46 Marvels of Pond-Life. 



had to be put on so as to squeeze tlie animal just 

 enough to keep it still without doing it any damage, or 

 completely stopping its motions. This was a trouble- 

 some task^ and often a little overpressure prevented its 

 success. 



Some observers always use in these cases an instru- 

 ment called a compressorium, by which the amount of 

 pressure is regulated by a lever or a fine screw ; but 

 whether the student possess one or not, he should learn 

 to accomplish the same result by dexterously manipu- 

 lating a well-made live-box. We will suppose the 

 Pterodina successfully caged, and a power of about one 

 hundred and fifty linear brought to bear upon her, for 

 our specimen is of the " female persuasion.^^ This will 

 suffice to demonstrate the disposition and relation of 

 the several parts, after which one of from four hundred 

 to five hundred linear may be used with great advan- 

 tage, though in this case the illumination must be 

 carefully adjusted, and its intensity and obliquity fre- 

 quently changed, until the best effect is obtained. 



We find, on thus viewing the Pterodina, that .it is 

 a complex, highly-organized creature, having its body 

 protected by a carapace, like the shell of a tortoise, but 

 as flexible as a sheet of white gelatine paper, which it 

 resembles in appearance. Round the margin of this 

 carapace are a number of little bosses or dots, which 

 vary in different individuals. The cilia are not disposed, 

 as at first appeared, in two separate and distinct disks, 

 but are continuous, as in the annexed sketch. Down 

 each side are two long muscular bands, distinctly 

 striated, and when they contract, the ciliary apparatus 



