THE BRANCHIOPODA 



49 



process of the mandible is feebly developed. The paired frontal 

 sense-organs in the form of papillae develop at an early stage, 

 though not present on hatching. The maxillulae and maxillae are 

 generally not marked off till after the succeeding limbs have been 

 differentiated. The " dorsal organ " or nuchal gland is very large 

 in the early stages even in those forms in which it is greatly 

 reduced or absent in the adult. The trunk-somites and their 

 appendages become differentiated in regular order from before 

 backwards. 



In the Conchostraca the earliest larva has no trace of the 

 trunk - somites. The antennules are greatly reduced and the 

 labrum is very large. The nauplius of Limnetis is remarkable for 

 the broad dorsal shield and for the peculiar cruciform shape of the 

 front of the head. In the Conchostraca and in the Cladocera the 

 shell develops from paired rudiments. The later larval stages of 



Fio. 31. 



Metanaupliiis of Li'jitodom, hate-lied fiom a " winter "-egg. c', antennule ; ", antenna ; md, 

 mandibular palp; ol, labrum; p'-p", rudiments of the six pairs of trunk-limbs. (After Sars, 

 from Korsehelt and Heider's Embryology.) 



the Conchostraca, as, for instance, Estheria, correspond very closely 

 with the adult structure of the Cladocera. 



In the Cladocera the eggs are usually large and rich in yolk, 

 or, when the reverse is the case, a special provision is made for the 

 nourishment of the developing embryos within the brood-chamber. 

 The egg-membrane, which in the parthenogenetic ("summer") eggs 

 is very thin, is early cast off, and the developing embryos lie free 

 within the brood-chamber. In the sexually produced ("winter" 

 or " resting ") eggs the whole development is gone through within 

 the egg-membrane. A distinct nauplius stage is passed through, 

 and, at least in some cases, is marked by the formation of a cuticle 

 which is cast off later. The rudiments of the maxillulae and 

 maxillae do not appear until after some of the trunk-limbs have 

 already appeared, and the maxillae afterwards become reduced and 

 disappear in the adult. In the embryos of some Cladocera in 

 which only five pairs of trunk-limbs are present in the adult, six 

 pairs of rudiments are formed, but the last pair disappears later. 



