564 ZOOLOGY SECT. 



appendages of the body-segments number 10 to 27 pairs. The 

 caudal styles are in the form of unjoin ted, curved claws. 



In this order are included Esther ia, Limnctis (Fig. 448) and one 

 or two other genera. 



ORDER 4. CLADOCERA. 



Branchiopoda of small size with a bivalved carapace which 

 encloses the trunk but not the head. The eyes are sessile and 

 united together. The antennre are biramous and used as 

 swimming appendages. Only 4 to 6 trunk appendages ; caudal 

 styles unjoin ted, claw-like. 



To this order belong Daphnia, Polyphemus, Leptodora (Fig. 

 449), etc, 



Sub-class II. Ostracoda. 



Crustacea with unsegmented, or indistinctly segmented, body, 

 bearing not more than four pairs of appendages on the trunk, the 

 limbless posterior part provided with a pair of caudal styles. 

 There is a well-developed bivalved carapace. Paired eyes may be 

 present or absent. Both antennules and antennae are used in 

 swimming ; the latter are generally biramous. The mandibles 

 have a palp. The young escapes from the egg as a nauplius. 



In this sub-class are comprised Cypris, Cy there, etc. (Fig. 450). 



Sub-class III. Copepoda. 



Crustacea with elongated, distinctly segmented body, bearing 

 usually five pairs of limbs, the last four having the character of 

 biramous swimming appendages, sometimes with a sixth pair 

 which may be vestigial : the posterior region (abdomen) without 

 appendages, provided with a pair of caudal styles. The cephalic 

 dorsal shield is not extended backwards, but usually coalesces with 

 the exoskeleton of the first (and sometimes also the second) body- 

 segment. Paired eyes are absent except in the Branchiura. Both 

 antennules and antennaa are usually well developed, and the latter 

 are sometimes biramous : they may both be used as swimming 

 organs or for prehension. The mandibles may be provided with a 

 pal]). The young is a nauplius. In the parasitic forms more or 

 fewer of these general characteristics may become lost in the 

 adult. 



ORDER 1. EUCOPEPODA. 



Free or parasitic Copepoda without paired compound eyes. 

 The appendages of the body-segments are devoid of a flagellum. 

 The genital apertures are situated on the seventh body-segment. 

 In this group are included (a) free-swimming forms, such as 

 Cyclops (Water-flea) (Fig. 451) and (b) parasitic forms or Fish- 

 lice c.g, Ergasilus, Chondr acanthus. Lernwa (Fig. 452). 



