MORPHOLOGY OP DRAIN AND SENSE ORGANS OF LIMULUS. 67 



seen whole as a transparent object these cells appear as a 

 very conspicuous narrow band^ showing clearly the peripheral 

 boundaries of the semicircular lobes. This narrow dark band 

 is best seen in larvse about four or five inches long^ but may 

 be distinguished even in the adults. After the stage shown in 

 fig. 46 the semicircular lobes become narrower and relatively 

 m aller (fig. 49). Its cortical layer is always smooth, and 

 contains many large ganglion-cells. Within the lobes is a 

 narrow stratified band of fine dense medullary substance, 

 something like a horseshoe in shape. Its arms are directed 

 forward and outward, and terminate blindly in the apex of 

 the lobes : the curved posterior part constitutes the anterior 

 commissure (fig. 43, a. com.). 



The optic ganglia, soon after their first appearance, divide 

 into four medullary cores or centres, each covered with a 

 cortical layer of ganglion-cells. Three of these lobes are seen 

 in fig. 49; the fourth, which differs in some respects from the 

 rest, is situated at the base of the ganglion, concealed by the 

 semicircular lobes. 



D. Comparison with Vertebrata. — It cannot be justly 

 denied that the brain of Limulus has a strong superficial 

 resemblance to a Vertebrate brain, and it is equally certain 

 that there is nothing else like its adult condition among other 

 Arthropods. Is this resemblance real — that is, is it pro- 

 found and varied, indicating genetic relationship, or merely a 

 superficial resemblance, which on closer inspection is seen to 

 be due to some deceptive quirk, but being in other respects 

 totally and irreconcilably different? It must be either one 

 or the other. Either Limulus is closely related to the Verte- 

 brates, and will show this by a fundamental similarity 

 of structure, or else it is a world apart, and will show this 

 also by an equally profound difference in structure. The 

 resemblances already pointed out can hardly be called super- 

 ficial, involving as they do the similarity in the number of 

 neuromeres, nerves, and sense organs. I shall not discuss 

 these points now, but shall consider the various vesicles and 



