184 



INVEETEBEATA 



CHAP. 



with large clear nuclei ; these are the rudiments of the ganglia of the 

 nervous system. The first of these pairs of ganglia is connected with 

 a similar mass of cells which forms a kind of focal Kne, surrounded by 

 the concentric parabolic curves of cells which make up the cephalic 

 lobes. This focal mass of cells is the rudiment of the primary 

 cerebral ganglion or protocerebrum, to which later the anten- 

 nulary ganglion or deuterocerebrum adds itself. The two cerebral 

 ganglia are connected by a bridge in front of the labrum. To the 



compound mass on each 

 side there is added, at 

 a later period, the an- 

 tennary ganglion or 

 tritocerebrum (tr.c, 

 Fig. 137). The outer 

 part of the cephalic 

 lobe gives rise to the 

 eye -stalk, the ecto- 

 derm covering which 

 gives rise to the visual 

 thabd cells of the compound 

 eye ; at its base there 

 is a deep groove, the 

 cells lining which, in 

 later stages, bud off 

 the cells which form 

 the optic ganglion. 

 This groove may be 

 termed the cerebral 

 groove. 



The mouth has now 

 made its appearance 

 as a groove behind 

 the labrum and leads 

 into a narrow stomo- 

 daeum, which descends 

 vertically towards the endodermic sac but does not yet reach it. 

 Behind the mouth there is found a median groove of ectoderm 

 extending backwards between the ganglia of opposite sides. The 

 cells forming this groove proliferate and form between each pair of 

 appendages a thickening, two or three cells deep, which later enters 

 into the formation of the transverse commissures between the 

 ganglia of the double ventral nerve cord (Fig. 133). 



The primary mesoderm forms a compact mass, in which, however, 

 some indications of a division into segmental masses corresponding 

 to the appendages are to be seen. This is one of the points on which 

 a renewed investigation is very desirable, because Eeichenbach's 

 statements on this point have been overlooked by subseqtient workers, 

 and it has been generally assumed that Crustacea are distinguished 



Fig. 132. — The "Nauplius" stage hi the development of 

 Astaciisfluviatilis viewed from the ventral side. (After 

 Reichenbach.) 



an, anus ; ail, rudiment of first antenna ; ai^, rudiment of second 

 antenna ; car, ridge marking tlie iirst trace of the carapace ; c.l, 

 cephalic lobe ; Za&, labrum ; m, mouth ; mn, rudiment of mandible ; 

 •pr.c, protocerebrum ; th.ahd, rudiment of thorax and abdomen. 



