772 DIGITAL AND OPHTHALMIC TENTACLES. 



the sinuations underwent concrescence, thus producing a thick matrix in the midst of 

 which the tentacles occur, in places, three or even four deep. By slicing off the upper 

 portion of the sheaths of the outer tentacles, it may be at once seen that the sequence 

 of the numbers one to eight and nine to thirteen, in my enumeration, is exactly such as 

 would result from the concrescence of a frilled border, and closer examination will show 

 that the remaining numbere follow suit in the order given. 



Not only are number and plan of distribution of the outer tentacles identical 

 in male and female, but they are also the same in the three species, iV. pompilius, 

 N. inacromphalus, and 3'. umhilicatus. In all cases the outermost of the series are numbers 

 5, 6, 7, and 8, of which the first three appear to be grafted upon the external surface 

 of the circlet, and are much shorter than the rest of the tentacles whose sheaths form 

 the outer wall of the crown, number 6 especially lagging far behind the others, its sheath 

 being produced into a prominent lobe behind. The prism-shaped sheath of the 7th tentacle 

 is always accurately wedged into a deep gi-oove between the 9th and 10th ; that of the 

 6th fits into a similar groove between the 5th and 11th, while the 5th itself is ensconced 

 at a higher level between the 4th, the 12th and the 11th. There is a shallow concaWty 

 between the sheaths of the 9th and 17th tentacles which receives that of the 8th; 

 the inner border of this concavity is furnished by the sheath of the I7th, and so 

 shuts off the 9th from participation in the lateral wall of the great ventral conca\'ity 

 of the annular lobe which I have called the fossa infundibularis ; this wall is actually 

 formed by the coalescent sheaths of the 8th, 17th and 19th tentacles. 



If we now look for some corroboration of the sequence of tentacles which I have 

 suggested above, from a consideration of the nerves which are distributed among them, 

 we shall find to a certain extent a welcome confirmation. My series 5 — 8 inclusive, 

 represented in Professor Vayssiere's scheme by the numbers 7, 10, 14 and 19, are found 

 to receive their nerves in absolute sequence (PI. LXXXI. fig. 6, and PI. LXXXII. fig. 6). 

 The most direct manner of procedure for exposing these nerves, and I believe the 

 technically correct method, is to remove one of the alae infundibali, and then by 

 horizontal incisions through the cartilage and coriaceous tissue at the base of the 

 cephalopodium gradually to expose the sense-centres of one side. In this way it is 

 impossible to overlook the otocyst which escaped both Owen and Valenciennes, being 

 first found by Macdonald (1855, Phil. Trans.). In front of and partially concealed by 

 the otic capsule appears the dark mass of the pedal ganglion. Upon exposing this portion 

 of the "sjTicerebrum'" without too much displacement of neighbouring organs, three 

 nerve-roots almost simultaneously appear to the view of the dissector, and upon tracing 

 these to their destination I was somewhat agreeably surprised to find that they were 

 the nerves belonging to the sixth, seventh and eighth digital tentacles according to my 

 system of enumeration (PL LXXXI. fig. 6). Continuing the dissection by turning aside 

 the olfactory labyrinth (rhinophore), we find that contiguous and level ^-ith the root of vi 

 is that of V (PL LXXXII. fig. G). The large nerve lying at a slightly deeper level 

 next to V in the dorsad direction is iv, at the base of which the root of ill arises 

 apparently as a branch, but the apj^earance of branching is attributable to the fact that 



* The term applied bj Professor LaDkester to a compound cephalic ganglionic mass iu contrast with 

 " archicerebrum " where the cephalic centre comprises the cerebral ganglia only. 



