The optic nerve, after entering the sclerotica, ex pands into a large ganglion, from the peripheral surface of which issue numerous nervous filaments. These pierce the choroides by as many holes, and go to form by their reunion the retina. This important membrane ex tends to the ciliary zone, and like it appears to unite it self with the groove of the lens.
The vitreous humour is contained in a peculiar vesi cle, having the lens seated in a concavity on its external surface. The lens divides easily into two parts, the line of separation being the groove w hich receives the ciliary ligament. Each portion consists of a number of con centric layers of variable thickness, composed of radiated fibres, becoming less and less distinct towards the cen tre, near which the laminated and radiated appearances cease to be perceptible. An imperfect representation of this structure is given by Sir E. Home, probably from preparations by :Nlr. John Hunter, in the Phil. Trans. vol. lxxxiv. tab. 5 p. 26.
The conjunctiva supplies the place of a cornea, and covers directly the crystalline lens, as there is no aque ous humour. This membrane in sonic is continuous with the skin, but in others there are imperfect eye-lids, formed by its duplicature, previous to passing over the leas. The skin, at the opening of the pupil, formed by the sclerotica, in the absence of an :Inca and iris, is strengthened by a membrane which appears to be mus cular, and probably assists in the contraction or enlarge ment of the aperture.
The animals of the cephalodous class, besides contain ing complicated eyes, are likewise furnished with ears. These are situated in the annular cartilage which sup ports the arms. In this cartilage there are two cavi ties. In each of these there is a bag filled with a gela tinous transparent fluid, and containing a calcareous substance, differing in its consistence, according to the species, from the brittleness of starch to the hardness of bone.
The auditory nerve penetrates the walls of this laby rinth, and ramifies on the membranous bag which it contains. There is no external opening, nor any ap parent alteration in the thickness of the investing in teguments.
The digestive system of the Cephalopoda exhibits several appearances by which it may be distinguished. The arms which surround the mouth, seize the animals which are to serve as food, and bring them to the mouth. The mouth is situated in the centre of the disk, round which the tentacula are arranged. It is
surrounded with a slight fold of the skin, which may be compared to lips, and which is rough on the central aspect. Within these are the two mandibles, of a deep brown colour, hard horny consistence, and in form re sembling the beaks of a parrot. Where free, they are conico-tubular, but where covered, they are open at the central side. The under beak, unlike the same organ in birds, is the largest, the most crooked, and embraces the upper, or the one on the dorsal margin of the mouth. These jaws are merely able to open and shut, as they possess no lateral motion. They are supported by the muscular bed of the mouth, which serves as a mould to fill the cavity towards the point. The tongue is situated between the beaks, and is armed with reflected teeth. These teeth, in consequence of the undulatory motion of the substance of the tongue, expedite the progress of the food into the gullet.
The salivary glands are four in number ; the first pair, seated on each side the muscular bed of the mouth, are each divided into numerous lobes, whose excretory ducts pour their fluid into the beginning of the gullet. The second pair, seated lower down and below the eyes, are not so much divided, and send out each a canal. These unite, and pour their contents into the mouth.
The gullet is furnished with a lateral expansion, not unlike the crop of gallinaceous birds. The stomach is muscular, like the gizzard of fowls, and the cuticle is thick, and separates easily from the other membranes. At the pyloric opening of the stomach, there is another aperture equally large, which leads into the aniral sto mach, or ceecum, as it has been improperly termed by some anatomists. It may with greater propriety be denominated the duodenum, as it performs some of the offices of that pact of the gut in the higher orders of animals. This stomach is conical, closed at the dis tal extremity, and performs about a turn and a half, like a spiral shell. Its inner surface is covered with a ridge, which traverses it in a closely spiral direction. The bile flows into it near the apex, and towards its base, glandular orifices pouring out a thick yellow fluid may be observed. The intestine, after leaving the pyloris, in some species makes one or two turns, in others it proceeds directly to the anus. This opening is seated at the base of the funnel, on its posterior or dorsal side.