Protozoa <ui(l llotifvrd at IIara)ta, III. 347 



perception of light than for the formation of images. While 

 the eyes are absent in the adults of some species, they may 

 always be found in the embryonic or young forms. 



Rotifers are dioecious and dimorphic ; that is, the sexes are 

 represented by separate individuals, and the males differ 

 greatly from the females in size and organization. Descrip- 

 tions of rotifers apply only to the females, as the males are but 

 little known and may be found for only a few days once or 

 twice a year. The male is usually inferior in size to the 

 female, and consists of a transparent sac with a circle of cilia 

 around the anterior border. It has no internal structures 

 except a sperm sac and copulatory organ, eats nothing, and 

 disappears in a few days. 



Rotifers are usually oviparous, although there are forms, 

 like Rotifer and Asplanchna, that are viviparous. There are 

 three kinds of eggs. One of these is the ordinary egg, from 

 which the female hatches, and which is sometimes encased 

 in a spiny or roughened shell. Another form, from which 

 the males are hatched, is smaller and smooth-shelled. These 

 two are the so-called summer eggs. The third is the "lasting" 

 or "ephippial" egg, which is larger than either of the other 

 forms and has a hard, thick shell, well able to withstand 

 drought and frost. The eggs are usually carried about at- 

 tached to the posterior part of the lorica just above the foot. 



Experiments were long ago made which show that the adult 

 rotifer can be revived after it has been subjected to a severe 

 drying process. Interesting results were obtained by us by 

 starting aquaria with earth from the bottoms of dried-up 

 ponds and lakes. Within two days after filtered water had 

 been placed on the dried mud, full-grown specimens of BracJi- 

 ionus with eggs were found, which goes to prove that the 

 rotifers had been dried with the mud and had revived when 

 the water was added. Further experiments along this line 

 might be profitably carried on to ascertain just how long 

 rotifers can remain in this condition without having their 

 vitality impaired, and to find out whether they will survive 

 after being subjected to the hard frosts of a severe winter. 



