CHAP. VI.] NERVOUS SYSTEM OF ARTHROPODS. 



97 



Lobster and the Crayfish, the ganglia of the thorax, which 

 supply nerves to the limbs, are distinctly larger than those 

 of the abdominal segments, though these are also of 

 good size, since the tail- segments are actively called into 

 play during locomotion. 



In the Prawn a further development and concentration 

 of the nervous system is seen. The thoracic ganglia are 

 fused into a single elliptical mass, whilst 

 those of the abdominal segments still re- 

 main separate. 



But in the ordinary edible Crab (fig. 36) 

 and its allies, an even more remarkable 

 concentration of the nervous system is met 

 with. All the thoracic and all the abdo- 

 minal ganglia are here fused into one large 

 perforated mass of nervous matter (c, c), 

 situated near the middle of the ventral 

 region of the body.* From this large 

 and compound ganglionic mass nerves are 

 received from, and given off to, the limbs, 

 to the abortive tail, and to other adjacent 

 parts. The brain (a) of the Crab is repre- 

 sented by a rather small bilobed ganglion. 

 It receives nerves from the pedunculated 



FIG. 36. Nervous 



compound eyes, from the two pairs of system of a crab 



T f> ii 1*1 (Ptilinurusvulftans). 



antennae, and from the palpi-bearing man- V Fused cerebra i 

 dibles. The posterior antennae (or anten- ganglia receiving 



, . _ x optic, tactile, and 



nules, as they are sometimes termed) con- olfactory (?) nerves ; 



tain in their basal joint a body which is 



J * cords ; c, c, great 



supposed to represent an olfactory organ, ventral gangiiomc 

 though others have regarded it (on very ^ s .-, (Mi 

 insufficient grounds) as an organ of hear- 

 ing. This small bilobed brain is, indeed, thought by 

 * A large artery passes through the aperture in this ganglion. 



