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Elementary Biology. 



at the other by a swollen head, bearing in the centre a 

 mouth, and fringed with a circle of tentacles. The head 

 is conical, and the tentacles spring from its broadest portion. 

 The conical eminence over the origin of the tentacles is 

 known as the hypostome. The hypostome bears the termi- 

 nal mouth aperture leading into the interior of the zooid. 

 The cavity into which the mouth leads we shall term the 



FIG. 114.-- PORTION OF A COLONY OF Hydractinia. (Glaus.) 



a, alimentary zooid ; d, tentacular zooid ; e, reproductive zooid 



enteron, but defer the explanation of that term until we 

 have completed our study of the general morphology. 

 Another kind of zooid is seen near the alimentary zooid, 

 possessing a body and swollen head ; but the head carries 

 no tentacles, and there is no mouth. This degraded form 

 we shall term a tentacular zooid. Yet another type is to 



