COMPARISON WITH VERTEBRATES. 171 



During the closing of the medullary plate, the olfactory organs may or may 

 not unite in the median line; but they invariably move forward either to the 

 median neural surface (cyclostomes), or still farther forward onto the anterior 

 haemal side of the forehead. (Fig. 4.) 



In most ostracoderms (Bothriolepis, Tremataspis, Cephalaspis), the olfactory 

 organs and the median and lateral eyes unite to form a very compact group on 

 the neural surface of the head, very similar to the grouping in Apus and other 

 phyllopods, where they may be located on either the neural or haemal surface. 

 (Figs. 5, 8, 12 and in.) 



In the cyclostomes, all the pro-cephalic sense organs are on the neural 

 surface, but they are not so compactly arranged as in the ostracoderms or in the 

 phyllopods. 



7. The Olfactory Lobes. — Arthropods. In the arachnids, the olfactory 

 lobes make their appearance as a deep transverse infolding on the very an- 

 terior margin of the medullary plate. They soon sink below the surface and 

 move backward onto the haemal side of the forebrain. The posterior wall of the 

 infolding gives rise to the olfactory neurones; the anterior wall is membranous, 

 and later disappears. The cavity of the infolding, as long as it can be recognized, 

 communicates with the spaces between the hemispheres, and with those under 

 the palium, i.e., with the potential first and second ventricles. (Figs. 46 and 47.) 

 The roots of the median olfactory nerve and the parietal eye nerves may be 

 located in the olfactory lobes. 



Vertebrates. The olfactory lobes arise as deep transverse infoldings across 

 the anterior margin of the open medullary plate (frog), (Figs. 25 and 26), 

 therefore from precisely the same location and in the same manner as 

 in the arachnids. The lobes are finally located on the anterior haemal margin of 

 the forebrain, and their cavities communicate as in Limulus. They are the only 

 brain lobes that have a conspicuous connection with both the olfactory organ and 

 with the parietal eyes. (Figs. 43, 44.) 



8. Function. — The olfactory organ of fishes is recognized to be an olfactory 

 organ largely on morphological evidence. Whether or no it actually has what 

 is commonly understood to be an olfactory function, whatever that may be, rests 

 on surmise. Nevertheless, it would still be proper to speak of it as an olfactory 

 organ, even if it were experimentally demonstrated that it reacted to sound or to 

 light, because we know that it is the true homologue of the olfactory organ in the 

 mammals. 



It is well to bear this in mind in comparing the olfactory organ of arthropods 

 with that of vertebrates. Although our case rests primarily on morphological 

 evidence, the evidence afforded by function, while meager, is confirmatory. 

 Stimulation of the olfactory organ of Limulus with various kinds of food, with acids 

 and with ammonia, does not usually produce any characteristic reflexes. Even 

 drops of rather strong hydrochloric acid, or ammonia, have no more effect than 

 when applied to other parts of the body; they cause a slight start, nothing more. 



