CARAPACE OF CLADOCERA 



39 



in Ceriodaphnia. In a few forms the whole surface is irregularly 

 covered with spines or scales. The hinder edge of the carapace 

 is often produced into a median dorsal spine {Daphnia, Fig. 19), 

 or more rarely there are two spines, one at each ventro-lateral 

 corner (ScapJioleberis, Fig. 20). 



The cuticle of the carapace is often separated from that of 

 the head by a cervical suture, as in Simocephalus (Fig. 10, C'./S'.), 

 and near the line of demarcation many forms exhibit patches of 



Fig. 10. — Simocephalus vetulus, x 30. Side view of female, showing tlie arrangement 

 of tile principal organs. A.S, Second antenna ; O.S, cervical suture ; li, fused 

 compound eyes ; ]T, lieart ; L, forwardly-directed gastric caeca ; iV, dorsal organ. 



glandular ectoderm which seem to be homologous with the 

 dorsal adhesive organs of the Limnadiidae. The commonest 

 condition is that of a median dorsal pit (Fig. 10, K), by means 

 of which the animal can fix itself to foreign objects. Certain 

 forms may remain for long periods of time attached by the 

 dorsal organ to plants, or to the sides of an aquarium, the only 

 movement being a slow vibration of the feet, by which a current 

 of water, sufficiently rapid for respiratory purposes, is established 

 round it.^ In Sida crystallina (Fig. 11) the dorsal organ is 

 represented by three structures ; in front there is a median raised 



^ Simocephalus vetuhts anchors itself to weeds, etc., by a modified seta on tlie 

 exopodite of the second antenna. It does not enjploy a dorsal organ for purposes 

 ofaxation. [G. S.] 



