92 Elementary Manual of Zoology. 
the gills on the ventral surface below the mouth, taking care not to 
injure the blood-vessels which lie in the closed pericardial chamber 
below. Cut off the operculum on the side which is uppermost and the 
pericardial chamber will be easily explored. Notice the thin membrane 
(pericardium) which envelopes the heart, very much in the way that the 
peritoneum envelopes the viscera, 
Make out the following parts of the heart from behind forwards :— 
(1) The large thin-walled sack (sinus venosus) into which the blood 
passes from the liver, kidney and other parts of the body. 
(2) The thin-walled vessel (auricle) which lies in front of the 
sinus venosus. 
(3) The thick-walled pinkish organ (ventricle) which lies in front 
of the auricle and almost completely conceals it. 
(4) The little pink thick-walled organ (bulbus arteriosus) which lies 
in front of the ventricle. It is the commencement of the 
vessel through which the blood is driven into the branchial 
arteries. 
By pulling back the bulbus arteriosus with forceps and dissecting 
away the tissues carefully in front, the continuation of this vessel can 
easily be made out to where it gives off the paired branchial arteries to 
the gills. From the gills the blood is gathered up by arteries, which 
unite to form the dorsal aorta. The dorsal aorta is a straight pinkish tube 
which runs down the back immediately beneath the vertebral column, 
It can readily be traced by dissecting away the kidney. After oxidisa- 
tion in the gills the blood passes down the dorsal aorta, which supplies 
the tail and all the posterior of the body. The blood from the tail 
returns vid the kidney, where it is partially purified. That from the 
digestive organs returns v7d the liver, where the nutriment absorbed from 
the digestive tract is elaborated. From both ktdney and liver the blood 
passes into the sinus venosus, whence it is driven by the heart.to the gills. 
Cut away all the ventral portion of the specimen, leaving only the 
head and the first inch or two of the back. Pin this portion, dorsal 
surface upwards, on to the weighted bark, and cut away the back of the 
head with scissors, so as to expose the brain. This dissection is best 
performed upon a specimen which has been hardened in alcohol, but most 
of the points can be made out without much difficulty in the fresh state. 
The brain is small and lies deep in the skull. After eutting away the 
bone, pick off with forceps the tissue which surrounds the white nerve- 
masses of the brain, and make out the following from behind for- 
wards :— 
(1) The cerebellum, This is the large median lobe at the posterior 
of the brain. Behind it can be seen the medulla oblongata, 
which is little more than a slight expansion of the spinal 
cord. 
