ENTOMOSTRACA. 



37 



same name is applied. In some the first pair of appendages are absent, in others they 



are well developed. Some have the segments of the body behind the mandibles well 



indicated, w T hile in others no indication of segmentation 



is visible. From this point the development is more or 



less direct, no startling metamorphosis being introduced. 



With succeeding moults new segments are introduced, and 



new limbs appear, until the adult condition is reached. 



In the Limniadiadge at an early age the carapax of the 



nauplius becomes folded so as to form the bivalve shell 



of the adult. It is uncertain whether all of this shell is 



moulted, but the evidence adduced seems to us to indicate 



that at least in Estheria it is not wholly cast, and that 



the concentric lines upon the outside are in reality lines 



of growth, like those so familiar among the Mollusca. 



An interesting feature of the eggs of the Phyllopods 

 is their ability to withstand dessication; in fact, the only 

 species so far as known whose ova have not this power is 

 Lepidurus productus of Europe. This power is not, how- 

 ever, confined to this group, but is possessed by Cypris, Cypridinia, Daplmia, and 

 many Copepoda. It is apparently a necessary provision for the perpetuation of these 



forms, for the puddles in which they dwell 

 usually become dry in the summer, and 

 were it not for this peculiarity of the 

 eggs the species would soon disappear. 

 When the mud is wet again by the spring 

 thaw the eggs hatch, but it is not neces- 

 sarily the ova deposited by last year's 

 brood which people the puddle, for Dr. 

 Semper found that mud which was taken 

 from a pond in which he had found Apus 

 gave no young the first year, and but few 

 the second ; but in succeeding years he was 

 able to hatch out nauplii in great numbers. 

 In 1872 Professor Zittel collected mud in 

 the oasis of Dahel, which was sent to Pro- 

 fessor von Siebold, and produced nauplii 

 of Artemia in 1877, but none in previous 

 years. A still further peculiarity is the fact 

 that, at least in several species, it is neces- 

 sary that the mud containing the ova should 

 become dry in order that the eggs may 

 hatch. So far as known a single species of 

 Chirocephalus is the only form in which 

 this dessication is not an absolute essential ; and further, the researches of Professor 

 Brauer show that the eggs of several Phyllopoda develop most rapidly, and, perhaps, 

 only, when they have previously been exposed to a temperature near the freezing 

 point. The length of time required for hatching and development varies here, as in 

 other groups, with the temperature. Branchipus and Apus eggs require several weeks at 



FIG. 49. Lepidurus productus. 



