MOEPHOLOGY OF MELIBB 517 



structure ; they are sometimes wholly retracted within a 

 permanent sheath.' ' At the base of these tentacular organs, 

 when they are retracted (PL 27, fig. 8 and PI. 29, fig. 15, K), is 

 a small knob. In sections (PI. 29, fig. 15) the lamellar structure 

 is indicated by certain lobations along the outer part (S). The 

 knob (K) seems to be made up of a mass of large and small 

 nerve-cells (PI. 29, fig. 16), which fibres connect with similar 

 cells distal in position to the former {Nc, Nes, Nfb, Nfi), and 

 finally by innervation in the epithelium of the lamellar external 

 parts of the organ (Nfp). From the distal border of the knob- 

 like ganglion, fibres commimicate with the base of the tentacle ; 

 these fibres are not made up of nerve-fibres only but also of 

 muscle-fibres {Nmf). This is known from their staining 

 reaction and also from the fact that some of these fibres 

 communicate with fibres within the organ which are decidedly 

 nervous, while the other fibres terminate on the organ. The 

 muscle-fibres help to retract the organ below the surface of 

 the tentacle, that is, to withdraw the tentacle within its stalk, 

 which in that case serves as a sheath ; the nerve-fibres to 

 convey stimuli. There is no permanent sheath, as indicated 

 by Gould, save that part of the tentacular stalk which sur- 

 rounds the organ, and acts as a sheath when the tentacle 

 is retracted. 



The function of these tentacular organs in nudibranchs 

 according to various authors is olfactory. Thus, Alder and 

 Hancock (1845 : 19) say : ' The dorsal tentacles are the 

 organs of smell, and, judging from their development, this 

 sense must be more acute in most of the nudibranchs than it 

 is in many other molluscs, with the exception, perhaps, of 

 Nautilus.' Hancock and Embleton (1852 : 242), discussing 

 a Doris, say : ' The dorsal tentacles, which have never been 

 observed to be used as tactile organs, we believe to be the seat 

 of the sense of smell ; and this belief is strengthened when we 

 reflect that these sense organs are most highly developed and 

 minutely laminated ; that they are plentifully supplied with 

 nerves from the ganglia placed in front of all the rest of the 

 cerebral masses ; that they are externally covered with 



