i66 



THE OLFACTORY ORGANS AND THE OLFACTORY LOBES. 



We may recognize two sets of organs in the phyllopods, the paired dorsal 

 ones and the unpaired ventral ones. They probably correspond to the stemmata, 

 or frontal ocelli of insects. 



Branchipus. 



In Branchipus the dorsal or paired frontal-organs consist of a compact mass 

 of small ganglion cells, with one or two large ones situated on either side of the 

 ocelli. (Figs. 95, no, B.) The terminal cells are in contact with the unthick- 



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b r. 



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Fig. 1 10. — The parietal eye and olfactory organs, or frontal organs, of Branchipus. A, The median olfactory 

 nerve of a young larva, showing at the base of the nerve, the ganglionic enlargement, w, formed on the anterior 

 surface of the forebrain; B, a more mature specimen, showing the breaking up of the lobes into a nerve plexus 

 containing ommatidia-like clusters of cells; C, one of the cell clusters more highly magnified. 



ened epidermis in the center of a faint rounded elevation. They are connected 

 with a small compact nerve, that runs parallel with the ocellar nerves, and that 

 arises from the anterior surface of the brain near the root of the lateral eye ganglion. 



The embryonic organ is formed by the separation, from the base of the lateral 

 eye ganglion, of a small patch of neuro-epithehum, which then migrates under 

 the epidermis toward the anterior median line of the head. 



The history of this organ, therefore, is practically identical with that of the 

 lateral, or primary olfactory placode of Limulus. 



