682 FRESH-WATER BIOLOGY 



Little study has been given to the senses of the Cladocera, except 

 that of sight. As special organs of touch there are the abdominal 

 setae, which are sometimes very long (Fig. 1090) ; sensory hairs on the 

 basal joint of the antenna near the body (Fig. 1075), or near the apex 

 (Figs. 1051, 1053, 1089) ; the lateral sense hair of the antennule (Figs. 

 1089, 1 1 17, 1 1 54); the flagellum on the antennule of the Sididae 

 (Figs. 105 1- 1 05 7), which is often fringed with fine hairs; and the 

 frontal sense hair of Bosmma (Fig. 1096). Any of the innumerable 

 hairs and setae may also serve this sense, though not specially modi- 

 fied for that purpose. There is no auditory organ. Whether the 

 olfactory setae really give sensations of smell and taste is doubtful, 

 although the structure of the sense rods is such that they may well 

 serve a chemical sense. They lie at the entrance of the valves in 

 the current of water which is coming in under the impulses of the 

 feet, and may take cognizance of the particles of food, etc., which 

 come along with the water. The Cladocera are certainly able to 

 discriminate between different kinds of particles brought in by the 

 legs, eating some and rejecting others. They have decided tastes 

 in the matter of diet, preferring some forms of algae to others. In 

 general, the diatoms are eaten in preference to the blue-green algae. 

 In the selection of food, the Cladocera are aided also by sensations 

 which arise in the mouth, since they may reject particles which 

 have been brought into the mouth and partially chewed. 



The eye is obviously the visual organ. It is sensitive to light and 

 can no doubt distinguish objects by the shadows which they pro- 

 duce, although its lenses are by no means numerous enough, or 

 perfect enough, to give sensations of form. The constant motions 

 of the eye are for the purpose of moving the lenses so that they will 

 cover the entire field of vision, and the animal no doubt directs its 

 movements by sensations which it receives through the eye. The 

 Cladocera respond differently to light of different intensities and 

 various colors. Most of them react positively to a weak light and 

 negatively to a strong one. There is, however, much difference in 

 this respect. Drepanothrix, for example, is vigorously repelled by 

 the light of a lamp, which will attract all the other Cladocera in 

 the vessel with it. Newly hatched Cladocera are attracted by Hght 

 which will repel older forms. On a bright, calm day a few inches 



