The apparatus for secreting the inky fluid, formerly regarded as characteristic of the class of Cephalopods, is wanting in the Nautilus, which, as it has a large and strong shell to pro tect its body, stands less in need of such a means of defence : the ink-bag is, however, present in the Argonauta.
The ink-bag (l, fig. 221) varies in its re lative position in different Dibranchiata: in the Cuttle-fish it is situated near the bottom of the pallial sac, in front of the testicle or ovary. In the Calamary it is raised close to the termination of the intestine; we have found it similarly situated in the Argonauta, Sepioteu this, and Rossia. In the Octopus it is buried in the substance of the liver, a small part only of its parietes appearing on the anterior sur face of that gland, from which its duct is con tinued forwards to terminate in this genus im mediately behind the anus.
From this connection of the ink-bag with the liver in the Poulp, Monro was led to sus pect it to be the gall-bladder. What its real nature may be still remains doubtful ; De Blain ville and Jacobson regard it as a rudimental urinary apparatus :* Sir Everard Home ± com pares it to the secreting sac which opens into the rectum in Rays and Sharks, and this we consider to be the true homology of the ink bag. It is interesting, indeed, to observe that corresponding anal glandular cavities in the Mammalia are in many instances modified to serve by the odour of their secretion as a means of defence, just as the part in question operates in the Cephalopods by reason of the colour of the ejected fluid.
When the ink-bag is laid open and well cleansed of its contents, its inner surface is seen to be composed of a fine cellular or spongy glandular substance : its exterior coat is of a tough white fibrous texture, and its outer surface commonly exhibits a peculiar glistening or silvery character.
The ink-bag probably attains its largest pro portional size in the genus Sepiola, where it presents a trilobate form. It is of an oblong pyriform shape in Sepia, Sepioteuthis, and Loligo. It is relatively larger in Sepia than in Octopu.s, and the quantity of water which its contents will discolour is very surprising: it behoves the anatomist, therefore, to be very careful not to puncture this part during the dissection of It Cephalopod.
In the living Cephalopods the inky fluid is secreted with amazing rapidity ; we have seen an Octopus, which had previously discoloured the water for a considerable extent around it, immediately after its capture continuing its black ejections several times in quick succes sion, and ultimately expelling in convulsive jets a colourless fluid, when the powers of secreting the black pigment were exhausted.
In every species of Cephalopod which pos sesses this organ, the tint of the secretion cor responds, more or less, with the coloured spots on the integument. The Italian pigment, called Sepia,' and the Chinese one, com monly called 4 Indian Ink,' both of which are the inspissated contents of the organ above described, afford examples of different shades of this singular secretion.
If the Cephalopods are enabled thus to con ceal themselves during the day, they have also the power, by means of another secretion, to render themselves conspicuous by night by means of a phosphorescent exhalation.t The Liver.—This gland is remarkable in the Cephalopods, as in the other classes of the Mol luscous Sub-kingdom, for its great proportional size. In the Nautilus the liver (q, q, fig. 219) extends, on each side of the crop, from the cesophagus to the gizzard. There is a parallelism of form, as will be afterwards seen, between this gland and the Respiratory organs, for it is divided into four lobes, and these are connected by a fifth portion, which passes transversely below the fundus of the crop. All these larger divisions are subdivided into numerous lobules of an angular form, which vary in size from three to five lines. These lobules are immediately invested by a very delicate capsule, and are more loosely sur rounded by a peritoneal covering common to this gland and the crop.
The liver is supplied by large branches which are given off from the aorta, (r, fig. 219,) as that artery winds round the bottom of the sac to gain the dorsal aspect of the crop. It is from the arterial blood alone, in this,. as in other Mollusks, that the secretion of the bile takes place, there being but one system of veins in the liver, corresponding to the hepatic, which returns the blood from that viscus, and conveys it to the vena cava at its termination. The colour of the liver is a dull red with a violet shade ; its texture is pulpy and yielding. When the capsule is removed by the forceps, the surface appears under the lens to be mi nutely granular or acinous, and these acini are readily separable by the needle into clusters hanging from branches of the bloodvessels and duct. The branches of the duct arising from the terminal groupes of the acini, form, by repeated anastomoses, two main trunks, which unite into one at a distance of about two lines from the laminated or pancreatic cavity.