50 



COELENTERATA 



[CH. 



with justice say that the polyp is really simply a tube closed at 

 one end and open at the other : further, the tentacles can be seen 

 with the microscope to be nothing but thin hollow tubes, opening 

 into the central one (Fig. 17). The central space is often termed a 

 stomach, and in the case of Hydra the idea suggested by this 



B 



FIG. 16. Hydra fusca-x. about 12. 



A.. Expanded condition. This specimen is budding off a yonng Hydra. It 

 contains a large food mass in its coelenteron, probably a Daphnia or some 

 other fresh-water Crustacean. B. Eetracted condition. 1. Mouth. 

 2. Oral cone. 3. Tentacles. 4. Bud. 5. Endoderm. 6. Foot. 



term is correct. In other Coelenterata the space performs other 

 functions besides those of the human stomach, and the term 

 coelenteron, which does not imply any function, is preferable. 



With the microscope, however, we can make out a number of' 

 further points. If the edge of the animal be carefully focused it 

 can be seen that the body-wall consists of twa layers, an outer 



