Encyclopaedia Britannica, 11th Edition, "Celtes, Konrad" to "Ceramics" / Volume 5, Slice 6

by Various

The lobes of the fore-foot of Nautilus and of the other Cephalopoda require further description. It has been doubted whether these lobes were rightly referred (by T.H. Huxley) to the fore-foot, and it has been maintained by some zoologists (H. Grenadier, H. von Jhering) that they are truly processes of the head. It appears to be impossible to doubt that the lobes in question are the fore-portion of the foot, when their development is examined (see fig. 35), further, when the fact is considered that they are innervated by the pedal ganglion. The fore-foot of Nautilus completely surrounds the buccal cone (fig. 6, e), so as to present an appearance with its expanded tentacles similar to that of the disk of a sea-anemone (Actinia). A.G. Bourne, of University College, prepared from actual specimens the drawings of this part in the male and female Nautilus reproduced in fig. 6, and restored the parts to their natural form when expanded. The drawings show very strikingly the difference between male and female. In the females (lower figure), we observe in the centre of the disk the buccal cone e carrying the beak-like pair of jaws which project from the finely papillate buccal membrane. Three tentaculiferous lobes of the fore-foot are in immediate contact with this buccal cone; they are the right and left (c, c) inner lobes, and the inferior inner lobe (d)—called inferior because it really lies ventralwards of the mouth. This inner inferior lobe is clearly a double one, representing a right and left inner inferior lobe fused into one. A lamellated organ on its surface, known as Owen’s organ, probably olfactory in function (n), marks the separation of the constituent halves of this double lobe. Each half carries a group of fourteen tentacles. The right and the left inner lobes (c, c) each carry twelve tentacles. External to these three lobes the muscular substance of the mouth-embracing foot is raised into a wide ring, which becomes especially thick and large in the dorsal region where it is notably modified in form, offering a concavity into which the coil of the shell is received, and furnishing a protective roof to the retracted mass of tentacles. This part of the external annular lobe of the fore-foot is called the “hood” (figs. 2, 3, m). The median antero-posterior line traversing this hood exactly corresponds to the line of concrescence of the two halves of the fore-foot, which primitively grew forward one on each side of the head, and finally fused together along this line in front of the mouth. The tentacles carried by the great annular lobe are nineteen on each side, thirty-eight in all. They are called “digital,” and are somewhat larger than the “labial” tentacles carried on the three inner lobes. The dorsalmost pair of tentacles (marked g in fig. 6) are the only ones which actually belong to that part of the disk which forms the great dorsal hood m. The hood is, in fact, to a large extent formed by the enlarged sheaths of these two tentacles. All the tentacles of the circumoral disk are set in remarkable tubular sheaths, into which they can be drawn. The sheaths of some of those belonging to the external or annular lobe are seen in fig. 3, marked n. The sheaths are muscular as well as the tentacles, and are simply tubes from the base of which the solid tentacle grows. The functional significance of this sheathing arrangement is as obscure as its morphological origin. With reference to the latter, it appears highly probable that the tubular sheath represents the cup of a sucker such as is found on the fore-foot of the Dibranchiata. In any case, it seems to the writer impossible to doubt that each tentacle, and its sheath on a lobe of the circumoral disk of Nautilus, corresponds to a sucker on such a lobe of a Dibranchiate. W. Keferstein follows Sir R. Owen in strongly opposing this identification, and in regarding such tentacle as the equivalent of a whole lobe or arm of a Decapod or Octopod Dibranch. The details of these structures, especially in the facts concerning the hectocotylus and spadix, afford the most conclusive reasons for dissenting from Owen’s view. On the ventral side an extensive part of the internal surface of the muscular ring is laminated, forming the so-called “organ of Valenciennes,” peculiar to the female and serving for the attachment of the spermatophores. We have so far enumerated in the female nautilus ninety tentacles. Four more remain which have a very peculiar position, and almost lead to the suggestion that the eye itself is a modified tentacle. These remaining tentacles are placed one above (before) and one below (behind) each eye, and bring up the total to ninety-four (fig. 3 v, v).

