SENSE ORGANS I 53 



ception of stimuli in general. Sensibility is one of the primary 

 functions of undifferentiated protoplasm. 



358. Almost the same may be said of hydra. The organism 

 is sensitive to the same stimuH and to no others. So far as is 

 known there is no localization of sensory function. All the 

 cells of the body consist largely of undilTerentiated protoi)lasm, 

 which is probably the organ of general sense as in ama-ba. It 

 is true that some of the cells of the ectoderm project beyond the 

 surface by slender protoplasmic processes which are probably 

 organs for the reception of stimuli. Since these processes are 

 more exposed they are more readily affected by stimuli and 

 hence may be regarded as incipient sense organs. 



359. In some other Coelenterates as, e. g., the medusa', the 

 sensory cells and the nervous elements generally, are better 

 developed. Many cells of the ectoderm are provided at their 

 free surfaces with sensory hairs while the opposite ends of the 

 cells are prolonged into long fibres which extend for some 

 distance under the ectoderm. 



360. In nereis the sense cells are clearly differentiated 

 from the other cells of the epidermis. Each sense cell projects 

 through the cuticula by a single protoplasmic process, while 

 the remainder of the cell is elongated into a fibre which extends 

 deep into the body to the central nervous system. The nucleus 

 of the cell often lies in the epidermis, but it may also lie immedi- 

 ately beneath the epidermis or even at a considerable distance 

 beneath the surface. In the latter case a number of such 

 nuclei may be collected into a group which constitutes a gang- 

 lion, and if the fibres run parallel in a bundle they form a 

 nerve. 



361. The sensory cells of Arthropods arc very much like 

 those of nereis, but they do not always have the exposed proto- 

 plasmic terminations. Instead, the fibre may end at the base, 

 or in the axis of one of the cuticular sensory hairs mentioned 

 above. (Page 146.) The hairs serve mechanically to trans- 



