X. PHYLUM COELENTERATA 



A. HYDRA 



i. General structure. Note the appearance of the animals in the culture. 

 They are sessile, that is, attached to the glass, plants, etc., by one end while 

 the rest of the body hangs free in the water. Obtain a living Hydra and place 

 in a watch glass with sufficient water to cover the animal. Examine with the 

 lowest power of the microscope or with a hand lens, and note the following 

 parts (Hegner, chap, viii, pp. 116-38). 



The body or column of the animal is a cylindrical elastic tube, capable of 

 great extension and contraction. From one end of this, which is the oral, 

 anterior, or distal end, arise a number of radially arranged slender outgrowths, 

 the tentacles. The number of these varies from four to ten, but is generally five 

 or six. The oral extremity between the bases of the tentacles forms a conical 

 elevation, the hypostome, in the center of which the mouth is located. The 

 mouth when closed has a star-shaped appearance but is usually difficult to see, 

 unless the hypostome happens to be turned directly upward. The flattened 

 base of the animal is the posterior, aboral, or proximal end, is often designated 

 as the foot or basal disk, and secretes a cement-like substance by which the 

 organism attaches itself. Column and tentacles are hollow, inclosing a cavity 

 known as the gastrovascular cavity because it has both digestive and circulatory 

 functions. Under the low power the gastrovascular cavity appears outlined by 

 brownish lines in most specimens; it is usually an extensive cavity only in the 

 anterior half of the column, being reduced posteriorly to a slender canal. 



Observe the tentacles more closely and note that they bear numerous pro- 

 tuberances, each of which contains a collection of very small oval sharply out- 

 lined bodies, the stinging cells or nematocysts. Such a collection of stinging cells 

 is called a battery. At the end of each tentacle is a great mass of stinging 

 cells. They also occur sparingly on the column. 



Hydra is not bilaterally symmetrical, like the frog, where there is only one 

 possible plane that will divide the animal into similar halves, but is radially 

 symmetrical, that is the parts of the body radiate from a common axis so that 

 a number of planes of symmetry could be passed through the animal. How do 

 you think the differentiation and development of the anterior end compare 

 with that of the frog? Does it seem to have any special structures different 

 from the rest of the body? 



Draw a Hydra in the extended state showing the above-mentioned 

 details. 



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