OLFACTORY ORGAN OF BRANCHIPUS AND APUS. 167 



The ventral frontal-organ is unpaired and lies just in front of the ocelli. 

 In larvae about lo mm. long, the organ is merely a rounded area, without any local 

 thickening of the chiten or epidermis, in which terminate a great many fine nerve 

 fibers, B, m.f.o. In very young larvae the latter arise from the united anterior 

 ends of two thick ridges, or lobes, on the anterior surface of the f orebrain. (Fig. 

 no, A,w.) These lobes are solid masses of cells Hke those in the forebrain and 

 undoubtedly arise as an outgrowth from it. In the later stages, therefore long 

 after the ocelli are fully formed, they increase greatly in size, expanding laterally 

 and forward, thus forming two wing-like plates, which still later break up into 

 many scattered sensory buds united by a nerve plexus, B, w. 



Each sensory bud contains several radiating cells; the latter are clear on the 

 periphery, and their pointed inner ends are granular and capped by refractive 

 plates or rods, like those on the retinal cells. (Fig. no, C.) These buds, 

 therefore, resemble the isolated ommatidia arising from the lateral olfactory 

 nerves in Limulus. 



In the adult Branchipus, the buds are united with the brain by loose nerve 

 strands containing dark colored bipolar cells, the remnants of the stalk by which 

 the median olfactory lobes were connected with the brain. A small cluster of 

 nuclei, g., at the base of the median nerve represents the remnant of the unpaired 

 portion of the lobes. 



Nowikoff, '05, also recognizes the resemblance of these cell clusters (in Lim- 

 nadia) to groups of retinal cells, as I had previously done for Limulus in 1893. 

 He regards them as detached retinal cells belonging to the median ocellus. But 

 the development of these cells in Branchipus, long after the ocelli are formed, and 

 the development of the lateral olfactory organ in Limulus, show clearly enough 

 that the isolated ommatidia are formed from the breaking up of independent sense 

 organs, quite distinct from the median eye. 



The median frontal organ of Branchipus clearly corresponds to the median 

 olfactory organ of Limulus, not only in its position, but in its development as a 

 ganglionic outgrowth of the forebrain. There is, however, this difference, that in 

 Branchipus there are no recognizable hemispheres, and the sensory buds are 

 formed from the median olfactory outgrowth, while in Limulus they are formed 

 from the lateral one. 



Apus. 



In Apus (Fig. in), the frontal organ is represented by a thick oval sclerite 

 behind the eyes. Here the underlying ectoderm is thickened and contains ver- 

 tical fibers crossed by several layers of horizontal ones. Between these coarse 

 fibers is a network of large ganglion-like cells, that appear to be connected with 

 the branches of two large nerves l.f.n. (lateral olfactories), containing numerous 

 scattered ganglion cells. These nerves arise from the base of the lateral eye gan- 

 glia and are distributed over a wide area, behind and between the lateral eyes, 

 including the thickened ectoderm beneath the sclerite. 



