128 Report of the Microscopical Section. [Sess. 
its passage from the heart to the gills loses part of its oxygen, 
and takes up a corresponding quantity of carbonic acid. This 
latter must be got rid of and the former renewed, which inter- 
change is effected by means of the gills. In these there are 
vascular channels which break up into a fine network, and the 
blood brought from the arteries by the sinuses is forced into 
these channels, where it is separated by a very thin chitinous 
membrane from the surrounding water in the branchial chamber. 
Here the blood becomes aerated, giving off its carbonic acid and 
taking up a corresponding quantity of oxygen. Thus renovated, 
it is forced by the succeeding currents of blood into other sin- 
uses, which in turn conduct it into the pericardial sinus, so that 
it may again with its life-supporting oxygen enter the heart. 
The nervous system of the crayfish runs in the middle line 
of the ventral aspect of the body close to the integument from 
the eyestalks to the telson. At this hinder end the nerve 
cord is divided up into several distinct branches, each proceed- 
ing to a different part of the telson and adjacent places. At 
each of the abdominal somites there is a ganglionic mass giving 
off branches to the muscles: equally in the thorax there are 
six ganglionic masses giving off branches to the muscles of the 
various appendages. When the main nerve cord reaches the - 
gullet, it divides into two branches, one running on each side 
of the gullet, afterwards uniting in front of it into a mass 
called the cerebral ganglion, from which nerve fibres run out 
to the antennz and eyestalks. 
The organs of vision—These are borne on the first somite 
of the head, and consist of a pair of compound eyes placed in 
a cavity at the end of comparatively long eyestalks. These 
eyestalks are calcified the same as other parts of the exo- 
skeleton, and at their end is placed the cornea, a transparent 
substance divided into a large number of minute squares, 
which, under the microscope, resemble mosaic work. Attached 
to the inside of each of these squares or facets is a rod called 
the visual rod, which runs back to a bulb in the eyestalk. 
This bulb is the expansion of the optic nerve. ‘The luminous ~ 
sensations are carried along these rods to the optic nerve, — 
whence they are conveyed to the brain. On examining a 
longitudinal section of the eye under the microscope, it is — 
found that the visual rods consist of several bands or zones of 
