XI 



PHYLUM ARTHROPODA 



573 



from one to another at the hinge, and are closed by a large 

 adductor muscle (>.), which extends transversely from valve to valve, 

 its insertions giving rise to markings on the shell (A, m.\ often 

 of systematic value. 



At the anterior end is a median eye (e), and in some forms 

 compound eyes are present as well. There are only seven pairs 

 of appendages. The aritennules (ant.l) and antennae (ant.%) are 

 large and uriiramous. The mandible (md.) has a large leg-like palp 

 and a flabellum-like offshoot. The first maxilla (mx.l) also bears 

 a large plate resembling a flabellum of Apus. The last cephalic 



FIG. 450. A, external view of Cypris ; B, the same with the appendages exposed by the removal 

 of the left valve of the shell ; C, transverse section ; D, a single sperm, abd. abdomen ; 

 a nt, 1, antennule ; ant. 3, antenna; <l.gl. digestive gland; e. median eye ; f.l, /., thoracic 

 feet; int. intestine ; m. adductor muscle; md. mandible; mx.l, m.r.2, maxilla*; nr. ovary; 

 xh. shell ; t. testis. (After Gerstaecker.) 



appendage (second maxilla, mx.2) is jaw-like in some forms 

 (Cypris), leg-like in others (Cythere). The only thoracic appendages 

 are two pairs of slender legs (f.l, /.). The abdomen (abd.) is 

 devoid of appendages, and is terminated by a pair of small 

 caudal styles. 



The diversity of form among the Oopepoda is so great that it 

 will be advisable to consider separately the free-summing 

 Eucopepoda, the parasitic Eucopepoda, and the Brancliiura. 



The free -swimming Eucopepoda are well represented by the 

 common water-flea (Cyclops), found everywhere in fresh and 

 brackish water, and easily recognisable, in spite of its minute 

 size, by its elongated form, its rapid, jerky movements, and by the 

 egg- sacs of the female. 



