388 INVERTEBRATE MORPHOLOGY. 



into the anterior abdominal region. Lateral eyes are present 

 in addition to the unpaired median eye. In Branchipus they 

 are situated upon the sides of the head upon well-defined 

 stalks, but in Apus they are closely approximated on the 

 dorsal surface of the cephalo-thoracic carapace, while in the 

 shelled forms they are united together to form a single eye 

 whose double nature is revealed only by a study of the details 

 in its arrangement. 



A peculiar feature in the life-history of the members of this group is the 

 comparative iufrequency of males, their proportion to females being so 

 small that for some time they were not known to exist. The females are 

 able to reproduce parthenogenetically males appearing only under certain 

 conditions which are not as yet satisfactorily understood. The eggs de- 

 velop generally in brood-pouches situated upon certain of the thoracic ap- 

 pendages (Apus, Limnadia) or else are affixed to filamentar processes of 

 these appendages (Mstheria). 



2. Suborder Cladocera. 



TheCladocera are distinguished from the Branchiopoda by 

 the segmentation of the body being much less clearly defined 

 and by the small and more definite number of appendages, 

 there being only from four to six pairs of thoracic limbs. A 

 bivalved shell arising from the maxillary segments and pro- 

 vided with an adductor muscle is always present ; it does not 

 enclose the head, but the rest of the body may be completely 

 withdrawn within it except in some genera, such as Evadne 

 and Polyphemus, in which it is transformed into a brood- 

 chamber, leaving the body almost unprotected. 



The antennules are always small unjoiuted structures pro- 

 vided with a bunch of olfactory hairs usually terminal in po- 

 sition, and the antennae are strong biramous locomotor organs. 

 The mandibles are simple toothed plates without palps, and 

 the second maxillae are usually entirely wanting in the adults. 

 The thoracic limbs are six in number in the genus Sida and 

 are all lamellate and abundantly supplied with marginal setae, 

 but, in Daphnia (Fig. 175), Moina, and allied forms the number 

 is reduced to five, and the more anterior ones are more or less 

 modified towards simple cylindrical jointed appendages, a 

 condition found in all the four thoracic appendages of Evadne 



