32 



ESSENTIALS OF ZOOLOGY 



Endoderm Ectoderm 



that processes of the body wall are developed which act as 

 grasping organs or tentacles. 



Hydra is irritable and contractile. The body when fully 

 expanded is about J in. long, and the tentacles may be produced 

 to a greater length, becoming fine and filamentous. Both 

 the body and the tentacles when the animal is disturbed are 

 retracted, the body to a small knob and the tentacles almost 

 to the point of disappearance. The result in the case of the 

 many-celled Metazoon, Hydra, is thus very similar to that 



of the Protozoon, Vorticella. 

 In the undisturbed, ex- 

 panded state Hydra is able 

 to capture food, which is 

 introduced to the mouth by 

 the tentacles, and so to the 

 gastric cavity or enteron 

 where it is digested. In 

 this state also it is able to 

 move by gliding along upon 

 the cells which form the 

 base of attachment or pedal 

 disc, and which then develop 

 pseudopodia. Quicker 

 movements are produced 

 by the animal looping like a 



Endoderm Ectoderm 



FIG. 14. Diagram of general structure 

 of Hydra and of a gastrula. 



caterpillar with the aid of 



the tentacles, the cells of which may also develop pseudopodia, 

 or even by somersaulting the base over the temporary 

 fixation obtained by the tentacles. They may quit their 

 hold of the weed and attach themselves by the base to the 

 surface of the water. Earely they quit their hold and float 

 freely in the water. 1 



,The Hydra may thus externally be resolved into a basal 

 disc, a body or column, a ring of tentacles, a projecting cone 

 bearing the mouth and termed the hypostome. From the 

 above it is alreadjr obvious that a section of the body, or of 

 one of the tentacles, is essentially the same. The section 



1 1744. Trembley, Mem, pour serv, a VHist. d'un genre de Polypes d'eau 

 douce. 



