356 



CRUSTACEA. 



eighteenth century to an early stage in the development of 

 Cyclops, under the supposition that it represented a distinct 

 generic type. It is now used for a larva, to whatever group of 

 Crustacea it may belong, having the following characters. 



The body is oval in shape, wider in front than behind, and 

 shows no trace of external segmentation. A dorsal shield is usually 

 absent, though it occurs in the nauplius of some Cirripedes and 

 of the Cypridae (Ostracoda) (Fig. 256). A pair of setae projects 

 on either side of the hind end. Three pairs of appendages are 



borne by the nauplius 

 larva ; the anterior (Fig. 

 242, a] the 1st antennae 

 of the adult, and placed 

 in front of the mouth, 

 being unbranched, while 

 the two posterior, which 

 become the 2nd antennae 

 and mandibles, are bira- 

 mous and paroral and 

 postoral respectively. 

 The 2nd antennae carry 

 a masticatory e n d i t e 

 directed inwards and 

 acting as a jaw, but 

 the mandibles are, as 

 yet, usually without such 

 a process. A large upper 

 lip projects in front of 



FIG. 242. Nauplius of Cyclops, dorsal aspect (from f ; mrmfh nrl fV,^ oli 

 Korschelt and Heider, after Claus). a' first antenna ; tne HlOUtn, and the all- 

 a" second antenna; at antennal gland ; ds mid-gut rnckT1 f QT , Tr OOTIQ! T-IT^C 

 with excretory cells ; md mandible ; o median eye. Hientary Canal presents 



a division into oeso- 

 phagus, mid- and hind-gut, though the anus is not always open 

 on hatching. The innervation of the 2nd antennae is from a 

 postoral pair of ganglia, as in some adult Branchiopods. The 

 unpaired median eye is the sole organ of vision, and a heart 

 is not yet formed. The little larva moves rapidly through 

 the water, by the strokes of all three pairs of limbs. 



When it is recognized that a nauplius stage occurs in the 

 development of the free-swimming Copepods and of their un- 

 gainly parastic relations, in the Barnacles and Sacculina, in 