Fig. 6.—Male (upper) and female (lower) specimens of Nautiluspompilius as seen in the expanded condition, the observer lookingdown on to the buccal cone e; one-third the natural size linear. Thedrawings have been made from actual specimens by A.G. Bourne,B. Sc., University College, London.
a, The shell. b, The outer ring-like expansion(annular lobe) of the circumoralmuscular mass of thefore-foot, carrying nineteententacles on each side—posteriorlythis is enlarged toform the “hood” (markedv in fig. 1 and m in figs. 2and 3). giving off the pairof tentacles marked g in thepresent figure. c, The right and left inner lobesof the fore-foot, each carryingtwelve tentacles in thefemale, in the male subdividedinto p, the “spadix”or hectocotylus on the leftside, and q, the “anti-spadix,”a group of fourtentacles on the right side—itis thus seen that the subdividedright and left innerlobes of the male correspondto the undivided right andleft inner lobes of the female. d, The inner inferior lobe of thefore-foot, a bilateral structurein the female carrying twogroups, each of fourteen tentacles,separated from one anotherby a lamellated organn, supposed to be olfactory infunction—in the male theinner inferior lobe of thefore-foot is very much reduced,and has the form ofa paired group of lamellae(d in the upper figure). e, The buccal cone, rising fromthe centre of the three innerlobes, and fringing the protrudedcalcareous beaks orjaws with a series of minutepapillae.	f, The tentacles of the outercircumoral lobe or annularlobe of the fore-foot projectingfrom their sheaths. g, The two most posterior tentaclesof this series belongingto that part of the annularlobe which forms the hood(m in figs. 2 and 3). i, Superior ophthalmic tentacle. k, Inferior ophthalmic tentacle. l, Eye. m, Paired laminated organ oneach side of the base of theinner inferior lobe (d) of thefemale. n, Olfactory lamellae upon theinner inferior lobe (in thefemale). o, The siphon (mid-foot). p, The spadix (in the male), thehectocotylized portion of theleft inner lobe of the fore-footrepresenting four modifiedtentacles, eight being leftunmodified. q, The anti-spadix (in the male),being four of the twelvetentacles of the right innerlobe of the fore-foot isolatedfrom the remaining eight,and representing on the rightside the differentiated spadixof the left side. The fourtentacles of the anti-spadixare set, three on one baseand one on a separate base.

In the adult male nautilus we find the following important differences in the tentaculiferous disk as compared with the female (see upper drawing in fig. 6). The inner inferior lobe is rudimentary, and carries no tentacles. It is represented by three groups of lamellae (d), which are not fully exposed in the drawing. The right and left inner lobes are subdivided each into two portions. The right shows a larger portion carrying eight tentacles, and smaller detached groups (q) of four tentacles, of which three have their sheaths united whilst one stands alone. These four tentacles may be called the “anti-spadix.” The left inner lobe shows a similar larger portion carrying eight tentacles, and a curious conical body behind it corresponding to the anti-spadix. This is the “spadix.” It carries no tentacles, but is terminated by imbricated lamellae. These lamellae appear to represent the four tentacles of the anti-spadix of the right internal lobe, and are generally regarded as corresponding to that modification of the sucker-bearing arms of male Dibranchiate Siphonopods to which the name “hectocotylus” is applied. The spadix is in fact the hectocotylized portion of the fore-foot of the male nautilus. The hectocotylized arm or lobe of male Dibranchiata is connected with the process of copulation, and in the male nautilus the spadix has probably a similar significance, though it is not possible to suggest how it acts in this relation. It is important to observe that the modification of the fore-foot in the male as compared with the female nautilus is not confined to the existence of the spadix. The anti-spadix and the reduction of the inner inferior lobe are also male peculiarities. The external annular lobe in the male does not differ from that of the female; it carries nineteen tentacles on each side. The four ophthalmic tentacles are also present. Thus in the male nautilus we find altogether sixty-two tentacles, the thirty-two additional tentacles of the female being represented by lamelliform structures.

Musculature, Fins and, Cartilaginous Skeleton.—Without entering into a detailed account of the musculature of Nautilus, we may point out that the great muscular masses of the fore-foot and of the mid-foot (siphon) are ultimately traceable to a large transverse mass of muscular tissue, the ends of which are visible through the integument on the right and left surfaces of the body dorsal of the free flap of the mantle-skirt (fig. 1, l, l, and fig. 3, k). These muscular areae have a certain adhesion to the shell, and serve both to hold the animal in its shell and as the fixed supports for the various movements of the tentaculiferous lobes and the siphon. They are to be identified with the ring-like area of adhesion by which the foot-muscle of the limpet is attached to the shell of that animal. In the Dibranchs a similar origin of the muscular masses of the fore-foot and mid-foot from the sides of the shell—modified, as this is, in position and relations—can be traced.

