30 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
assuming that the act of rotation of the axes occurring in the former has 
been leapt over in the ontogeny, the mouth and anus arising at once on 
the pole opposite the blastopore. 
Arthropoda. 
Facetted Eyes of Crustacea and Insects.* — Prof. S. Exner has 
published a summary account of the important observations on the 
physiology of the facetted eyes of Crustacea and Insects, to which the 
attention of the Society has been from time to time directed. 
For the examination of the dioptric apparatus of the facetted eye 
those are best adapted in which the crystalline cone is fused with the 
cornea ; fig. 1 represents such an eye from a Limulus. A is the dioptric 
Fig. 1. 
apparatus consisting of cornea and crystalline cone, while B is the re- 
tinula. An eye of this kind can be easily removed with a fine knife and 
so freed of its pigment that objects may be looked at through it from 
behind. By the aid of the refractometer Exner found that the dioptric 
apparatus of a facet-segment does not consist of a homogeneous mass, 
but of cylindrical layers whose refractive powers decrease from the axis 
to the mantle. Lens-cylinders of this kind function just like lenses, 
but they are better adapted for the facetted eye. Thus the optic power 
of lens-cylinders is almost independent of the fluid surrounding them, 
* ‘ Die Physiologie der facettierten Augen von Krebsen und Insekten,’ Leipzig 
und Wien, 1891, large 8vo, 198 pp., 7 pis. See Biol. Centralbl., xi. (1891) pp. 581-8. 
