30 A. S. PACKARD, JR., ON THE ANATOMY 



rounded ; on the upper side is a broad, 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, arising very near the upper side of the brain, one on each side of the median 

 furrow, so that the second and third sections made by the microtome, pass through them. 

 Next below (from above downwards), is the origin of the single nerve sent to the two 

 ocelli. We have not traced this nerve as far as the ocelli, but Milne-Edwards states that 

 near the ocelli it divides into two branches. One of these two branches we figure in the 

 drawing of the ocellus (plate 5, fig. 14). On each side of the ocellar nerve, and in nearly 

 the same plane, arise two tegumental nerves, and directly below them a second pair of 

 larger nerves (fronto-inferior tegumental) descend ventrally. 



No nerves arise from the inferior half or two-thirds of the brain, which is smooth and 

 rounded, with no indications of a median furrow. 



It will thus be seen that, as stated by A. Milne-Edwards, there are no antennal nerves, 

 such as usually exist in Arthropods with the exception of the Arachnida. This we have 

 proved in the same manner as MUne-Edwards (though at the time ignorant that he had 

 pursued the same method), by laying open with fine scissors the envelop (arterial or peri- 

 neurial) which reaches to the posterior end of the brain, and seeing that the fibres of the 

 nerves sent to the first pair of gnathopods originate quite independently of the brain 

 itself. Moreover, after making sections of several brains, it is easy to see that only the 

 commissures connecting the brain with the oesophageal ring are present ; the nerves to 

 the first pair of gnathopods not arising from the brain itself, but from the anterior and 

 outer part of each side of the oesophageal ring, i. e., where the ring joins the brain ; the 

 commissure is very short in the larva, and obsolete in the adult. 



Internal structure and histology of the brain. Most of the numerous stained and 

 unstained transverse sections threw but little light on the topography ; the nerve-fibres 

 and ganglion-cells being apparently arranged horizontally, and mostly confined to the 

 upper part of the brain ; at any rate it was not until I had studied the horizontal sections, 

 that I could gain an insight into the relation of parts as shown by sections cut vertically 

 from in front backwards. 



Finally a series of about fifty sections each, from two .brains, cut by Mr. Mason hori- 

 zontally from above, downwards, and carefully mounted in consecutive order, each section 

 being numbered, has enabled me to arrive at a tolerably complete idea of the topography 

 of the brain, so that I could mentally construct a model of the brain of Limulus, and 

 compare it with the normal arthropod brain. 



The histological elements are four in number : — 



1. Large ganglion-cells, filled densely with granules, and with a well defined nucleus 

 similarly filled and containing a granulated nucleolus. These cells (plate 7, fig. 3c) may 

 be crowded or loosely grouped ; the granular contents varying in density, and the walls 

 of the cell thick and loose or thinner and dense ; they terminate in large nerve fibres. 

 They are similar in form and size, though not in topographical arrangement, to the 

 large ganglionic cells of the lobster's brain (see plate 7, fig. lb). 



