34 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
crystalline cones or cells, (b) two retinal-pigment-cells (four in GalatJiea ), 
while (c) the retinule-cells themselves are more or less pigmented 
according to illumination. The changes observed according to illumi- 
nation are described. Except in Lucifer and Sergestes , a granular, 
dirty-yellow “ tapetum ” was observed, associated with two cells in the 
posterior joart of each ommatidium. 
Following Exner, the author distinguishes and discusses eyes forming 
an apposition-image, a superposition-image, or the former in light and 
the latter in darkness. 
The rods are quite independent of the cones, the former arising from 
a group of retinule-cells, the latter from the epidermic crystalline cells. 
Variation in Portunns depurator.* — Mr. E. Warren has made on 
this crab seven’ measurements on each individual male, corresponding 
to those made by Prtf. Weldon on the male of Carcinus msenas. The 
total number of crabs measured was 2300, and the measurements were 
recorded to the tenth of a millimetre. Since the crabs were growing 
and varied much in size, it was found necessary to reduce the measures 
to percentages of some standard dimension. The carapace length was 
selected, and all the measurements are expressed as thousandths of this 
dimension. On glancing down the mean of the several dimensions it 
will be seen that a stable equilibrium is in no way reached, except, 
perhaps, in the case of the total breadth ; thus, throughout life, this crab- 
is gradually changing its shape, and it appears to be clear that, when 
all its organs are in a rapid state of change, the crab can protect itself. 
Here is an argument, Mr. Warren thinks, against the transmission of 
acquired characters, for otherwise the earlier brood would tend to have 
a somewhat different shape to the later, and this is scarcely probable. 
From the measurements which the author records, he believes himself 
to have proved that the mutual relationships of the organs measured 
are almost as closely similar between the two genera Porlunus and 
Carcinus as one or two more sharply marked off races of a single species* 
Of course, a considerable number of such comparisons would have to be 
made before any safe conclusion could be drawn, and the meaning of 
the differences observed by such comparative treatment of a large series 
of genera. The larger duration probably indicated real differences in 
the correlation constant, and are possibly associated with changes in the 
habit or environment. For example, it is conceivable that a crab which 
swims might require to be more symmetrical than one that only crawls 
between tide-marks. 
Changes in the Carapace of Carcinus Msenas.f— Mr. H. Thompson 
has made a study of certain changes observed in the dimensions of parts 
of the carapace of this common crab. The methods followed were those 
of Prof. Weldon. On comparing two sets of measurements it appeared 
that the average size of the “ frontal breadth ” of the crabs collected in 
the year 1893 exceeded that of the crabs collected in 1895. The de- 
ficiency in frontal breadth in 1895 appeared to be compensated for by 
the possession of a larger right dentary margin. As the results seemed 
to indicate that a change in regard to these dimensions was taking place 
in the species, it was thought desirable to compare similar measurements 
* Proc. Ptoy. Soc. Loud., lx. (1896) pp. 221-14 (6 figs.), f Tom. cit., pp. 195-8. 
