98 THE APODID^ PART i 



above the unpaired '' eye " (see Fig. 27, p. 105). At the 

 posterior end of the unpaired " eye " it gives off a 

 branch which runs into that organ, as will be described 

 later. The sacs then widen out over the eyes, as 

 shown in Fig. 24. 



This water-layer probably facilitates the move- 

 ment of the eyes by the ring of muscles attached 

 round their rims. It is not improbable that the 

 sacs further serve as lenses, but this cannot be stated 

 with certainty. As for the mechanism by which the 

 water is drawn in and out, a contraction of the whole 

 ring of muscles at once would draw the water in, 

 while a general pressure of the body fluid under the 

 eyes, caused by muscular contractions in other parts 

 of the body, would expel it through the canal. 



This whole structure has probably been developed 

 in the following way. The original head showed con- 

 strictions between the segments of which it was 

 composed, i.e., folds of the skin projecting inwards. 

 The eyes, in travelling backwards, would necessarily 

 have to pass by these folds. The first fold of all 

 would be that between the prostomium and the 

 first segment. It is clear that the eyes must either 

 be stopped in such a fold, or else carry it back with 

 them. This latter is what we suppose took place at the 

 posterior edge of the prostomium. The eyes came 

 against the fold of the first segment, which generally 

 overlaps the prostomium when the body is at all 

 contracted. Under this the eyes would disappear.^ 



1 It is clear that while the bending round of the anterior segments 

 •would so stretch the dorsal integument as to obliterate all such constric- 



