jrORPHOLOGY OF BRAIN AND SENSE ORGANS OP LIMULTJS. 43 



thickenings, which are highly suggestive of the rods on the 

 ommatidial cells of the lateral eye. 



Very similar figures are seen in the inner mandibles of 

 immature specimens, as in fig. 8. Here one can see a distinct 

 fibrous process passing outward from the ganglion-cell, which 

 in this instance lies well to one side of the central cavity. 



These buds, which in the young are found in all parts 

 of the body, and at first are everywhere alike, finally 

 undergo various modifications. Some may degenerate, some 

 may become olfactory buds, others gustatory, and still 

 others temperature organs. The resemblance of these buds to 

 ommatidia is so striking that we must include them both in 

 the same category. We can therefore reduce the whole 

 system of sense organs either to isolated sense 

 cells or sense buds, or aggregations of the same. 

 This agrees with my conclusion concerning the sense organs 

 of insects (see * Zool. Anz.,^ 1890, Nos. 13, 14), where I main- 

 tained that the ommatidia of the compound eye were nothing 

 more than specialised cell clusters, which in other parts of the 

 body were supplied with various forms of spines or hairs, and 

 served as tactile, auditory, gustatory, or olfactory organs. 

 Moreover, in my earlier observations on the ''Eyes of 

 Molluscs and Arthropods," I showed that the eyes even of 

 Molluscs were composed of circles of cells or ommatidia, which 

 were also widely distributed over the surface of the body. In 

 Vertebrates we have evidence of the same condition. Isolated 

 sensory cells are there widely distributed, and, while essentially 

 alike in structure and appearance, have very diverse functions. 

 The olfactory and gustatory organs are but aggregates of 

 sense buds like those widely distributed over the body. The 

 presence of similar sense buds in the eye is shown by the 

 circles of rod-cells surrounding the cones. 



Part II. — On the Morphology of the Arthropod Brain. 

 The Brain of Insects and Myriapods. 

 Among the most remarkable features of the brain of Limulus 



