

 layers are found in the body wall. The cells of both layers 

 are differentiated in relation to definite functions. All of the 

 coelenterates are aquatic, mostly marine, and chiefly sessile in 

 the adult state. 



Class Hydrozoa (Hydra). — Specimens of Hydra may usually 

 be found in quiet ponds or streams where they are attached by 

 one end to aquatic vegetation. 



Examine a living individual in a watch glass with a hand lens, 

 or with the low power of the compound microscope. 



Jar the watch glass and note any response to such a mechanical 

 stimulus. With the point of a needle very gently touch one of 

 the tentacles of an extended animal. Is there any evidence 

 that the stimulus is transmitted from one tentacle to another or 

 to any other part of the body? Compare the form of the body 

 when extended and contracted. If a hydra should be found 

 moving along the object to which it is attached note the type of 

 locomotion employed. Reflecting upon the hydra's behavior, 

 what deductions might be made with regard to some details of 

 organization? 



General Structure. — The body of the animal in a moderately 

 extended state is tubelike in form increasing slightly in diameter 

 toward the free end so that under certain conditions it is vaselike. 

 At the fixed end the body expands into a hasal disc for attachment. 

 A circle of slender tentacles arises from the free end. These may 

 vary in number. In the arrangement of bodily parts Hydra is 

 radially symmetrical, that is, similar parts are repeated about 

 an axis as a center. 



The region encircled by the bases of the tentacles projects 

 as a conical elevation termed the hypostome. At its apex is 

 located the mouth, which leads into the digestive cavity. The 

 mouth is usually closed and difficult to distinguish. 



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