OCELLATE HYDRO-MEDUSA. 39 



filled with a saturated solution of picric acid in fresh 

 water, and left for eight or ten hours. Each specimen 

 should then be placed, by itself, in a small bottle of very 

 dilute alcohol ; about forty per cent. In about half an 

 hour this should be poured off and renewed, and the pro- 

 cess repeated until the alcohol shows no trace of a yellow 

 color. After the specimen has remained for about half 

 an hour in the last alcohol, pour off all but enough to 

 cover it, and add strong alcohol, a few drops at a time, 

 at intervals of about five minutes, until the bottle is 

 filled. 



The specimen should be examined in some of the fluid 

 from its own bottle. 



I. The General Structure. Examining a specimen in a 

 watch-crystal, with a low power of the microscope, or 

 with a hand-lens, notice : 



1. The transparent gelatinous umbrella (Fig. 21, ft, 

 25, a) which makes up the greater part of the body. The 

 outlines are sharp and regularly curved in a living speci- 

 men, but they are usually somewhat shrunken and dis- 

 torted in a preserved specimen. 



a. The portion of the umbrella which is at the top in 

 Fig. 21. and which, from its relation to the mouth, may 

 be called the ab-oral portion, is greatly thickened, and the 

 outer and inner surfaces are separated from each other by 

 the elastic gelatinous substance of the umbrella. 



b. At the lower or free edge (Fig. 21, b), the gelatin- 

 ous substance gradually diminishes in thickness. 



2. The sub-umbrellar cavity or space (Fig. 25, b) under 

 or inside of the umbrella. 



3. The velum, or muscular horizontal diaphragm (Figs. 

 21 and 25, c) which runs inwards around the lower edge 

 of the umbrella, over the opening of which it forms a flat 



