NEBALIELLA 



219 



which presses backwards against a knob just above the attachment of the antennule. 

 This knob has been described and figured by Thiele (1905, PI. 2, fig. 3) but the speci- 

 men from which the figure was drawn must have been badly distorted. 



Attached to the outer surface of the eye at about the middle of its length and near the 

 lower margin is a muscle which runs backwards and inwards to attach to a median 

 point just below the hinge of the rostrum. By contraction of this muscle the eye would 

 tend to be drawn backwards and inwards. However, its backward movement is re- 



Fig. 7. Nebaliella extrema. 



A. Side view of rostrum, eyes, antennules and antennae, the right half of the carapace having been partly cut away. 



B. Oblique front view of eyes and rostrum. 



ant. I, antennule; ant. 2, antenna; e. eye; e.m. eye muscle ;/.r. flange of rostnun; h.a. hook on second joint 

 of antenna; h.e. heel of eye; k.a. knob above antennule; k.r. keel of rostrum. 



stricted by the posterior heel. From the lateral view in Fig. 7 A, it will be seen that this 

 heel is lodged in a gap between the antennulary knob and the antennule itself. Contrac- 

 tion of the eye muscle can, therefore, only have one action. It must cause the eye to 

 pivot about this point. This will make the anterior margins of the eyes rotate inwards 

 towards the middle line. Now the eyes form the lateral margins of the anterior entrance 

 to the filter chamber — the only entrance by which food-bearing water is sucked in — 

 and their rotation inwards will diminish the size of this entrance. There is thus in the 

 eyes a mechanism by which the amount of water, and hence the amount of food, en- 

 tering the carapace can be controlled. 



