Practical Course. 79 
(2) Females with broad abdomens, to which are attached four pairs 
of bifurcated swimming appendages. 
In the female make out the paired genital openings, which are easy 
to see on the ventral portion of the cephalothorax, opposite the inser- 
tions of the last pair but two of large ambulatory legs. 
In the male look for the genital openings, which are situated at 
the base of the last pair of legs. They are much more difficult to make 
out than the genital openings of the female. 
Lift up the jointed appendages which surround the mouth and 
notice on either side the anterior opening into the branchial chamber, 
Also lift up the whole cephalothoracie shield as far as it will go with- 
out tearing the membranes underneath it, and see the posterior openings 
above the last pair of ambulatory legs, through which the water enters 
the branchial chamber. 
Now make a sketch of the crab, both from above and from below 
and mark in the names of what has been seen. 
With a pointed pair of scissors cut away one side of the shield of 
the cephalothorax very carefully, above the bases of the ambulatory 
legs, beginning at the anterior opening of the branchial chamber. 
This will expose the large branchial chamber. Notice the numerous 
branchie which occupy the chamber ; they are attached just above the 
bases of the legs. 
Examine one of the branchiz with a lens and see how it consists 
of a central portion to which are attached a number of delicate pro- 
cesses arranged like the leaves of a book. 
With a pair of scissors cut away the edges of the shield which 
covers the cephalothorax. Dissect off the whole shield, a little piece 
at a time, so as not to damage the organs which lie below it. In doing 
this be very careful not to tear any of the tissues, for the heart lies 
immediately below and is liable to be torn away with the muscles by 
which the stomach is attached to the centre of the shield. 
If small pin the animal, dorsal surface upwards, on to the weigh- 
ted piece of bark, inserting the pins into the bases of the legs, If large 
it will be better not to pin it down, Now immerse the crab in water 
deep enough just to cover it. 
Notice that the whole space which has been exposed by dissecting 
off the cephalothoracic shield is covered with a delicate dark-coloured 
membrane. Dissect off the whole of this membrane very carefully 
under water, removing any thread-like parasites (Nematodes) which you 
come across. 
The following organs will now be visible :— 
(1) On either side the large branchial chamber, which we have 
already examined from below. 
