74 ZOOLOGY. 
While the jelly-fishes should be studied alive, the larger ones can be 
preserved in alcohol, after being killed by the gradual addition of 
alcohol to the sea-water in which they are living. The small meduse, 
as well as Noctiluca and the Ctenophores, have been preserved with suc- 
cess by E. Van Beneden, by the use of a solution of osmic acid or of 
picric acid. Osmic acid hardens the tissues so that fine sections can 
be made, and it colors black the greasy matters, and especially myeline, 
a chemical substance usually found in the nervous system, and enables 
us to trace well the limits of the cells. The small jelly-fishes may be 
placed in a very weak solution of osmic acid (i to 7g per cent. of 
water) varying with the size of the animal, for from fifteen to twenty- 
five minutes, when the animal turns brown. This brings out clearly 
the gastro-vascular canals. The specimen can then be placed in strong 
alcohol, without losing its form and transparence. These animals and 
all other transparent animals can be well kept in a concentrated, watery 
solution of picric acid. Professor Semper tells us that all soft animals, 
worms as well as hydroids and polyps and mollusks, may be killed ex- 
panded in chromic acid (1} per cent), or in acetic acid of variable 
strength, and then preserved in alcohol. 
Cuass I].—TuHe Actinozoa (Sea-Anemones and Coral 
Polyps). 
General Characters of Actinozoans.—So persistent is the 
form and structure of the body in these animals, that a 
study of the common sea-anemone will enable the student. 
to readily comprehend the leading and most fundamental 
characteristics of the class. 
The common Actinia of our coast (Metridium marginatum) 
is to be found between tide-marks on rocks under sea-weeds, 
or in tidal pools, but grows most luxuriantly on the piles of 
bridges. It readily lives in aquaria, where its habits may 
be studied. An aquarium may be improvised by using a 
preserve-jar or glass globe, covering the bottom with sand, 
with a large flat stone for the attachment of the sea-ane- 
mone. By placing a green sea-weed (ulva) attached to a 
stone in the jar, and filling it with sea-water, the animal 
may be kept alive a long time. After observing the move- 
ments of the crown of tentacles as they are thrust out or 
