The kidneys (fig. 226, rr)are in the form of spongy cellular organs developed upon the two posterior branches of the vena cava. The circulatory organs consist of a systemic central heart (fig. 226, c) which drives the aerated blood to all parts of the body. The blood finds its way into the veins mostly through the intervention of a system of capillaries, but also by means of sinuses and lacunae amongst the tissues. The two great trunks which carry the venous blood to the branchiae, are further provided, in the Cuttle-fishes, with special contractile dilatations, situated one at the base of each gill, and known as the "branchial hearts " (e e).
The respiratory organs are in the form of two (Cuttle-fishes) or four (Nautilus) plume-like gills, placed symmetrically on the sides of the body within the pallial sac. The gills (fig. 226, b b) consist each of a central stem, bearing finely-divided lateral vascular laminae; and as they are not ciliated, the necessary respiratory currents are maintained by the alternate contractions and expansions of the muscular walls of the mantle-sac. In each expansion the water finds its way into the pallial chamber by the opening between the rim of the mantle and the neck; and in each contraction it is expelled through the tube of the funnel, which is so constructed as to allow of the egress but to prevent the ingress of the water.
The organs of sense are a pair of large and very highly developed eyes, and a pair of auditory sacs. The great oesophageal nerve collar is protected by a cartilaginous plate, which foreshadows the cranium of the Vertebrata; this also sends out prolongations which strengthen and defend the eye, and the auditory chambers are excavated in its substance.
Fig. 226. - Central organs of the circulation, gills, and renal organs of Sepia officinalis. (After John Hunter). a Aorta; v Vena cava; v v' Visceral veins; c Systemic heart; d d Dilatations of branchial veins on entering the heart; e e Branchial hearts; b b Branchiae ; r r Renal organs.
The sexes in all the Cephalopoda are in different individuals, the males and females generally being more or less unlike externally. In this order the ducts of the generative organs open into the pallial chamber, and each individual, besides the essential organs of reproduction (testis or ovary), generally possesses an accessory gland; that of the female secreting a viscid material which unites the eggs together, whilst that of the male coats the spermatozoa, and aggregates them into peculiar worm-like filaments, from six to eight lines in length, termed "spermatophores," or the "moving filaments of Need-ham." The spermatophore is filled with spermatozoa, and possesses the power of expanding when moistened, rupturing, and expelling the contained spermatozoa with considerable force. During the congress of the sexes the male transfers the spermatophores to the pallial chamber of the female, true intromission not being possible, but the mode in which this transference is effected differs in different cases, and is not universally known.
In the males of many of the Cuttle-fishes, one of the arms is peculiarly modified, and is said to be "hectocotylised," but the extent to which this modification is carried differs in different cases, and it is not always the same arm in different species which is thus affected. In some cases, the "hectocotylised" arm is little altered from its ordinary form, and though the alteration be primarily sexual, the arm is not known to play any part in the reproductive process. In other cases, again, such as Octopus carina (fig. 227), Tremoctopus violaceus (fig. 228, b), and Argonanta argo (fig. 228, a), the "hectocotylised" arm is the efficient agent in the impregnation of the female.
It is, in these forms, longer and thicker than the other arms, and possesses posteriorly a sac which is filled with spermato-phores. During the reproductive act the "hectocotylised" arm is actually detached by the male, and deposited, with its freight of spermatophores, within the pallial chamber of the female. When thus detached (fig. 228, b), it is capable of independent movement, and when first found in this free condition within the mantle-cavity of the female Argonaut, it was regarded as a parasitic worm. Cuvier gave the name of "Hectocotylus Octopodis" to it, under this belief as to its nature. Hence the name of "hectocotylus" (in allusion to the suckers which it carries) is still applied to the detached arm; whereas the arm, if not detached, is simply said to be "hectocotylised."
Fig. 228. - a Male of Argonaut a argo, with the hectocotylised arm still contained in its enveloping cyst, four times enlarged (after H. Muller); b Hectocotylus of Tremoc-topus violaceus. (After Kolliker.)
In those cases in which the hectocotylised arm is not detached, it is asserted by Steenstrup that it is employed by the male in the direct transference of the spermatophores to the pallial chamber of the female; though it is still uncertain how the spermatophores find their way from the seminal ducts to the sac in the interior of the arm.
The eggs of the Cuttle-fishes are enclosed, singly or many together, in special capsules, which are generally attached in bunches to some foreign body. The ovum undergoes partial segmentation, as in Birds and Reptiles, and the unsegmented portion of the yolk is gradually absorbed by the growing embryo.
The shell of the Cephalopoda is sometimes external, sometimes internal. The internal skeleton (fig. 229) is known as the "cuttle-bone," "sepiostaire," or "pen" (gladius), and may be either corneous or calcareous. In some cases it is rendered complex by the addition of a chambered portion or " phragmacone," which is to be regarded as a visceral skeleton or "splanchnoskeleton." In Spirula (fig. 229, c) the phragmacone is the sole internal skeleton, and is coiled into a spiral, the coils of which lie in one plane, and are near one another, but not in contact. It thus resembles the shell of the Pearly Nautilus, but it is internal, and differs, therefore, in this respect from the external shell of the latter, though resembling it in the fact that the last chamber lodges part of the viscera. The only living Cephalopods which are provided with an external shell are the Paper Nautilus (Argonauta), and the Pearly Nautilus (Nautilus pompilhis); but not only is the structure of the animal different in each of these, but the nature of the shell itself is entirely different. The shell of the Argonaut (fig. 230) is involuted, but is not divided into chambers, and it is secreted by the webbed extremities of two of the dorsal arms of the female. The arms are bent backwards, so as to allow the animal to live in the shell, but there is in reality no organic connection between the shell and the body of the animal. In fact, the shell of the Argonaut, being confined to the female, and serving by its empty apex as a receptacle for the ova, may be looked upon as a "nidamental shell," or, as it is secreted by a modified portion of the foot, it may more properly be regarded as a "pedal shell." The shell of the Pearly Nautilus (fig. 233), on the other hand, is a true pallial shell, and. is secreted by the body of the animal, to which it is organically connected. It is involuted, but it differs from the shell of the Argonaut in being divided into a series of chambers by shelly partitions 'or septa, which are pierced by a tube or "siphuncle," the animal itself living in the last chamber only of the shell.
Fig. 229. - a Internal Skeleton of Sepia ornatal b Pen of Histioteuthis Bonelliana; c Shell (" phragmacone ") of Spirula fragilis ; d Animal of Spirula Peronii.