. . d, in the majority of the descendants of Apus,facilitates the wandering to the sides. The case ofLimulus is particularly interesting. Here, as will bedescribed later, the eyes wandered from the first, notforwards and upwards, but sideways and upwards, sothat the brain could not follow as in Apus, but, beingdrawn in two opposite directions, remained where itwas, the extraordinary length of the optic nervesshowing clearly that the eyes must have wanderedconsiderably. Almost more interesting, however, are the water-sacs which spread
. . d, in the majority of the descendants of Apus,facilitates the wandering to the sides. The case ofLimulus is particularly interesting. Here, as will bedescribed later, the eyes wandered from the first, notforwards and upwards, but sideways and upwards, sothat the brain could not follow as in Apus, but, beingdrawn in two opposite directions, remained where itwas, the extraordinary length of the optic nervesshowing clearly that the eyes must have wanderedconsiderably. Almost more interesting, however, are the water-sacs which spread out over the eyes of Apus,between them and the integument. These have never,we think, been described before, at least in detail,and here deserve particular attention as lendingsupport to our theory of the migration of the eyes. SKCT. VI THE SENSORY ORGANS 97 Fig. 24 is a diagram of these water-sacs. A smallpore, in the shape of a fine transverse slit (seeFigs. 24 and 69, p. 303) in front of the eyes, is, inlarge specimens, visible to the naked eye. This leads. oe FIG. 24.—Diagram to show the water-sacs over the eyes of Apus. s, water-sacs;c, canal leading into the same ; /, pore ; e, paired eye ; /;, brain ; a\ cesophagalcommissure ; og; optic ganglion ; w, eye-muscles. The eye is drawn in thesection ; in reality a median section passing through the pore passes between theeyes ; (?2, unpaired eye receiving a branch from the water-canal. into a canal, which runs along the dorsal surface ofthe unpaired eye. Here it widens out con-siderably, its upper and lower chitinous membranesbeing in close contact, except in the median line II 98 THE APODID^: PART i above the unpaired eye (see Fig. 27, p. 105). At theposterior end of the unpaired eye it gives off abranch which runs into that organ, as will be describedlater. The sacs then widen out over the eyes, asshown in Fig. 24. This water-layer probably facilitates the move-ment of the eyes by the ring of muscles attachedround their r
Size: 1573px × 1587px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublisherlondo, bookyear1892
