138 THE POPULAR SCIENCE MONTHLY 



finally cover its tentacles and mouth by puckering in the oral edge of 

 its column. In this contracted state it may remain hours at a time, 

 and when it eventually expands it does so by relaxing its muscles and 

 refilling its body with sea water. A beam of strong sunlight, if thrown 

 upon an expanded Metridium several feet under water, will usually call 

 forth the same contraction as mechanical stimulation does. 



When the exterior of a Metridium is tested locally, its receptiveness 

 for certain stimuli is found to be quite diverse. The animal makes no 

 - yJ .,i...j T u- movements when dissolved food-substances are cau- 



tiously discharged upon the external surface of its 

 column, though this very area is sensitive to mechan- 

 , 1 1 i ical stimulation. Precisely the reverse is true of the 



1 e lips; these organs are easily stimulated by dissolved 



' food-products, but no reaction occurs even when they 



j are punctured by a needle. Both mechanical and 



1 chemical stimulation, however, are effective on the 



:t::^(4 tentacles and vigorous responses can be called forth 



from even distant parts of the body by the application 

 of either of these forms of stimuli to the tentacles. 



;:(?'■• 



mi 



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: :-#-|. : i;.'! m Since these reactions, as just intimated, often involve 

 xyt^A^ responses in very different parts of the animal from 



those to which the stimulus is applied, it follows that 

 Fig. 2. Ectoderm we are dealing with a process justly regarded as nerv- 



FBOM THE Ten- „ , ...... . . . , 



tacle ok a Sea- 0US j * or transmission m this case is not accompanied 

 anemone (Metri- w ith any observable motion. The surface of a sea- 



dium) ; e, epithelial ,. , . . , . , 



layer ; m, muscular anemone may then be pictured as a true receptor sur- 

 layer ; n, nervous face partly differentiated in different regions for par- 

 lameii'a/ P ° ' % ticular classes of stimuli, but not so far specialized 

 that it can be described as made up of sense organs. 

 An examination of the structure of the ectoderm (Fig. 2) will do 

 much to make clear the mechanism by which the reactions of sea-ane- 

 mones are carried out. The ectoderm of these animals is a modified 

 epithelium in which three definite layers can be distinguished. The 

 outermost of these forms more than half the thickness of the total layer 

 and is a true columnar epithelium. It contains, in addition to ordi- 

 nary epithelial cells, gland-cells and nettle-cells, and, what is of more 

 importance to us, sense-cells. These sense-cells are long, narrow bodies 

 whose distal ends are armed with a sensory bristle which, under ordinary 

 conditions, projects into the surrounding sea water and whose proximal 

 ends run out into finely branched, nervous processes which intermingle 

 with similar processes from other cells. The complex made by the 

 interweaving of immense numbers of these processes constitutes the 

 second layer of the ectoderm, the nervous layer, and this layer often 

 contains in addition to the large amount of fibrillar material derived 



