SPONGES AND HYDROIDS 369 



alcohol giving the best result). By hanging them in 

 the fluid the strength of the alcohol surrounding them 

 is kept as high as possible. They should be transferred 

 to fresh alcohol after a few hours, and again in the 

 course of a day or two. 



They may also be well preserved by plunging them 

 into osmic acid (1 per cent, solution), in which they 

 should remain for from three to five minutes. Small 

 specimens taken from a rock-pool and put immediately 

 into tubes containing osmic acid give excellent pre- 

 parations. Larger sponges will need to be cut into 

 small pieces before being placed in the preserving 

 fluid, and the sooner this is done after the sponge is 

 taken from the sea, the better the result will be. 



Hydroids may be easily killed and preserved by 

 simply placing them in formalin 10 per cent., but one 

 must not expect to see the hydranths nicely expanded 

 by this method. 



Most gymnoblastic hydranths can be killed in a 

 fairly good state of expansion by quickly plunging the 

 expanded colony into warm corrosive sublimate, and, 

 moving it slowly up and down for about a minute. 

 Wash afterwards in water to remove the corrosive, 

 and transfer to formalin or alcohol. A safer method 

 for catching the hydranths nicely expanded is to 

 anaesthetize with menthol, which takes many hours, 

 and then to kill with corrosive. Many calyptoblastic 

 forms can be killed in a fair state of expansion by 

 simply plunging the colony into warm corrosive, and 

 giving it a gentle shaking. They are more sensitive 

 to the presence of anaesthetics than the gymnoblastic 

 forms, and to kill the hydranths well expanded requires 

 a considerable amount of practice. 



Hydromedusae. — The successful preservation of 

 medusae, or any other delicate, soft-bodied animals, 



24 



