GRUSTACEASENSQRY OBOANS 



353 



outer or inner side is not yet quite certain, with a fibre of the optic 

 nerve, and must therefore be considered as a retinal cell. 



The gi-eat similarity in structure between 

 suola- a single eye and the eyes of Flatodes 

 should not be overlooked. The three -fold 

 structiu'e of the unpaired Crustacean eye seems 

 to be characteristic. Occasionally, e.g. in 

 Branch iims, 3 separate nerves leave the brain 

 to run to the 3 single eyes. 



The structure of the paired lateral 

 eyes of the Crustacea (stalked and un- 

 stalked) is much more complicated. 

 We have here the compound eye so 

 characteristic of the Arthropoda. Even 

 though, in single divisions of the 

 Crustacea, it presents important modi- 

 fications and complications in its 

 structure, we nevertheless evidently 



, . \ , ..T i- . Fig. 237.— Eye of CalaneUa mediter- 



have to do (with a lew exceptions to ranea J Juv, from below (after Gren- 

 be mentioned later) with homologous aoher). p, pigment plates ot the paired ;j)i, 



visual organs. Let us take for descrip- °' *« "''p^"-«i p°/m°° : >;'-. ^f^^ >=«"« ; 



o _ i:" 110, optic nerve ; nf, nervi frontalis. 



tion the paired eye of BrancMpus, 



which presents in a tolerably simple manner the typical structure 

 of the compound eye. The movable stalk of the hemispherical eye of 

 Branchijms (Fig. 238, £) contains the optic nerve; this swells 

 in the stalk into a ganglion, the ganglion optieum, which must 

 be reckoned as belonging to the brain. The optic ganglion is 

 followed at the distal end near the base of the eye by a second 

 ganglion, the retinal ganglion. Nerve fibres radiate towards the eye 

 from the nerve cells of this retinal ganglion. The eye itself is 

 separated from the cavity of the eye stalk by a thin basal membrane. 

 The nerve fibres which radiate from the retinal ganglion penetrate this 

 membrane to enter the retinal cells immediately on the other side 

 of it. The eye represents the half of a hollow sphere with thick 

 walls, whose outer spherical surface corresponds with the outer surface 

 of the eye, and whose inner (concave) surface corresponds with the 

 basal membrane. It consists of numerous closely packed single eyes 

 arranged radially. In each single eye or ommatidium (Fig. 238, E) 

 we distinguish three chief constituents : — 



1. The Retinula, i.e. that portion of the whole retina of the 

 compound eye which belongs to each of the single eyes. This is the 

 proximal portion coming next to the basal membrane. 



2. The crystal cone, and 



3. The hypodermal elements with the superjacent chitinous 

 cuticle, the cornea of the Arthropodan eye. 



A. The Retinula consists of 5 long cells regularly grouped 

 VOL. I 2 a 



