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sections. Owing to interruptions these sections, made from unstained 

 alcoholic specimens, were not examined: duiing the past winter I have 

 been able with the aid of Mr. N. N. Mason, of Providence, to take 

 up the study afresh. Mr. Mason has kindly made sections, both trans- 

 verse and horizontal, stained with osmic acid ; also sections of the brain 

 of the supra-oesophageal ganglion of the lobster, stained with picro- 

 carmine, for comparison. The following results, then, are based on 

 over two-hundred sections of the supra-oesophageal ganglion of Limu- 

 lus, but more especially on two brains cut each into fifty sections, from 

 Viooo to Vöoo of ail i"cli in thickness. The examination of a few sections 

 of the lobster's brain, enabled me to comprehend more readily the re- 

 cent papers ofUietl, Newton and Krieger on the brain of the 

 Decapodous Crustacea and of the insects, and thus give me a standard 

 of comparison by which to study the topography and histology of the 

 brain of Limiilus. 



General Anatomy of the lirain. — The singular relations of the 

 central nervous system of the adult Limulus, have been fully described 

 and beautifully illustrated by A . M i 1 n e - E d w a r d s , and Dr. D o h r n 

 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 distri- 

 buted 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 

 supra-oesophageal 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 gang- 

 lionic 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 similar 

 arterial coat, but this we have failed to find ; the brain is protected by 

 a thick membrane (»perineurium« of Krieger) formed of fibrous con- 

 nective tissue, and the nerves are protected by a continuation of this 

 membrane, as several longitudinal sections of these nerves have taught 

 \is. The brain in a Limulus ten inches long, exclusive of the caudal 

 spine, is about five or six millimetres in diameter; it is flattened 

 slightly above, and on the upper side 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 



