Eyes of Molluscs and Arthropods. 723 



ones venders it certain tbat tliey have an important plij'siological func- 

 tiou to perforili. 



The gland cells in Mollusca contain a transparent substance which 

 cxpands enormonsly when broiig-ht in contact with water ; — I b clic ve 

 tbat tbeyalso possess the power toabsorb to a great degree 

 gases or otber substances beld in solution. It bas been suggest- 

 ed tbat tbe slime cells are closely united witb sense bairs so tbat tbe 

 sligbtest Irritation would cause tbe immediate secretion of tbe slinie, 

 wbicb miglit bave a poisouous property. Tbis explanation sounds fairly 

 reasonable wbeu applied to tbe Mollusca ; but it loses its force wben 

 applied to tbe sense organs of tbe lateral line, or to tbe organs oftaste 

 in tbe Vertebrates. Wbatever effects tbis absorption bas upon tbe 

 gland cells , causiug contraction or expansion , or concentration of tbe 

 foreign gases or fluids. — could tben be transmitted to tbe adjacent 

 sense cells. It is difficult to imagine just bow an odor, a gas, or any 

 foreign substance beld in solution . would affect a sense cell , but tbey 

 surely do affect tbem in some way, causing distinct sensations. If we 

 bad certain cells placed around tbe sensory one, for tbe absorption of 

 tliese gases or fluids, tben tbe effect would be intensified, or mucb 

 smaller quantities of foreign matter would cause sensations. If tbis be 

 true, tben one of tbe sense papillae of Haliotis, or groups of cells in 

 Lima, migbt be compared witb an ommatidium. In botb tbere are central 

 sense cells surrounded by pigmented, or glandulär ones, for tbe absorp- 

 tion, and tberefore intensification, of tbat particular form of Vibration 

 for wbicb the organ is developed. 



It is extremely difficult to express in words tbe difference between 

 a respiratory , or locomotive cell , wbicb is simply irritated to renewed 

 activity by tbe presence of irritating substances in tbe water, and one 

 wbicb is a pure sense cell, or destined simply for tbe reception of such 

 Stimuli, wbicb are tben transferred to otber cells. But we may say tbat, 

 ingenerai, all cells are susceptible to Stimuli from witbout. 

 Each one may receive directly only Stimuli enougb to sus- 

 tain its own vital processes; when, bowever, it receives 

 more Stimuli than are necessary, tbe surplus beiug trans- 

 mitted to otber cells, it becomes a sense cell, or a cell 

 specially adapted for tbe reception of Stimuli. 



Animals receive Stimuli, or energy, in various forms; from tbe sun, 

 sound, movemeuts and cbeniical Compounds, and tbey bave various 

 forms of organs for the reception of each form of energy. Plants, how- 



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