Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/774

738 CUTTLEFISH plate, which surrounds the gullet, and incloses and protects the great oesophageal nerve-collar, and which sends off prolongations which strengthen and protect the eye, thus discharging the functions of the orbits of higher animals. Fin. 3. a, Internal skeleton (" seplostaire ") of Sepia nrnata, Rang.; b. Internal skeleton ("pen") of Histioteuthis Bunelliana, D Orb.; c. Internal skeleton (" phragmacone ") of Spirula fragilis, Lamarck ; <Z, Animal of Spirula Peronii. The integumeiitary skeleton of the cuttle-fishes consists of an external shell only in the Paper Nautilus (Argonauta), but in all others in which it is present at all it is composed of certain horny or calcareous structures, which are lodged in the substance of the mantle, and are therefore concealed from view. This internal skeleton, so characteristic of the Dibranchiates, is well developed in all the Decapods, but is either absent or rudimentary in the Octopods, in which it never consists of more than " two short rudimental styles encysted in the dorso-lateral parts of the mantle" (Owen). In the typical Decapods (such as Lolitjo, Sepioteuthis, JZnoploteuthis, Ilistiotenthis, <fec.), the skeleton is horny, and consists of a feather-shaped " pen " (b, fig. 3) composed of a central shaft and two more or less extensively developed lateral expansions or wings, the whole imbedded in the mantle in the middle line of the back. In some cases (Ony- choteuthis, Ommastrephes, Loligopsis) the hinder end of the pen is developed into hollow conical appendix or cup, form ing a rudimentary "splanchnoskeleton." In the genus Se2)ia, the internal skeleton (a, fig. 3) consists of a horny oval plate, strengthened by calcareous matter, which is deposited principally on its internal surface, and consists of numerous thin plates separated by vertical fibres. The " cuttle- bone," or " sepiostaire," is of a porous, spongy consistence, and is concave on its inner surface behind, terminating posteriorly in a small cone (" mucro "), from which a thin, wing-like margin is prolonged forwards on both sides. In the singular genus Spirula the internal skeleton (c, fig. 3) has the form of a calcareous and nacreous tube, coiled up into a flat spiral, the coils of which are not in contact. The internal cavity of the shell is partitioned off by a succession of pearly septa, which are perforated on the ventral or con cave side of the shell by a tubs (" siphuncle" ") running the whole length of the spiral. In its general construction, the skeleton of the Spirula is very like the shell of the Pearly Nautilus ; but it is quite certain that its relations to the animal are quite different. Though the shell itself is exceedingly abundant in certain regions, the animal is at present only known by some very imperfect examples, and its connection with the shell is not precisely clear. The last chamber of the shell, however, is little or not at all larger than those behind it, and it is certain that the animal in no sense lives in the shell. On the contrary the last chamber simply lodged the extremity of the visceral sac, and the shell is to all intents and purposes an internal one, though possibly it is only partially concealed from view by folds of the mantle, and is not absolutely encysted. Of the three types of internal skeleton characteristic of living cuttle-fishes namely, the horny pen of the Calamaries, the calcareous " bone " of the Sepice, and the spiral cham bered and siphunculate shell of Spirula two appear under various 7- forms in the Secondary and Tertiary rocks, whilst the third, comprising the Spirulae, is so far unknown in the fossil condition. Thus, we find fossil " pens," in all essential respects identical with those of the ordinary living Decapods, to be by no means very rare in deposits of Secondary age, and such genera as Teiidopsis, Leptoteuthis, Geoteuthis,. and Beloteu- this have been founded on these remains. Similarly, the calcareous " cuttle-bones : from the Tertiary rocks, upon which are founded the genera Spirulirostra, Beloptera, and Belemnosis, appear to be referable to the Sepiadce. In Spirulirostra, however, the skeleton consists partly of a spirally bent, chambered, and siphonate " phragmacone," protected by a pointed calcareous " guard," and it thus reminds us on the one hand of the living Spirula, and on the other hand of the extinct Be- lemnites. By far the most import ant of the fossil cuttle-fishes, how ever, are those which form the family of the Belemnitidw, a group wholly Secondary in its distribution, which has a type of skeleton peculiar to itself. This skeleton consists of a Conical chambered FIG. 4.- Diagram of Belemnito shell the " phragmacone " which is partitioned off by calcareous septa into distinct air-chambers, pierced ventrally by a tube or "siphuncle." The conical phrag macone is placed in a corresponding excavation in the anterior end of a longer or shorter, sub-cylindrical, calcareous, and fibrous structure, which is known as the " guard," or "rostrum," and which protects the delicate phragmacone from injury. The guard is tie part of the skeleton which is most frequently found in the fossil condi tion, and in perfect specimens it is prolonged forwards anteriorly into a longer or shorter horny or shelly plate, which corresponds with the front portion of the " pen " of the Calamaries, and is known as the " pro-ostracum." The various genera of the Belemnitidce Belemnit> s, Bdemnitella, Xiphoteuthis, Conoteuthis, Acanthoteulhis, Belemnoteuthis are founded on differences in the natur3 of the internal skeleton. We know, however, from specimens preserved in such fine-grained deposits as the Oxford clay, that the cuttle-fishes of this family possessed lateral fins, and two " tentacles " in addition to the eight proper " arms ; " that the suckers were provided with horny hooks ; and that there was an ink-sac. As before remarked, the only known Dibranchiate (after Phillips), r, horny per or" pro-ostrncum;" a, coni cal cavity or " alveolus," in which the chambered " phragmacone " ( />) is con tained ; g, " guard, or "ros

trum."