30 Mr. A. S. Packard, Jim., on the Internal 



) " ""•* 



lobster, stained with picrocarmine, for comparison. The 

 following results, then, are based on over two hundred sec- 

 tions of the supraoesophageal ganglion of Limulus, but more 

 especially on two brains, which were cut by Mr. Mason each 

 into over fifty sections, from -nuro to jfo of an inch in thick- 

 ness. The examination of a few sections of the lobster's 

 brain enabled me to comprehend more readily the recent 

 papers of Dietl, Newton, and Krieger on the brain of the 

 Decapodous Crustacea and of the Insecta, and thus gives me 

 a standard of comparison by which to study the topography 

 and histology of the brain of Limulus. 



General Anatomy of the Brain. — The singular relations of 

 the central nervous system of the adult Limulus have been 

 fully described and beautifully illustrated by A. Milne-Ed- 

 wards ; and Dr. Dohrn and myself have described its general 

 anatomy in the larval stage. The central nervous system of 

 Limulus consists of an oesophageal collar, mostly made up of 

 six pairs of ganglia, from which nerves are distributed to the 

 six pairs of foot-jaws (gnathopods) , while the ring is closed 

 or completed in front by the brain, or what corresponds to the 

 supraoesophageal ganglion of normal Crustacea and insects. 

 In these Arthropoda the brain is situated in the upper part 

 of the head, in a plane parallel to but quite removed from that 

 of the rest of the ganglionic chain ; in Limulus , however, the 

 brain is situated directly in front of and on the same plane 

 with the rest of the central nervous system. Milne-Edwards 

 states that the oesophageal ring, as well as the posterior part 

 of the nervous system, is enveloped by an arterial coat. He 

 also states that the brain and nerves are enveloped in a simi- 

 lar arterial coat ; but this we have failed to find. The brain is 

 protected by a thick membrane (" perineurium " of Krieger) 

 formed of fibrous connective tissue ; and the nerves are pro- 

 tected by a continuation of this membrane, as several longi- 

 tudinal sections of these nerves have taught us. The brain 

 in a Limulus 10 inches long, exclusive of the caudal spine, 

 is about 5 or 6 millims. in diameter ; it is flattened slightly 

 above, and on the upperside has a shallow median furrow, 

 indicating that it is a double ganglion. Three pairs of nerves 

 and a median unpaired one (the ocellar) arise from the upper 

 third of the anterior face of the brain. The two optic nerves 

 are the largest ones, arising one on each side of the median 

 furrow, so that the fifth to fifteenth sections made by the 

 microtome pass through them. Next below (from above 

 downwards) is the origin of the ocellar nerve, which, as 

 described by A. Milne-Edwards, is single, arising from the 

 median line ; on each side, and in nearly the same plane, 



