124 THE PRESERVATION OF MARINE ANIMALS
former is killed in the chrom-osmic mixture, in which it is to remain
fifteen to thirty minutes, before being transferred to alcohol. Beroe
is simply placed directly in alcohol, 70 per cent., but before
immersion in this a short piece of wide glass tube is placed in the
gastric cavity, a bubble of air being left in the tube so as to cause
the specimen to float in the alcohol. The tube keeps the specimen
in its natural cylindrical form, but great care must be exercised in
the manipulation.
Echinodermata.—In order to prepare star-fishes with the
ambulacral feet distended, they should be allowed to die in alcohol
of 20 to 30 per cent., with their ambulacral surface uppermost.
Ophiuroidea should be killed in fresh water and placed in alcohol
when dead.
Sea-urchins are to be placed in a small quantity of sea water, and
when their tube-feet are extended a mixture of acetic acid 100 parts,
chromic acid (1 per cent.) ten parts, is poured suddenly over them.
They must be removed into alcohol immediately afterwards, so that
the acid may not have time to act on the shell. When placed in the
alcohol two small holes should be made at opposite points of the
shell to allow the alcohol to enter the interior, and care must be
taken in transferring the specimens to stronger alcohol, that the
interior is emptied and then refilled with the latter. To prepare sea-
urchins dry, holes should be made in them in the same manner, and
then they should be placed in 70 per cent. alcohol for a day or two,
after which they can be taken out and left to dry.
Holothurians are extremely difficult to fix in their natural
extended condition. The method to be followed with large speci-
mens is to seize them when expanded with finger and thumb behind
the base of the tentacles and plunge the anterior part of the body
into pure acetic acid, while another person injects alcohol of 90 per
cent, into the specimen per unum with a syringe. As soon as it is
dead, the specimen is placed in 70 per cent. alcohol, a cork being
placed in the anal orifice to prevent the escape of the injected liquid.
Smaller specimens are treated in the same way, omitting the injection,
and using forceps instead of the fingers.
Planarians are to be killed by pouring over them boiling corro-
sive sublimate in saturated solution, turning them out immediately
they are dead into cold fresh water, and after washing them placing
them in alcohol.
Nemerteans should be narcotized in a solution of chloral