XI 



PHYLUM ARTHROPODA 



559 



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 antemmles (ant. 1) and antennae (ant. 2) are 

 large, and the latter usually biramous. 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 appendage (second maxilla, mx.2) is 

 jaw-like in some forms (Cypris), leg-like in others (Cythere). The 

 only thoracic appendages are two or four pairs of slender legs 

 (f.l, f.2). The abdomen (abd.) is devoid of appendages, and is 

 terminated by a pair of small caudal styles. 



FIG. 457. 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; ant.l, antennule ; ant.2, antenna; d.r/l. digestive gland; e. median eye; 

 f.l, f.2, thoracic feet; int. intestine ; m. adductor muscle ; md. mandible ; mx.l, mx.2, 

 maxillae ; ov. ovary ; sh. shell ; t. testis. (After Gerstaecker and Zenker.) 



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

 will be advisable to consider separately the free-swimming 

 Eucopepoda, the parasitic Eucopepoda, and the Branchiura. 



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. 



Cyclops (Fig. 458, 1) has been compared in form to a split pear, 

 the broad end being anterior, and the convex surface dorsal. 

 The first thoracic segment is fused with the head, and the 

 cephalothorax (c. th.) thus formed is covered with a carapace 



