296 CEPHALOPODA. 



The Olfactory Organs. 



These organs, unlike the eyes and the auditory vesicles, appear in the 

 embryo at a very late stage. In Sepia, in which they were observed 

 by ZERNOFF (and previously by KOLLIKER) at a time when all the 

 arms, the funnel and the chromatophores have already formed, there 

 appears, behind each of the eyes, a round prominence. The edges of 

 this rise and curve over towards the centre and thus an invaginatioii 

 of the ectoderm is brought about which at first is rather shallow and 

 sac-like, its floor being much thickened, while the covering consists 

 of thinner cell -layers. Some of the ectoderm al elements in the floor 

 of the olfactory sac take the form of spindle-shaped sensory cells pro- 

 duced into stiff setae. This is the condition in the adult Elwlw, 

 while in Sepia and Loligo the intermediate and supporting epithelial 

 cells become much lengthened and invested with movable cilia. The 

 organ at a later stage, becomes deeper and sac-like this being the 

 usual adult condition. 



The papillae which, in Argonauta, usually take the place of the 

 olfactory pits, are considered by KOLLIKER as the equivalent of 

 the prominences which, in other Cephalopods, ontogenetically precede 

 the invagination, this author consequently regarding them as a lower- 

 stage of development of the olfactory organ. 



These organs cannot be homologised with the osphradial olfactory organs 

 which are so strikingly developed in the Prosobranchia, and in a lesser degree 

 in many other Molluscs, since the latter are found in the mantle-cavity near 

 the gills, whereas the former occur on the head near the eye. In Nautilus, 

 true osphradia occur near the gills. [According to WILLEY, two pairs of 

 these organs are present. This author also described two pairs of olfactory 

 tentacles, the pre- and post-occular tentacles.] 



Otocysts. 



The rise of the otocysts has been observed in most of the forms 

 of which the ontogeny is known, and these organs have already been 

 depicted in many of the figures here given (Figs. 1 16-118, p. 260, 122, 

 p. 265, 126 p. 270 and 128 p. 273). KOLLIKER examined in detail 

 their (later) structure, and they were subsequently carefully studied 

 by KAY LANKESTER and GRENACHER. 



The position of the otocysts in the embryo may be ascertained 

 from the above-mentioned figures. They form as depressions of the 

 ectoderm (Fig. 141 A) which gradually deepen and become vesicular 

 (Fig. 141 B and G). The aperture of invagination does not close 



