The Retina and Optic Ganglia in Decapods, especially in Astacus. 65 
(Fig. 62), could by division (Fig. 63) become four (Fig. 65), and by 
their elongation the rhabdome [rhl] would come to lie some consi- 
derable distance from the cone proper (Fig. 64). These two changes, 
the increase in number and the elongation, are the chief modifica- 
tions noticeable in the cone cells. The changes in the retinular cells 
are more complex. Each of the five retinular cells in the simpler 
type (Fig. 61) gives rise to a part of the rhabdome and forms a por- 
tion of the sheath of the cone. In the complex type, these two 
functions are carried out by separate cells, the distai retinular cells 
formiug the sheath of the cone and the proximal ones producing 
the rhabdome. Beginning with the simpler type, it is conceivable 
that one of its retinular cells (Fig. 62, 9 — 10] might come to be 
chiefly concerned with sheathing the cone, while the remaining four 
might become more or less limited to the rhabdome. If the retinular 
celi that partially envelopes the cone were to divide, the ommatidium 
would then contain two distai retinular cells and four proximal ones, 
a condition precisely that found in Serolis (cf, Parker, 91, pag. 89). 
From this stage, by a division of the four proximal retinular cells 
into eight, a stage in which the numerical relations of the cells are 
exactly those in Astacus would be reached. The elongation of the 
cone cells would separate the proximal from the distai retinular cells, 
the latter being limited to the cone, the former to the rhabdome 
(Fig. 64). The details of this process are clearly enough shown in 
the diagrammatic fìgures already referred to, and require comment in 
only one particular. The position in the simpler ommatidial type of 
the celi (Fig. 62, 9 — 10)^ from which the two distai retinular cells 
of the more complex type were derived, is such that it is probable 
that the two descendants of this celi do not remain in the same 
ommatidium, but that one of them [10) applies itself to an adjoining 
ommatidium, while thè other [9] remains with the ommatidium to 
which it originally belonged (cf. Figs. 63 and 65). 
The type of optic ganglia found in BrancMpus has already been 
shown to be a primitive one, and the exclusive association of omma- 
tidia containing only five retinular cells with this type is evidence in 
favor of the primitive nature of the ommatidia. In a similar way, 
the regulär occurrence of ommatidia of ten retinular cells with a 
derived type of ganglionic structure, as in Astacus^ supports the con- 
clusion already reached that these ommatidia, like the connected 
ganglia, are also structures of a derived type. Deductions such as 
these lead to the general conclusion that, beginniug with a stage 
Mittheilungen a. d. Zoolog. Station zu Neapel. Bd. 12. 5 
