LESSON 73.] NERVOUS SYSTEM IN CRUSTACEA. 



243 



FIG. 366. 



FIG. 36T. 



Nervous system of the 

 Cymothoa. 



Nervous system of 

 Sand-hopper, a, Ce- 

 phalic ganglia ; 5, CE- 

 sophagus ; c, Cau- 

 dal ganglia. 



trunk, in a longitudinal di- 

 rection. 



1106. The same form of 

 the nervous columns is seen 

 in the highest Crustacea, 

 while yet in their embryo 

 condition in the ovum. In 

 the short and broad trunk 

 of the Cymothoa, where the 

 legs are still equally devel- 

 oped along the sides of the 

 body, the nervous columns 

 (Fig. 367) have already ap- 

 proximated to touch each 

 other on the median plane, 

 and the ganglia on the two 

 sides have coalesced to form 

 a single chain along the 

 middle of the abdominal 

 surface of the body. The 

 ganglia are still nearly equi- 

 distant, and equally developed along the columns ; but where the 

 minute posterior segments occur, the ganglia are closer 



together and smaller in size. The transverse concen- 

 tration of the columns and ganglia towards the median 

 plain, thus seen in the lowest Crustacea, is succeeded 

 in higher species by a longitudinal movement of the 

 nervous matter, directed, as we have seen in inferior 

 classes of Articulata, chiefly to two points of the body, 

 the head and the thorax, from which the largest and 

 most important appendices of the body, whether for sen- 

 sation, mastication,or progressive motion, are developed. 



1107. In the long-tailed Crustacea, as the Lobster 

 (Astacus marinus), Cray-fish (Astacusfluviatilis),^ 

 only is the sympathetic system of nerves derived from 

 the lateral ganglia of the stomach, but the ganglia 

 and columns have coalesced and met transversely 

 along the whole body; in the region of the thorax, 

 from which the five pairs of large extremities are de- 

 veloped, the ganglia have enlarged in size above those 

 of the post-abdomen, and considerably approximated 

 to each other in a longitudinal direction (Fig. 368). 



FIG. 368. 



Nervous system 

 of the Lob'ster. 



