374 PRESERVATION OF MARINE ORGANISMS 



It is a distinct advantage, soon after killing the 

 medusa in formalin, to pour in some chromic acid — 

 about a 5 per cent, solution — -just enough to colour 

 the formalin. The chromic acid makes the jelly tough 

 and more pliable, so that there is less risk of breakage 

 when the specimens are handled for examination. The 

 yellowish-brown colour of the chromic acid may be 

 disregarded, because in the course of a few months the 

 colour changes to a pale bluish-green, owing to the 

 oxidation of the chromic acid. 



The successful preservation of Scyphomedusae, 

 especially the large ones, greatly depends upon using 

 plenty of strong formalin. 



Ctenophores are troublesome animals to preserve, 

 because nearly every genus requires special treatment. 

 There is, however, one general rule to be remembered — ■ 

 namely, that Ctenophores must not be stored away in 

 formalin, but always in alcohol 70 per cent. 



Pleurobrachia may be killed in formalin 5 per cent, 

 in sea water. The simplest method is to fill a large 

 test-tube or a tall cylindrical measuring-glass with the 

 formalin solution, and to drop the specimens in. Leave 

 them alone until they sink to the bottom, and then 

 remove to formalin 5 per cent, in fresh water. They 

 may be left in formalin for a few weeks, but it is best 

 not to delay their transference into alcohol too long. 

 Begin with very dilute alcohol, and gradually work up 

 to 70 per cent, strength. 



Beroe may occasionally be killed in a fairly good, 

 expanded condition by allowing it to swim in a small 

 glass pot, with just sufficient sea water to cover it, 

 and then, when fully expanded, pouring in very quickly 

 a large quantity of corrosive sublimate (saturated 

 solution in sea water). As soon as the specimen has 

 become white, pour off the corrosive and add fresh 



