HIGH SCHOOL ZOOLOGY. 



199 



A cominon form in spring pools is Braiichipus vernalis 



(Fig. 125), with eleven pairs of such 

 legs ; an allied genus, Artemia, is 

 veiy common in salt lakes. Other 

 genera are protected by a shell, which 

 may be horse-shoe shaped as in Apus, 

 or formed of two valves as in Estheria. Such shells are also found 

 in another group of Phyllopods — the Cladocera or water-fleas — ■ 

 (Fig. 126), in which there are only five pairs of legs, but which 



Fi^. 125. — Branchipus vernalis, 

 swimming on its back. x 3. 



Fig. 126 —I)a2)h7iia pukx. x 18. Fig. 127.— Cypr is Candida, x 16. 



are otherwise marked by the large second pair of antennae 

 taking on a locomotive function. Another order, Ostracoda, in- 

 cludes forms with a still shorter post-cephalic region, for only 

 two pairs of legs are to be found behind the jaws (Fig. 127). The 

 Copepoda, however, have a much longer post-ceplialic region 

 than this, there being five thoracic segments, the first of which 

 is coalesced with the head, and five abdominal segments term- 

 inating in a furca. The latter are footless, but the thoracic 

 segments bear biramous swimming feet, and the head-segments 

 the usual appendages, although the second pair of maxillae 

 separate on each side into two independent so-called foot-jaws, 

 which may undergo curious alterations. Many of the Copepoda 

 live a semi-parasitic or parasitic life ; in the free forms (Fig. 12S), 

 the jaws are formed for biting, but in the pai'asitic forms, the 

 parts of the mouth are more or less converted for sucking or ad- 

 hesion (Figs. 129 and 130), in which the posterior antennae may 

 assist. As a rule the parasitic Copepoda do not appear to injure 



