50 THE CRUSTACEA 



The young of the Conchostracan Cyclestheria have an embryonic 

 development very similar to that of Cladocera within the shell of 

 the parent. 



A remarkable exception to the rule of embryonic development 

 among Cladocera is afibrded by the aberrant genus Leptodora. The 

 parthenogenetic " summer "-eggs develop in the usual way Avithin 

 the brood-pouch of the parent. The " winter "-eggs, however, hatch 

 out as metanauplii (Fig. 31). The body is unsegmented, but the 

 six pairs of thoracic limbs are already visible as rudiments. The 

 antennules are very short. The antennae, on the other hand, are 

 unusually large, as they are also in the adult. They are without 

 any masticatory process. The mandibles have long unsegmented 

 palps. The compound eyes are not yet developed, but the nauplius 

 eye is present, and persists throughout life in the individuals 

 hatched from " winter "-eggs, Avhile it is absent in those hatched 

 from the " summer "-eggs. In the reduction of the antennules this 

 larva shows some resemblance to that of Estheria. 



Remarks on Habits, etc. 



The great majority of the Branchiopoda inhabit fresh water. 

 A few species of Cladocera, belonging to three genera, occur in the 

 sea, and the Anostracan Artemia is found in salt lakes and brine- 

 pools. The Cladocera form an important part of the plankton of 

 lakes and ponds, and the larger Anostraca, Notostraca, and Con- 

 chostraca occur chiefly in small ponds and rain-water pools. The 

 occurrence, throughout the group, of sexually produced, thick- 

 shelled " resting "-eggs, which can survive desiccation, in addition 

 to the thin -shelled eggs produced by parthenogenesis, probably 

 indicates the very «-ncient adaptation of the Branchiopoda to a 

 freshwater habitat. 



No parasitic Branchiopoda are known. 



The Cladocera are nearly all of microscopic size, and some species 

 which do not exceed 0*25 mm. in length are among the smallest 

 known Arthropoda. Of the other orders, the Notostraca comprise 

 the largest forms, some species of Apus reaching 70 mm. in 

 length. 



Palaeontology. 



The Conchostraca are well known as fossils, and forms 

 referred to the existing genus Estheria occur as early as the 

 Devonian. The Notostraca are more doubtfully represented by 

 P.rotocaris from the Lower Cambrian. From the delicacy of their 

 structure, the Anostraca are less likely to be preserved, and almost 

 the only undoubted example is Branchipodites of the Oligocene. The 

 Cladocera are not certainly known earlier than Post-tertiary deposits. 



