18 WILLIAM PATTEN. 



II. Olfactory Organs. 



A. Structure of the Olfactory Organ. — The olfactory 

 organ is visible from the exterior as an irregular yellowish- 

 browUj wart-like thickening of the cuticula, from 5 to 8 mm. 

 broad, and situated about 25 or 30 mm. in front of the 

 mouth. In specimens from 2 to 4 inches long it is usually 

 raised into a beak-like projection directed backwards. 



Directly beneath the ectoderm are a great many — at a 

 rough estimate from 1500 to 2000 — clear, flask-shaped sense 

 buds, each of which is connected by a narrow neck with a 

 cuticular canal. The distribution of the olfactory buds, as I 

 shall call them, is fairly well shown by surface views of the 

 olfactory region (PI. 2, fig. 19). In this preparation, which is 

 probably from an adult male, there are two unusually well- 

 defined median elevations containing many more canals than 

 elsewhere. The olfactory buds underlying this portion are sup- 

 plied by a large median nerve (fig. 18, m. ol. n.); and this fact, 

 together with the method of development, shows that it con- 

 stitutes a distinct part of the olfactory organ. The lateral 

 portions are clearer and smoother, and contain comparatively 

 few canals ; this is specially the case on the posterior lateral 

 borders immediately over the bulb-like termination of the 

 lateral olfactory nerve (fig. 19). In young individuals, 2 — 4 

 inches long, the cuticula of this part is more transparent than 

 elsewhere, and looks like a small lens. This fact, together 

 with the presence of pigment there in the early stages, was 

 what led me, in my paper on the '' Origin of Vertebrates,'' to 

 regard this organ as a degenerate pair of eyes. 



In the adult the cuticula over the olfactory organ often 

 appears a dirty silvery white in reflected light, and black by 

 transmitted light, owing to the inclusions of air in the ex- 

 tremely minute ''pore canals.'' These "pore canals" are 

 found equally abundant elsewhere, but they do not contain air. 

 The cuticula in the median part of the olfactory organ often 

 contains irregular cavities (fig. 19), as though some animal 



