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 medusae, 

 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-flshes may l)e 

 placed in a very weak solution of osmic acid (\ to -f^ 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 moUusks, may be killed ex- 

 panded in chromic acid (IJ per cent), or in acetic acid of variable 

 strength, and then preserved in alcohol. 



Class II.— The Actinozoa {Sea- Anemones and Coral 

 Polyps). 



General Characters of Actinozoans.— So persistent is the 

 form and strneture 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 aliye a long time. After observing the move- 

 ments of the crown of tentacles as they are thrust out or 



