Tentacles o/* Nautilus pompilius. 171 



All the tentacles of the Nautilus are built after a single 

 plan, and preserve the essential features of this even when 

 highly moditied. Ordinarily the tentacle is considered to be 

 formed by two parts — a fleshy sheath surrounding an exten- 

 sile cirrus. The cirrus is the essential structure and will be 

 spoken of as the tentacle, while the sheath seems to be merely 

 a fold of the skin which has been produced around the cirrus 

 for protective or sui)portive purposes. Surrounding- the head 

 of the Nautilus are thirty-eight tentacles, to which Owen 

 gave the name of digital tentacles. The sheaths of tiiese are 

 fused to each other, so that a complete Cephalic Sheath 

 (Owen), open ventrally only, is formed. These tentacles 

 have been described as having no regular arrangement ; but 

 an examination of fifty-one specimens proved that they are 

 arranged upon each side in a constant order. Only six 

 specimens showed a variation from the normal arrangement, 

 and this variation existed in each case upon one side only. 

 Whether the same tentacle always occupies the same position 

 or not cannot be decided till after further dissection of the 

 nerves going to the tentacles. 



The digital tentacles present the structural plan uniformly 

 and simply. The surface of the tentacle is marked by a 

 close series of annular grooves, which are deepest upon the 

 inner side of the tentacle. The tentacles are frequently 

 flattened upon the inner side. A large nerve-trunk occupies 

 the centre of the tentacle ; around this are the radial bundles 

 of longitudinal muscles. Closer examination of the nerve- 

 cord reveals that it is enlarged by collections of ganglion- 

 cells at regular intervals, each enlargement corresponding in 

 position to a segment of the tentacle included between two of 

 the annular grooves. From the ganglionic enlargements 

 nerves pass to the different portions of the segment, but espe- 

 cially to the inner side. The segmental structure of the 

 nerve-cord persists in cases where all traces of the external 

 annulations have disappeared. 



Dr. AVilley, in a recent article, described the great adhesive 

 power of the digital tentacles. This power seemed strange 

 until after a closer study of the segments of the tentacles. 

 The groove between the segments is much deeper upon the 

 inner face of the tentacles than elsewhere. The inner face of 

 the segment is flattened. On this side, between the epi- 

 thelium and the longitudinal muscles, are radial transverse 

 muscles — the only transverse muscles in the tentacles which 

 we can surely identify as such. 



Their open : ion would be somewhat as follows : — when the 

 flat surfaces of the segments of the tentacles are applied to 



12* 



