NERVOUS SYSTEM. INVERTEBRATA. 27 



D. 19. A Spider-Crab (Maia squinado) dissected from above. 

 The cerebral and cesophageal ganglia, with the nerves that 

 arise from them, are essentially similar to those of the 

 Lobster, except for the groat development of the integu- 

 mentary nerves given off from the posterior corners o the 

 cerebral ganglion. The circumcesophageal connectives are 

 remarkably long, owing to the position o the ganglia of 

 the ventral chain. The latter are concentrated to the 

 maximum degree in the longitudinal direction, and are 

 fused together to form an oval mass in the middle of the 

 cephalothorax, from which nerves radiate to the appendages 

 and body. The large appendicular nerves are composed 

 of bundles of small fibres without a common investment. 

 From the dorsal surface of the central mass, at its anterior 

 end, a pair of small nerves are given off to be distributed 

 to the roof of the gill-chamber, and from the posterior 

 end of the mass a bundle of nerves pass into the abdomen. 



O.C. 1303 A I. 



Audouin & Milne-Edwards, Ann. Sci. Nat., t. xiv. 1828, 

 p. 92. 



D. 20. A King- Crab (Limulus polyphemus) dissected from the 

 dorsal aspect. The entire cephalothoracic part of the 

 central nervous system is concentrated around the oral end 

 of the O3sophagus in the form of an oval ring. The prge- 

 oral part of the ring represents the cerebral ganglion ; it 

 projects anteriorly as a subconical, feebly bilobed mass 

 (protocerebrum) from whose anterior end nerves are given 

 off to the lateral and median eyes, and to a ventral integu- 

 mentary pit of unknown function situated in front of the 

 mouth. From its dorsal surface, near its union with the 

 lateral parts of the ring, arise a pair of delicate integu- 

 mentary nerves that innervate the ventral skin of the 

 cephalothorax external to the limbs. Between the 

 protocerebrum ana the lateral parts o the ring are a 

 pair o centres (indistinguishable superficially) united by 

 a small prae-oral commissure ; they form part of the 

 cerebral ganglion and give off nerves to the chelicerse 

 and viscera. The protocerebrum is remarkable for an 



