78 Elementury Mannal of Zoology. 
which are given off from each swollen place (ganglion). Immediately 
behind the mouth the students will be able to make out that a pair 
of large nerves are given off. These pass round the digestive just in 
front of the pharynx and connect the ventral chain with a pair or 
nerve masses (supra-zsophageal ganglia), which lie above and in front 
of the esophagus. 
The exeretary organs of the earth-worms consist of a number of 
little tubes (nephridia), which open to the exterior,a pair to each 
segment, They are very difficult to make out, however, in the Dehra 
species and the students will not be able to recognise them. 
DISSECTION OF THE FRESH-WATER CRAB. 
Fresh water-crabs (Zelphusa sp.) are common under stones in the 
beds of the Song and other rivers of Dehra Dun. They are known 
locally as gegra or kenkra. The largest available specimens should 
be procured. They can be killed almost instantaneously by dropping 
them into boiling water. If boiling water is not available, chloroform 
may be used. 
Examine a specimen closely and observe that it is covered with 
a hard armour (exoskeleton) which serves in all Arthropoda to support 
the muscles. Make out in it the following parts :— 
(1) The body, which consists of — 
(2) The broad anterior unsegmented portion (cephalo- 
thorax) with a curved groove across the dorsal 
portion of it. 
(2) The little posterior segmented portion (abdomen), 
which consists of six divisions folded up under 
the cephalothorax. 
(2) Five pairs of large jointed appendages attached to the 
ventral surface of the cephalothorax. 
(8) The jaws and other small jointed appendages which surround 
the mouth. 
(4) The mouth, which is to be seen by separating the appendages 
which surround it. 
(5) The pair of movable eye-stalks, and the two pairs of minute 
jointed antenne, which are situated in front of the 
cephalothorax. 
(6) The anus, or posterior opening of the digestive tract, situated 
on the lower surface of the last segment of the abdomen. 
(7) The little jointed appendages attached to the abdomen, 
Notice that the crabs are of two kinds, vzz. :-— 
(1) Males, with narrow abdomens, to which are attached two 
pairs of little white appendages close to the cephalo- 
thorax. 
