310 



MBMOIES OF THE NATIONAL ACADEMY OP SCIENCES, 



sliape and i)ositiou of the optic ganglia or lobes, and the similar relations in the mode of origin of 

 the optic nerves; both the Arachnida and Merostomata having median and lateral eyes with a, in 

 general, similar position in respect to the exterior of the head. Besides this, neither class pos- 

 sesses optic ganglia closely resembling those of insects and Decapod Crustacea. 



And yet there is a noteworthy difference between the brain of Limuliis and of Arachnida. In 

 the latter the cerebral ganglia are large and well developed, foriiiiug, we should roughly estimate, 

 two thirds to three-fourths of the braiu. The central medullary or white tibrous portion is largely 

 developed, forming the bulk of the cerebral ganglia, and is more or less spherical in shape, while 

 the cortical layers or masses of large ganglion cells are large and thick, the cells themselves being 

 abundant, whereas in Liuurlus they are scanty, forming isolated small groups. Reference to the 

 accompanying figures, copied from Saint Remy's work, will show the structure of the Arachnid 

 braiu together with the resemblance to and the difference from that of Limulus. 



A 

 sir. looiy 



oliplu ' , 1"P cxflol 



glpl 



ped.gaBg 



Fig. XII. — Brain of Lycosa narbonensis Walck., 

 horizontal section passing througli the middle of 

 the middle segment of the optic lohes and through 

 the lower medullary plate, and through the mid- 

 dle part of the lower medullary mass. I. o. n., 

 lower optic nerve; I. m. p., lower medullary 

 plate; a. I. ol., anterior fibrillar layer of the 

 lower optic lobes; gl. pi., glomerulated plate of 

 the lower optic lobule ; pf. lol., posterior fibrillar 

 layer of the lower optic lobule; L o. I., lower 

 optic lobe; cer. gang., cerebr.il ganglion. — After 

 Saint Remy. 



Fig. XIII.— Longitudinal (.sagittal) sectionoftbe same, passing through aplane situated 

 near the junction of the juter third with the middle third of the brain, str. body, 

 .stratified body ; u. m. o p. I., upper medullary- mass of the upper optic lobe ; /. u. o. I., 

 fibrillar layer of the upper ojjtic lobule; u. o. n., upper optic nerve; I. o. «., lower 

 optic nerve; l.m. p., lower segment of the lower medullary plate; l.m. 1., lower 

 medullary mass of the lower optic lobe; ^, head of the lower medullary mass ; chel.n., 

 cheliceral nerve; chel.gdvg., cheliceral ganglion: ped. gang., ganglion of the pedi- 

 palp. — After Saint Remy. 



A. Part of a transverse section through a plane near the middle of the brain, to show 

 the stratified body (sir. body), a, principal plate of the anterior segment of the strati- 

 fied body; b, accessory plate of a ; c, posterior segment of the stratified body. — After 

 Saint Remy. 



I have been unable to detect any trace in the brain of Limulus of the posterior stratified body 

 (Fig. xii-xiv, str. body) -:7hich is regarded by Saint Remy as the homologue of the "pedunculated 

 body" of insects. Neither does M. Viallanes refer to any organ similar to that discovered by Saint 

 Remy.* 



The position of this organ and its structure should be noticed. Both in spiders and the scor- 

 pion it has nearly the same situation as in insects, being i^laced in the uppermost region of the 

 head in a lobe-like swelling projecting above the general surface of the braiu. In the side view of 

 the scorpion's brain (Fig. xiv str. body) it is seen to be somewhat trilobate and divided into an 



' But iu his final paper, p. 449, he very briefly states that this organ, "although modified, is likewise recogniz- 

 alile in the Arachnids, where it has been described by M. Snint Remy under the name of organc stratifii." 



