SECT, vi THE SENSORY ORGANS 99 



It would be impossible for them to get further 

 than the posterior end of this fold unless they 

 dragged it after them, and thus, as we suppose, the 

 fold has travelled backwards with the eyes, the front 

 part gradually closing, till only a fine transverse slit 

 is left. 



This derivation of the water-sacs receives some 

 support from the ring of muscles round the eyes, 

 which are clearly bands borrowed from the longi- 

 tudinal musculature. Their sinewy attachment joins 

 the large muscle which runs down from near the 

 eyes to be attached near the prostomium. A com- 

 parison of Fig. 24 with Figs. 12 and 13, p. 56, makes 

 the origin of the eye muscles very evident. As the 

 eyes, which are hypodermal structures, travelled 

 backwards, dragging an intersegmental fold back 

 with them, they naturally took along with them some 

 of the longitudinal muscle bands attached to that 

 fold. This accounts for the way in which a hypo- 

 dermal structure, such as the original Annelidan eye, 

 became an independent organ, movable by a special 

 and apparently highly developed system of muscles. 



The development of the stalked eye from the eye 

 of Apus appears to us by no means such a simple 

 matter to understand. We are inclined to think that 

 it may have taken place in two different ways : 

 (i) By the gradual projection of the eye itself above 

 the surface of the body (we find such projections of 

 the eye stalks in many Trilobites). (2) By the 

 tions between them, this would not be the case with the fold between 

 the prostomium and the first segment. The labrum of Apus was 

 probably at first a movable organ. 



H 2 



