IV 



4 ZOOLOG Y. 



While the jelly-fishes should he studied alive, the larger ones can he 

 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 Nocliluca and the Ctenophores, have been preserved with suc- 

 cess hy E. ^ 7 'an Beueden, by the use of a solution of osmic acid or o{ 

 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 (5 to -^ percent, 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 (l 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 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 marginal um) 

 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 u 

 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 



