290 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



A number of these horizontal sections proved useful, as when the animal is young, of the 

 length indicated, the bilateral symmetry of the lobes and ganglion cell masses is more marked 

 than in fully grown sfiecimens. 



But my main reliance has been some excellent sections made for me three years ago by Dr. 

 H. C. Bumpus. These were transverse, longitudinal (sagittal), and horizontal sections of the brain, 

 removed from living individuals, about 8 inches in length, exclusive of the caudal spine. The 

 transverse and sagittal sections were stained with h;ematoxylin, the horizontal with carmine. I 

 have also made horizontal sections of the brain of the young Limulus, and especially horizontal, 

 sagittal, and transverse sections of the embryos and larva; in diiferent stages of development. 

 I have found the horizontal sections to be by far the most useful and the various carmine stains 

 the most desirable, since with this stain the central fibers of the lobes remain white or unstained, 

 while the chromatic cells composing what I have called the " nucleogenous bodies " are colored 

 deep carmine, and thus contrast with the other parts of the brain. 



In order to comprehend the topography of the brain, while my main reliance has been a study 

 of the horizontal sections, I found it useful to draw models of different sections on cardboard and 

 to mount them on a long insect pin. By the use of this simple device I could obtain a tolerably 

 clear idea of the relations between the lateral and median optic lobes and the cerebral lobes, rela- 

 tions by no means so easily understood as those prevailing in the brains of Crustacea and insects. 



Also, at the kind suggestion of Dr. Bumpus, I made a wax model of the brain, and for aid in 

 preparing the sheets of beeswax and for other suggestions I am also indebted to Dr. Bumpus. I 

 made a model from the horizontal, sagittal, and transverse sections; but, owing to the difficulty of 

 painting or otherwise marking the exact limits of the different very irregularly shaped lobes on 

 the sheets of wax, I have not had specially good success in locating on the wax the different lobes 

 and roots of the nerves proceeding from them. The portion of the clearly and distinctly stained 

 nucleogenous bodies can, however, readily be indicated, and as concerns the relations of these 

 masses to each other and to the brain in general the wax models have been quite serviceable. Such 

 models would undoubtedly prove useful in dealing with the brains of insects, Crustacea, and Arach- 

 nida, as well as Myriopoda. 



II. General Anatomy of the Brain op Limulus. 



The nervous system of the Limulus was first described and figured by Van der Hoeven in 

 1S38, the subject of his investigation being Limulus moUiccanus Latreille, represented by two speci- 

 mens from the Dutch possessions in the East Indian archipelago. Tliat of the American species, 

 Limulus poly pliemus Latr., has been described and figured by Sir Richard Owen and afterwards 

 more accurately and with many details by M. A. Milne-Edwards.* It is noticeable that the 

 brain and oesophageal ring, with their nerves, are identical in shape in the two species, there 

 being apparently but very little difference, externally, or internally, in the species of Limulus, both 

 living and extinct. 



The central nervous system consists of an oesophageal ring formed by the consolidation on 

 each side of the tesophagus of the six ganglia innervating the six pairs of cephalic appendages. 

 The ring is closed in front by the supra-cesophageal, or to speak more correctly, the pre-oesophageal 

 ganglia or brain; the brain is the partial homologue of the supra-cesophageal ganglia of other 

 Arthropoda. 



It will be remembered that in other arthropods the brain is situated in the upper part of the 

 head, in a plane somewhat different in direction from that of the rest of the ganglionic cord. In 

 the Arachnida the supra-oesophageal ganglion also is in e different plane ft'om that of the (esophag- 

 eal mass, forming a small, more or less bilobed subspher'cal mass resting directly over and upon 

 the front part of the oesophageal nerve-mass or oesophageal ring. In Limulus, however, as seen 

 by Fig. I (in the text), the flattened subovate brain is situated directly in ft-ont of and in the same 

 plane as the oesophageal ring and also the abdominal portion of the central nervous system. 



Hence the position of the brain is pre-cesophageal rather than supra-oesophageal. The brain 



' For references to the works of this and of other authors see the bibliography at the end of this article. 



