Eyes of Mollusca and Arthropods. 707 



little doubt that tbey terminate eitlier in contractile, or digestive cells, 

 within the body (PI. 32, flg. 153, c. d. e.). 



After, or coiueident witb, tbe production of digestive and sensi- 

 tive cells, certain of the still indifferent, and probably ciliated oues, 

 developed internal, amoeboid processes which were capable of contrae- 

 tions caused by Stimuli received from without. The contractile powers 

 of a single cell can only increase at the expense of the sensitive ones. 

 It is not unreasonable to suppose that two cells situated close together 

 would possess, in unequal degrees, the power of sensitiveness and con- 

 tractility: but the more one cell became contractile, the more it would 

 lose its power of receiving direct Stimuli , and would tend to absorb 

 them from that cell which had the sensitiveness to an unusual degree; 

 two such cells would then become dependeut upon each other, the one 

 increasing in contractility, and losing its power of receiving direct 

 Stimuli . while the other would lose its contractility and gain in sen- 

 sibility in the same proportion. The sum of the labor performed by both 

 cells in the indifferent condition would exactly equal that performed by 

 the two specialized ones. The communication between the two cells 

 would be at first one of ordinary contact. But in proportion as the con- 

 tractile cell became more contractile , those parts devoted to this pur- 

 pose, i. e. the inwardly directed processes, would be more developed, 

 while there would be a corresponding dimiuutiou of the outwardly di- 

 rected part, or that necessary for receiving impressions. In its com- 

 pauion cell , the sensitive one , the reverse would be the case. As the 

 contractile cell moved more toward the interior, it could retain its con- 

 tractility only as long as it received Stimuli from without; it could 

 therefore never separate itself from its partner, the sense cell, to whose 

 aid it owes its contractile powers. The contractile cell would, there- 

 fore. draw with it a part of the sensitive one, which, siuce it must 

 necessarily remain in contact with the outer world, would be drawn 

 out into a long fibre, its line of communication with its cell partner. 

 The two cells have now reached an unequal degree of diiferentiation , 

 since the contractile one is entirely adapted to contractility, while the 

 sense cell has not only to ree ei ve direct Stimuli, but trans mit them 

 to its partner. This is an ob\àous disadvantage, and Steps are soon takeu 

 to avoid it. In the Mollusca, as well as in the Coeleuterates, we have 

 Seen that the more highly sensitive the scuse cells are, the thinner they 

 become, uutil they finally communicate with the outer world by a very 

 fine bunch of sense hairs , while the cell body is reduced almost to a 

 fibre. We have also seen how the fine prolongations of the sense cells 



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