Fig. 7.—Minute structure of the cartilage of Loligo (from Gegenbaur, after Furbringer) a, Simple cells. b, Dividing cells. c, Canaliculi. d, An empty cartilage capsule with its pores. e, Canaliculi in section.

In Nautilus there are no fin-like expansions of the integument, whereas such occur in the Decapod Dibranchs along the sides of the visceral hump (figs. 15, 16). As an exception among Octopoda lateral fins occur in Pinnoctopus (fig. 38, A), and in Cirrhoteuthis (fig. 38, D).

In Nautilus there is a curious plate-like expansion of integument in the mid-dorsal region just behind the hood, lying between that structure and the portion of mantle-skirt which is reflected over the shell. This is shown in fig. 2, b. If we trace out the margin of this plate we find that it becomes continuous on each side with the sides of the funnel. In Sepia and other Decapods (not in Octopods) a closely similar plate exists in an exactly corresponding position (see b in figs. 10, 26). In Sepia a cartilaginous development occurs here immediately below the integument forming the so-called “nuchal plate,” drawn in fig. 8, D. The morphological significance of this nuchal lamella, as seen both in Nautilus and in Sepia, is not obvious. Cartilage having the structure shown in fig. 7 occurs in various regions of the body of Cephalopoda. In all Glossophorous Mollusca the lingual apparatus is supported by internal skeletal pieces, having the character of cartilage; but in the Cephalopoda such cartilage has a wider range.

In Nautilus a large H-shaped piece of cartilage is found, forming the axis of the funnel (fig. 8, A, B). Its hinder part extends up into the head and supports the peri-oesophageal nerve-mass (a), whilst its two anterior rami extend into the tongue-like siphon. In Sepia, and Dibranchs generally, the cartilage takes a different form, as shown in fig. 8, C. The processes of this cartilage cannot be identified in any way with those of the capito-pedal cartilage of Nautilus. The lower larger portion of this cartilage in Sepia is called the cephalic cartilage, and forms a complete ring round the oesophagus; it completely invests also the ganglionic nerve-collar, so that all the nerves from the latter have to pass through foramina in the cartilage. The outer angles of this cartilage spread out on each side so as to form a cup-like receptacle for the eyes. The two processes springing right and left from this large cartilage in the median line (fig. 8, C) are the “pre-orbital cartilages”; in front of these, again, there is seen a piece like an inverted T, which forms a support to the base of the “arms” of the fore-foot, and is the “basi-brachial” cartilage. The Decapod Dibranchs have, further, the “nuchal cartilage” already mentioned, and in Sepia, a thin plate-like “sub-ostracal” or (so-called) dorsal cartilage, the anterior end of which rests on and fits into the concave nuchal cartilage. In Octopoda there is no nuchal cartilage, but two band-like “dorsal cartilages.” In Decapods there are also two cartilaginous sockets on the sides of the funnel—“siphon-hinge cartilages”—into which fleshy knobs of the mantle-skirt are loosely fitted. In Sepia, along the whole base-line of each lateral fin of the mantle (fig. 15), is a “basi-pterygial cartilage.” It is worthy of remark that we have, thus developed, in Dibranch Cephalopods a more complete internal cartilaginous skeleton than is to be found in some of the lower vertebrates. There are other instances of cartilaginous endo-skeleton in groups other than the Vertebrata. Thus in some capito-branchiate Chaetopods cartilage forms a skeletal support for the gill-plumes, whilst in the Arachnids (Mygale, Scorpio) and in Limulus a large internal cartilaginous plate—the ento-sternite—is developed as a support for a large series of muscles.

Fig. 8.—Cartilaginous skeleton of Cephalopoda (after Keferstein.)
A, Capito-pedal cartilage ofNautilus pompilius. a points to the ridge whichsupports the pedal portionof the nerve-centre.	B, Lateral view of the same—thelarge anterior processesare sunk in the muscularsubstance of the siphon. C, Cephalic cartilages of Sepiaofficinalis. D, Nuchal cartilage of Sepiaofficinalis.