368 THE INVERTEBRATA 



tion appears in which males are present. In this sexual and asexual 

 reproduction go on side by side. The same thing occurs again in 

 autumn or at other times when, in unfavourable circumstances, such 

 as cold or starvation, males appear. It is interesting to note that, 

 since parthenogenesis is never suspended by all the females, there is 

 nothing to show that a sexual phase in the life cycle is necessary. 



The normal cladocerans which compose the tribes Ctenopoda and 

 Anomopoda are often united under the name Calyptomera in contrast 

 to the remaining two tribes, which are known as Gymnomera. These 

 are aberrant forms whose food consists of planktonic organisms 

 relatively much larger than the particles upon which Daphnia feeds. 

 Their carapace has shrunk till it forms only the brood pouch and 

 leaves free the comparatively slender, prehensile trunk limbs with 

 which the food is handled, and their eyes are prominent and adapted 

 to sighting moving objects. They are often bizarre in form. 



Polyphemus, a British freshwater genus, is an example* of the Tribe 

 Onychopoda. It has a long telson, but the head and "abdomen" are 

 not elongate and the carapace does not fuse with the hinder part of 

 the "thorax". The trunk limbs have gnathobases. In Evadne and 

 Podon, marine members of the tribe, the telson is not elongate. 



Leptodora (Fig. 247), the only member of the Tribe Haplopoda, is 

 a pelagic inhabitant of certain fresh waters in Britain and elsewhere. 

 The body is long and slender owing to elongation of the head and of 

 the " abdomen", in which the segmentation is distinct. The fore part 

 of the trunk bears six pairs of slender, jointed, uniramous limbs, 

 without gnathobases. The carapace has fused with all the somites of 

 this region and projects behind it as a brood pouch. The winter tgg 

 gives rise to a Nauplius, the only instance of a larva in the Cladocera. 



Class OSTRACODA 



Free Crustacea, with or without compound eyes; with a bivalve 

 carapace and an adductor muscle; a mandibular palp, usually bi- 

 ramous; and not more than two recognizable pairs of trunk limbs, 

 these not being phyllopodia. 



The small crustaceans which compose this class differ little in the 

 general form of the body but show very great variety in that of their 

 appendages. All their cephalic limbs are well developed and complex; 

 the trunk limbs are uniramous and one or both pairs may be lost. 

 The adductor is in the maxillulary somite. There is often a gastric 

 mill and usually a pair or more of hepatic coeca : the latter and the 

 gonads may (Cypris) extend into the shell valves. Both antennal and 

 maxillary glands are present, both have ectodermal ducts, and both 



