662 



FRESH-WATER BIOLOGY 



in the position of the eyes. In the Anostraca (Fig. ion) there is 

 no shell-fold and the body, composed of many distinct somites, has 

 an almost worm-like aspect; the Notostraca (Fig. 1012) are also 

 elongated and composed of numerous somites, but are flattened, and 

 their anterior portion is covered dorsally by a broad arched carapace ; 

 the bodies of the Conchostraca (Fig. 1013) tend to be laterally com 

 pressed and are enveloped in a bivalve shell that makes them look 

 like a small clam. The shell-fold is not attached to the trunk 

 somites which it envelops. It may be more or less corneous but 

 is never calcified. The eyes are elevated on movable peduncles in 

 the Anostraca but are sessile in all other phyllopods. A peculiar 



FIG. ion. Branchinecta paludosa, male and female, ai, first antenna; 02, second antenna; d, cerco- 

 pods or furcal rami; p, penis; t, telson. X 3. (After Packard.) , 



structure, the frontal (or haft) organ, is variously developed in 

 the different groups; in some it is only a sensory area and in others 

 it has a knob-like pediculated form. 



The head is distinct from the trunk and the number of trunk- 

 somites is variable. Some notostracans have as many as forty-two 

 trunk-somites; the Conchostraca have from thirteen to twenty- 

 eight, and the number in the Anostraca ranges from nineteen to 

 twenty-three. Apart from the head, the trunk of phyllopods 

 shows no differentiation into distinct regions. The terms " thorax" 

 and "abdomen" have been variously used to designate the pre- 

 or post-genital, or the limb-bearing or limbless, regions respectively. 

 But the limits of these regions do not coincide, even approximately, 

 except in the Anostraca; and "thoracic" and "abdominal" are 

 therefore not applicable to the group. The last segment, or telson, 

 usually bears a pair of appendages, the furcal rami or cercopods. 



The appendages are fairly uniform in character, except as they 

 are modified by sexual dimorphism. The first antennae are always 



