XXVII 



DEVELOPMENT 



345 



or aperture of the stomodaeal depression (Fig. 90) three eleva- 

 tions appear, the rudiments of the antennules (a 1 ), antennae 

 (<7 2 ), and mandibles (m) : in front of them is another pair 

 of elevations on which the eyes (A) subsequently appear. 



FIG. 90. Early embryo of Fresh-water Crayfish in the nauplius 

 stage. 



A in the upper part of the figure is the eye : /, the labrum overhanging 

 the mouth, on each side of which are the rudiments of the antennules 

 (a 1 ), antennae (a 2 ), and mandibles (m) : behind them is the rudiment of 

 the thorax and abdomen (TA) with the anus (A). The rudiments of 

 the first three pairs of ganglia (G, go*, gm] are seen through the trans- 

 parent ectoderm. 



(From Lang, after Reichenbach. ) 



An unpaired elevation (TA) behind the mouth, and having 

 the anus (A) or aperture of the proctodaeal depression at its 

 summit, is the rudiment of the thorax and abdomen. The 

 embryo is now called a nauplius. Many Crustacea are 



