304 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



i. e., two pairs of eye-lobes or ganglia, viz. — first, those which send nerves to the lateral eyes ; sec- 

 ond, those which innervate the median eyes; and, third, the cerebral ganglia, which, with the 

 mushroom bodies, form the chief portion of the brain. We have seen that the ganglia of the first 

 l^air of appendages are separate from the prestomial or i^recesophageal brain mass. Hence we 

 throwout these ganglia in our consideration of the constituents of the brain proper. 



Prof. Patton, though he has not published any observations on the structure of the brain of the 

 adult Limulus, in his article on the "Origin of- Vertebrates from Arachnids " briefly describes (pp. 

 330-346) the development of the ''cephalic lobes" and of the eyes of the embryo of Limulus. 

 Patton does not give details of the structure of the brain of the embryo, only stating such facts as 

 have a bearing on his remarkable theoretical conclusions,* reserving a fiill description for a future 

 paper. On pages 338 and 339, however, he gives " diagrams" and a " semidiagrammatic" view of the 

 brain of the embryo Limulus and refers to two pairs of median eyes, with their neuromeres, and 

 on page 343 gives a figure of the "fore and mid brain of young larva of Limulus (just hatched) 

 seen from neural surface," in which four preoesophageal neuromeres with their nerves are repre- 

 sented. Deferring to a few pages further on in this essay, the consideration of the number of 

 brain neuromeres in the embryo and larva of Limulus, we will simply remark that we have, after 

 repeated examinations, been unable to find more than a single pair of median eyest in the embryo 

 or larval Limulus, nor have we been able to detect more than three pairs of neuromeres in front 

 of those innervating the first pair of ajipendages, these three pairs of neuromeres finally consti- 

 tuting the brain. 



Our studies as regards the median-eye lobes entirely confirm those published by Kishinyoue, 

 in his excellent work on the development of the Japanese Limulus longispina. He remarks : 



About two weeks before the hatching of the embryo the brain proper becomes divided into two transverse por- 

 tions by a constriction (Fig. 69). Thus the brain of Limulus may be divided into four parts — the ganglion of the 

 median ej'es, the ganglion of the lateral eyes, and the anterior and posterior portions of the brain projier. 



These last two portions form apparently the cerebral lobes and mushroom bodies. Mr. 

 Kishinyoue has also written me, since his paper was published, that he had only observed a single 

 pair ,of median eyes. 



The only other author besides myself who has studied the brain of the adult Limulus is M. 

 Viallanes. An abstract of his paper read before the French Academy, December 1, 1890, appeared 

 in the Journal of the Eoyal Microscopical Society, for February, 1891. 



The author considers that the brain in Limulus polyphemus (this being the species investi- 

 gated by him) " gives origin to the oceUar nerve," to the nerve of the compound eye, to the cheU- 

 ceral nerve, to the stomatogastric nerve." We copy his description of the brain : 



structure du cerveau. — Le cerveau se compose de deux paires de centres ganglionnaires. La premiere est essen- 

 tiellement constitute comme \e proioc&rihron des autres arthropodes et doit etre d<^sign^e sons ce nom. Quant h la 

 seconde, quelques doutes pouvant encore subsister sur ses homologies, je la d^slgnerai provisoirement sous le nom de 

 cerveau postcrieur. 



Protocerebron. — II se compose d'une paire de nodules fibreux ou lobes protoc^r^braux relativemeut petite, rdunis 

 sur la ligne m^diaue par une commissure irreoesopliagienne. Les lobes protoo^r^braux sont partiellement revetus 

 d'une ^corce de grandes cellules unipolaires; et chacun d'eux donne uaissanc eau nerf ' de I'ocelle correspondant. 



Quant au nerf de I'ceil coraposd, il s'unit an lobe protoc^r^bral correspondant, non pas directement, mais par 

 I'intermediaire d'une formation comparable dans ses traits essentiels au lobe optique des insectes et des crustac^s, 

 car on y recounait les bomologues de la lame ganglionnaire, du chiasma externe, des masses m<;dullaires externe 

 et interne; mais chez le Limule ce lobe optique est tres petit relativemeut et enfoui au sein de la masse du cerveau, 

 au lieu d'etre i?,cart6 de colui-ci et en contact immfidiat avec I'ceil composd. Chez le Limule, le nerf optique est done 

 constitu^ par des fibres postr(5tinieune8 stirpes a I'extreme. 



A chacun des lobes protocdr^braux est annexe un organe qui, en raison de ses rapports anatomiques et de sa 

 structure histologiqiie, doit etre assimile au corps jx'doncule des insectes. 



• With Patton's attempt to derive the vertebrates from forms comparatively high, so specialized and modified 

 as the Hcnr)iioiis and spiders, to say nothing of Limulus, we have little sympathy, regarding them not only as 

 unsound, Imt :is 1 1 ndiu- tn liMd to inai-curate observations. On general grounds the attempt to derive the vertebrates 

 from any Imt ih- most primitive and generalized vormian forms would seem to be hazardous, the Arthropoda being 

 a specialized and cuiuplutud branch of the animal series. 



t Patton considers that a diverticulum of the median eyebulb "represents, in all probability, a pair of eyes 

 belonging to the first brain segment," and in the next paragraph refers to "the three fused ocelli of Limulus" (p. 344). 



