10 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



The antennae serve for locomotion. The internal organization includes 

 usually an alimentary canal, a muscular system, a nervous system, and 

 a pair of antennal excretory glands. The alimentary canal when fully 

 developed consists of an endodermal mesenteron and an ectodermal 

 stomodaeum and proctodaeum. The mouth is concealed above a large 

 labrum ; the anus is usually formed at a later stage. The nervous 

 system includes three pairs of ganglia corresponding to the append- 

 ages. Though there is no visible segmentation in the ectoderm, the 

 presence of appendages and ganglia shows that the nauplius is at 



rarl 



A 



B 



Fig. 2. — Nauplius and metanauplius of Apus cancriformis Bosc (from Claus, 



1873). 

 A, nauplius, with first antennae (lAnt), second antennae (^Ant), and man- 

 dibles (Md), rudiments of teloblastic appendages seen through cuticle. B, 

 metanauplius, appendages of teloblastic segments (IbSegs) exposed after first 

 moult. 



least a partly segmented stage of development. The region of the body 

 behind the mandibles is that in which later the other segments will be 

 formed, and their rudiments may be seen beneath the naupliar cuticle. 

 When these segments are formed, however, they are generated by a 

 different method from that which formed the anterior segments. 



The nauplius is derived from a very early stage of embryonic de- 

 velopment, represented in species that hatch at a later period by a 

 simple embryo with rudiments of three pairs of appendages. The 

 embryo still in the egg at this stage is clearly more simple in its 

 structure than is the nauplius. The nauplius, therefore, is not merely 

 an early hatched embryo — it has undergone a metamorphosis before 

 hatching to adapt it to a free life in the water. 



