674 JUSTUS CARRIERE. 



(ommatidium) of Musca^ while Berger first described tlie related 

 layers of the optic ganglion. Already, some years ago, I myself 

 examined both parts by means of thin vertical sections, and I 

 can in the first place confirm Grenadier's accounts concerning 

 the construction of the pseudoconus and of the retinula. 



The whole set of eye-units (ommatidiai) of Musca vomito- 

 ria are enclosed in a chitinous capsule, the exterior face of which 

 is formed of the cornea-lenses, while a thin chitinous membrane 

 limits the eye towards the brain, being full of holes to let pass 

 the nerve-fibres. It is only a little smaller than the cornea and 

 nearly concentric to it. The eyes being placed on both sides of 

 the head have their largest extension in the direction from above 

 to below ; the foremost retinulee stand almost perpendicular on 

 the basal membrane ; in the midst of the eye they take a posi- 

 tion more inclined backwards, and the last ones are not only 

 set quite slantwise, but even curved in order to catch still 

 a glimpse backwards, while all the other retinulse are quite 

 straight. With this setting of the eye-units the animal sees to 

 the side, and obliquely to the front and behind, but not 

 directly to the front. Experiments show also that a fly rests 

 often quietly sitting if one approaches it with a fine-pointed 

 object directly from the front; but upon approaching it from 

 the side mostly makes oflF. 



The eye is limited anteriorly by a fold in the membrane 

 (Chitinpanzer) x, which is narrow and directed towards the 

 inside, being specific to many insects. 



Also on the dorsal margin such a fold, but less deep, is met 

 with ; whereas the posterior margin of the eye is supported by 

 a round ledge (3/), the transverse section of which is shown by 

 the drawing. 



1 The term " ommateum " was introduced by Lankester in bis memoir on 

 tbe eyes of Scorpions to signify the entire soft parts of the non-segregate 

 (unicorneal) eye of Aracbnida and Hexapoda, consisting of a layer of uerye- 

 end cells, and nsually a layer of vitreous cells. Tbe similar term "ommati- 

 dium " is introduced in tbis paper to signify tbe units consisting eacb of a 

 retinula and a vitrella, togetber with their sheath of pigment cells, into which 

 tbe ommateum of the multicorneal (polymeniscous) eye of Arthropods, is 

 segregated. 



