Sensory Organs 



35 



processes of the animal's body, known as its 

 tentacles, act as feelers and prehensile struc- 

 tures. The external surface of the body and 

 tentacles of Hydra are covered by minute 

 bristle-like projections, some of which are 



FIG. 2. Sensory organs or cells connected with a nerve cell. C, 

 Protoplasmic fibre of a tactile sense-cell forming nodular enlarge- 

 ments. D. Cnidoblast connected with nerve cell by a protoplasmic 

 fibre. 



named palpocils; they are formed of an out- 

 growth of the protoplasm of the cylindrical cells 

 which constitute the ectoderm or outer skin 

 layer of the animal. (Fig. 2.) 



c 2 



