2 6o AQUATIC MICROSCOPY FOR BEGINNERS 



they are kept until the young are hatched, when the 

 latter make their escape into the water, to care for 

 themselves. 



In those forms without shells the eggs are passed 

 out of the body into one or two small, pear-shaped 

 sacks called external ovaries, where they remain until 

 hatched. In these cases, however, the egg masses are 

 carried about by the parent and become conspicuous 

 objects. It is a common occurance to find the little 

 animals apparently loaded with the burden of eggs, 

 and not uncommon to see the young escape. The 

 "common Cyclops" is an instance. No member of the 

 Entomostraca is so frequently seen nor so abundant as 

 the Cyclops, and hardly any other affords so good an 

 example of this method of depositing and caring for 

 the eggs in external overies, Cyclops having two of 

 the latter, while some other almost equally common 

 forms have but one. The external ovaries are usually 

 long, pear-shaped bodies attached one on each side 

 near the posterior extremity of the animal, where it 

 narrows to form its tail-like region. The eggs are 

 round, unless they are made polygonal by pressure, 

 almost black, and entirely opaque. In Canthocamptus 

 there is but one external ovary. Both kinds are 

 shown in Figs. 185 and 186. 



In many instances the young, when first hatched, 

 bear so slight a resemblance to the parent that some 

 of them have been described and named as entirely 

 different animals; and it was not until they were seen 

 leaving the egg while still attached, or in the external 

 ovary, that their true character was discovered. This 

 is especially true of Cyclops, the young of which is 

 shown in Figure 186. It changes its skin several 



