248 DK WALTER 11. GASKELL. 



the Ammoccetes or in any vertebrate, for the external molecular 

 layer (6, fig. 4) (Miiller's layer of Nerven-ansatze) is very short 

 and inconspicuous (in fig. 4 it is drawn too thick). 



Apart from the retina proper and from the dioptric apparatus, 

 two other striking differences are seen in these two pictures — 

 viz., in the Crustacean eye the retina is upright, in the Ammo- 

 ccetes inverted, and in the latter eye there is an additional 

 framework of supporting tissue elements — the Mullerian fibres 

 — which are not found in the former. Both these points require 

 explanation. 



Passing from the retina to the fibres of the optic nerve, we 

 again find a remarkable resemblance, for in Ammoccetes, as 

 pointed out by Langerhans and carefully figured by Kohl, a 

 crossing of the fibres of the optic nerve occurs as the nerve 

 leaves the retina, just as is so universally the case in all com- 

 pound retinas. To this crossing Kohl has given the name 

 chiasma nervi optici, in distinction to the cerebral chiasma, 

 which he calls chiasma nervorum opticorum. Further, we find 

 that even this latter chiasma is well represented in the Arthro- 

 pod brain ; thus Bellonci ^ in Sphteroma, Berger,'-^ Dietl,^ and 

 Krieger * in Astacus, all describe a true optic chiasma, the only 

 difference in opinion being, whether the crossing of the optic 

 nerves is complete or not. Especially instructive are Bellonci's 

 figures and description. He describes the brain of Sphteroma 

 (fig. 7) as composed of three segments — a superior segment, the 

 cerebrum proper, a middle segment, and an inferior segment; 

 the optic fibres, as is seen in fig. 7, after crossing, pass 

 direct into the middle segment, in the ganglia of which they 

 terminate. From this segment also arises the nerve to the 

 first antenna of that side — i.e., the olfactory nerve. The optic 

 part, then, of this middle segment is clearly the brain portion 

 of the optic ganglionic apparatus, and may be called the optic 

 lobes, in contradistinction to the peripheral part, which is usually 

 called the optic ganglion, and is composed of two ganglia, 

 Op. g. I. and Op. g. II., as already mentioned. These optic lobes 

 are therefore homologous with the optic lobes of the vertebrate 

 eye. 



^Op. clt., p. ISl, -Op. cit. 



^ Zcitsch. f. wisscns. Zool., VA. xxx., 1876. *Ibid., Bd. xxxiii. 



