GUSTATORY AND OLFACTORY TISSUES 267 



The osphradium of Sycotypus canaliculatus is a modified gill whose 

 parallel plates show much likeness to the gill-plates represented 

 in Chapter XVII. Figure 232 shows one side of a plate in vertical 

 transection near its base. To the right is seen the beginning of the 

 large extent of distal epithelium, which clothes the greater part of the 

 plate. Its tall columnar cells hold heavy granules of pigment in their 

 distal cytoplasm. The nuclei are at various proximal levels, and there 

 seems to be a double layer of irregular basal cells with rounder nuclei. 

 This epithelium is so like the gill epithelium that one must consider it 

 as respiratory. 



To the left and nearer the body of the animal is the layer of still less 

 differentiated epithelium, which lines the deep curvature of the fold. 

 It possesses some mucous cells, one of which is pictured, and its nuclei 

 are comparatively higher or more distal in the cells than those of the 

 outer epithelium. 



Between the two, and placed in a longitudinal belt, comes the olfac- 

 tory epithelium. The cells are longer and stouter, and their cytoplasm 

 is clearer. The cilia represent the olfactory rods seen in many other 

 gastropods, as Segaretus, where they are short and evidently not ciliated. 

 Their length in Sycotypus seems to be rather an indication of low special- 

 ization. The body of the lamella and the epithelium on but one side 

 of it are represented in Figure 232. The epithelium on the other side 

 is symmetrical. 



The peculiar organ, discovered by Van de Hooven in Nautilus and 

 named after him, is considered to be an olfactory organ, or one for testing 

 the water, as mentioned above. It consists of a region derived from 

 the mantle cavity and lined with a thick, heavy epithelium. Where 

 a section is teased and 

 mounted, the same effect 

 may be attained, as is shown 

 in Figure 233. The large, 

 heavy cells with proximal 

 nuclei are gland cells and 

 secrete a mucus. 



Another set of cells, more 

 numerous and much finer, lie 

 between the gland cells and 



extend thp samp Hktanrp FlG ' 2 33- Teased preparation from the olfactory 



me Same distance, epithelium of a Nautilus, gl.c., large gland cells 



from base tO distal edge. which have been shaken out from among the thread- 



These are the nerve or sen- g^ctory celb,rfur. ;./., nerw fiber. (After 

 sory cells, and they send ef- 

 ferent processes toward the central ganglia, while their afferent pro- 

 cesses bear perceptory cell organs. The efferent processes form a heavy 



