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 antennules (ant.l) and antennae (ant.2) are 

 large and uniramous. 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. oM. abdomen ; 

 ant.l, antennule ; an/.?, antenna; <Lgl. digestive gland; e. median eye; /.!,/. 2, thoracic 

 feet; int. intestine ; m. adductor muscle; itf. mandible; mx.l, mx.2, maxilbe ; or. ovary; 

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



appendage (second maxilla, m&.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 Copcpoda is so great that it 

 will be advisable to consider separately the free-swimming 

 Eucopepoda, the parasitic Eucopepoda, and the Branckiura. 



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. 



