i68 



THE OLFACTORY ORGANS AND THE OLFACTORY LOBES. 



Similar conditions to those in Branchipus and Apus prevail in other phyllo- 

 pods, but we need not consider them here. It is enough to show that the remark- 

 able olfactory organ of Limulus becomes more intelligible when compared with the 

 condition of the frontal organs in phyllopods. In both cases we may witness 

 important steps in the transformation of primitive segmental sense organs into 

 a very special condition preparatory for, and in part realizing, a new function. 



The causes lying back of this transformation are remote and probably in- 

 accessible. I formerly supposed that the unfavorable position of the organs in 



e. 



, c.e.v 



m 



fo 



.-V 



FIG. in. Head of Apus, showing the eye chamber c.e.v. and its external opening, o, the median frontal organ, 

 m.f.o, and the course of the lateral, frontal nerve, l.f.n. 



Limulus might have had something to do with their loss of visual functions, but I 

 now regard this as merely a coincidence, since their position in the free swimming 

 phyllopods is not unfavorable to their use as eyes, and yet they have suffered a 

 similar transformation. 



III. COMPARISON OF OLFACTORY ORGANS IN VERTEBRATES AND ARTHROPODS. 



i. Number of Placodes. In arthropods the olfactory organ arises from two 

 pairs of sensory placodes that still retain structures characteristic of visual cells. 

 According to the amount of median fusion that has taken place, the adult organ 

 may be regarded as a single unpaired one, (Limulus) ; or as three, a paired and 

 unpaired one, (Apus); or as two pairs, (Branchipus and other phyllopods.) 



In vertebrates the primitive olfactory organ has been regarded by various 

 authors as single, paired, or multiple. The first view has been widely entertained, 

 especially by the older anatomists, and was based largely on the condition in 

 the cyclostomes and in Amphioxus. Of more recent authors, Burckhart, 1908, is 

 inclined to regard the vertebrate olfactory organ as formed by the fusion of two 

 pairs of placodes. Kuppfer distinguishes three parts in Petromyzon, an unpaired 

 one at the point where the neuron last closes and one on either side. 



These conflicting views are intelligible on the assumption that the vertebrate 

 organ is derived from three or four separated anlagen, as it is in Limulus and the 



