ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
527 
the relation of its fibres to ganglionic cells is not easily determined, and 
in addition to the fibres, which are truly optic, there are eight or ten 
which are motor and a few special sensory fibres which resemble the 
nervi nervorum. Although the author’s observations are not as yet 
numerous, he thinks that a comparison of the optic ganglia in various 
Crustacea shows that the type represented by the optic ganglia in 
Branchipus is essentially Entomostracan, while that represented in 
Astacus is peculiar to the Malacostraca. It is of interest to observe 
that the optic ganglia in Hexapods are almost identical in their 
structural features with those of Astacus. After a discussion of the 
growth of retina, ganglia, and optic nerves, the author concludes with 
some remarks on the derivation of the ommatidia. The simplest and 
the most complex types are, without doubt, genetically connected, and it 
is probable that the simplest type is a primitive one, from which the 
other has been derived. The numerical relation of the cells indicates 
the way in which this derivation can have been accomplished. A cone 
composed of two cells can be easily converted into one formed of four, 
and the simplest group of five undifferentiated retinular cells could be 
differentiated, and division give rise to the two distal and eight 
proximal retinular cells of the more complex type. The simplest type 
of ommatidium appears to have been derived from a cluster of cells in a 
continuous unfolded epithelium, and by a process of cell-division and 
differentiation. The simpler type of ommatidium has given rise to the 
more complex. 
Conjugation of Cambarus.* — Mr. E. A. Andrews finds that there are 
important differences between the American crayfish Cambarus and the 
European Astacus. Certain structures hitherto known only as specific 
and generic characters are necessary accessory reproductive organs. In 
confinement C. affinis conjugated in November, and in February, March, 
and April. The process lasts several hours. During it the male 
exhibits great skill and is visibly excited, while the female shows 
scarcely any evidence of excitement. The sperm is introduced into the 
cavity in the annulus, which thus serves as a sperm receptacle. As in 
the American lobster the well-known hook on the ischipodite on the 
third walking leg of the male is used to firmly attach the male to the 
female. There is no wide distribution of sperm as has been described 
for Astacus. It is all placed in the annulus. The oviducts of the 
female are not concerned in the process of conjugation. For further 
details of the process the author’s own account must be consulted. 
Observations on Carcinus Msenas.-f— Prof. Weldon, as Secretary of 
the Royal Society’s Committee on the measurable characteristics of 
plants and animals, J made an attempt to measure the death rate due to 
the selective destruction of C. msenas with respect to two particular 
dimensions. About 7000 females varying in length from 7 to 13*95 
mm. were chosen at random except as regards their size, and two dimen- 
sions were measured in each. The results were then compared with 
those of the corresponding measurements made upon 1000 adult females 
taken from the same locality (Plymouth Sound). The results of Prof. 
* Zool. Anzeig., xviii. (1895) pp. 284-5. 
t Proc. Roy. Soc., lvii. (1895) pp. 360-79. % See ante , p. 514. 
