CHAPTER X. 

 THE OLFACTORY ORGANS AND THE OLFACTORY LOBES. 



The agreement between the olfactory organ of Limulus and that of verte- 

 brates may be traced in respect to so many different characters that the existence 

 of a genetic relationship between the marine arachnids and the vertebrates is 

 placed beyond a reasonable doubt. Indeed there is a greater difference in 

 respect to this organ, between Limulus and other invertebrates than there is 

 between Limulus and vertebrates. 



The olfactory organ of Limulus, in certain respects, stands in a class by 

 itself. Nevertheless it represents a modification of organs very widely distributed 

 in the arthropods, and known in insects as the frontal ocelli, or stemmata, and in 

 the phyllopods and other Crustacea, as the frontal sense organs. 



The history of these organs is an important lesson in evolution. It affords an 

 impressive illustration of the essentially unalterable character of the procephalic 

 sense organs, and it distinctly sharpens our perspective of the long series of inter- 

 mediate forms that connect the most primitive segmented animals with the modern 

 ones. 



I. The Olfactory Organ of Limulus. 



Structure in Adult Limulus. — Gross Structure. — In an adult Limulus, the 

 olfactory organ (subfrontal schlerite of Lankester) is a bi-lobed, wart-like thicken- 

 ing of the cuticula, from 5-8 mm. wide, situated in the median line, 30-40 mm. 

 in front of the mouth. (Figs. 38, 39, 70, ol.o.) It is innervated by three large 

 nerves, a median and two lateral ones. 



The olfactory cuticula is provided with a central cluster of sensory spines and 

 is perforated by many sensory and glandular openings. (Fig. 109, A.) The 

 under-lying ectoderm is pigmented, and just beneath it are many branching nerve 

 fibers, together with ganglionic or sensory cells, and a large number, about 

 1500, flask shaped, or spherical, slime buds. 



The most conspicuous parts of the olfactory organ are the slime buds, 

 which are, with few exceptions, sharply confined within the area of the olfactory 

 schlerite. They have the usual form and structure, as described in the chapter on 

 the gustatory organs (p. 116), the only noticeable peculiarity being the clusters 

 of small ganghonic or sensory cells lying near, or on, their outer surface. (Fig. 

 88, a.) 



Minute Structure. — The minute structure of the olfactory organ has not been 

 satisfactorily determined, especially the character of the nerve terminals. So 



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