A NAUPLIUS LARVA 53 



CRUSTACEA 



A LARVAL ENTOMOSTRACAN. A NAUPLIUS LARVA 



In an aquarium containing copepods or ostracods there are 

 sure to be numbers of the young larvae of these animals. They 

 are minute, free-swimming forms and are called nauplii, and may 

 be recognized by the triangular or oval, unsegmented body, 

 which bears three pairs of appendages and a median eye. 

 Nauplii of marine entomostracans may also be met with in 

 large numbers among the small animals obtained by skimming 

 the surface waters of the sea with a fine net. 



Examine in a watch-glass under a microscope water contain- 

 ing sediment taken from a jar in which are copepods or ostra- 

 cods. Find a nauplius ; the ostracod nauplius differs from that 

 of the copepod by being enclosed in the characteristic bivalve 

 ostracod shell. If marine plankton is at hand, look for several 

 kinds of nauplii in it. 



Study the structure of a nauplius. Observe the unseg- 

 mented body ; if the animal is not newly born, signs of segmen- 

 tation may have begun to appear. Observe the three pairs 

 of segmented appendages ; the segmentation, however, is often 

 indistinct. These appendages are homologous to the first and 

 second pair of antennae and the pair of mandibles of the adult 

 animal. As in the adult, the first pair is uniramous; the 

 second and third pairs are biramous. Both of the latter two 

 pairs are used for locomotion, although it is probable that they 

 also act as jaws. The median eye will be seen, and the straight 

 digestive canal. 



Exercise l. Draw a nauplius on a large scale and label all the 

 parts above mentioned. 



