678 FRESH-WATER BIOLOGY 



In the head is the large compound eye (Fig. 1050). This has nu- 

 merous or few lenses (Figs. 1059, 1076, 1169), and is capable of being 

 rotated by three muscles on each side. It is a most conspicuous 

 organ, by its size, its dark pigment, and its constant motion during 

 life. In the head are also the brain, the optic ganglion, with its 

 numerous nerves to the eye, the ocellus, or pigment spot, the an- 

 tennary muscles, and the anterior part of the digestive tract. The 

 head bears two pairs of appendages: (i) The antennules (Figs. 1051, 

 1079, 1114, 1152), which carry sense-rods, the olfactory setae, usually 

 placed at the end, and have also ordinarily one or more lateral sense 

 hairs; (2) the antennae, the main organs of locomotion, large swim- 

 ming appendages, with a stout basal joint bearing two branches or 

 rami, which, in turn, carry long plumose setae. The number of the 

 antennary setae may be expressed by a formula which shows the 

 number of the setae on each joint of each branch of the antenna; 

 the numbers for the dorsal branch occupying the place of the 

 numerator of a fraction. The formula thus constructed reads 



Daphnia (Fig. 1050), ~~ I ~ 3 ; that for Sida (Fig. 1051), 3lZ. 

 1-1-3 I ~4 



The antennae are moved by powerful muscles, which may occupy 

 a great part of the interior of the head (Fig. 1050) . On the size of the 

 antennae, the length and number of the setae, and on the size of the 

 muscles operating them, depends the type of locomotion. Latona 

 (Fig. 1052) leaps suddenly from point to point by single powerful 

 strokes of its broad antennae. The smaller Daphnidae (Fig. 1079) 

 hop, rather than leap, by more numerous and less vigorous strokes. 

 The heavier forms of this family (Fig. 1075), with smaller anten- 

 nae, have a rotating, unsteady motion, produced by rapid strokes. 

 Drepanothrix (Fig. 1104), whose antennae bear saber-like setae, 

 scrambles and pushes itself about, and the mud-haunting Ilyocryptus 

 (Fig. i no) crawls and pulls itself about among the weeds, rather 

 than swims. The members of the large family of the Chydoridae 

 have small antennae and move them very rapidly; while their 

 progress varies from a rapid whirling-motion, as in Chydorus (Fig. 

 1150), to a slower wavering and tottering progress, as in Acroperus 

 (Fig. 1121). In the Macrothricidae and Chydoridae the post- 



