CRUSTACEA OF FRESH WATERS 169 



(Fig. 58), which is found in the lakes of Scotland, 

 Ireland, Wales, and the Lake District of England. 

 In Bythotrephes the carapace does not enclose the 

 body, but is reduced to a small brood-sac; the 

 addomen, however, is drawn out into a long spine, 

 which may be two or three times as long as the 

 body. A further point of interest is the division of 

 the eye into a dorsal and a ventral portion, differing 



Fig. 58 — Bythotrephes longimanus, Female, with Embryos in the 

 Brood- SAC. x 12. (After Lilljeborg.) 



in structure in much the same way as do the two 

 divisions of the eyes in certain marine plankton 

 Crustacea (see p. 152). Another very remarkable 

 lacustrine form is Leptodora, the largest of all the 

 Cladocera, being sometimes more than half an inch 

 in length. In this case also the carapace is very 

 small, and does not enclose the body. The swimming 

 antennae are very large, and the abdomen is long and 

 divided into several segments. 



Leptodora is further remarkable on account of its 

 mode of development. The parthenogenetic eggs. 



