A NAUPLIUS LARVA 47 



Class : Crustacea. Division : Entomostraca 

 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 inclosed 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 unsegmented 

 body ; if the animal is not newly born, signs of segmentation may 

 have begun to appear. Observe the three pairs of segmented ap- 

 pendages ; the segmentation, however, is often indistinct. These 

 appendages are homologous to the first and second pair of an- 

 tennse 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 1. Draw a nauplius on a large scale and label all the parts 

 above mentioned. 



The nauplius larva is of great theoretical significance. It ap- 

 pears as the youngest, free-swimming, larval stage of almost all 

 the entomostracans and of several of the malacostracans, and 

 those malacostracans which are born in a later period of their 

 development pass through a nauplius stage (that is, a stage in 

 which the body is unsegmented and bears three pairs of append- 



