Tun animals which are included in this class, arc fa miliarly known by the name of Cutt/c-Fish. They at tracted the notice of the ancients by their curious forms and manners, and appear to have been examined by Aristotle with minute attention.
The cephalopoda, in reference to their external ap pearance, may be regarded as consisting of two parts. The tunic or sac, which contains the viscera, and the head surrounded by the tentacula. The sac is in some species in the form of a purse, destitute of any appen dages, while in others it exhibits fin like expansions. It varies considerably in its consistence, and in some it is strengthened on the back internally by corneous ribs or testaceous plates, in others protected externally by spiral shells. In some species it is connected with the head by an intervening space, which may be regarded as a neck, but in others the tunic and head are continuous behind. In all, it exhibits after death great changes of colour.
On the summit of the head there is a flattened disk, in the centre of which is seated the mouth. Round the margin of this oral disk, which is strengthened by a band of muscular fibres, ate placed the arms or tentacula. Beyond this circle of arms, in some species, there are situated two organs, larger in their dimensions than the arms, which may be denominated feet. Both the arms and feet are covered on their central aspect with nu merous suckers, by which they are enabled to attach themselves to different bodies, and to seize their prey, and in their axis both a nerve and artery may be observ ed. These arms and feet are capable of being moved, at the u ill of the animal, in every direction, and are the organs by which progressive motion is performed. In the space between the head and tunic in front, there is an opening of funnel with a projecting aperture. This funnel opens into the cavity of the sac, and serves to con vey water to the gills, and to early off the different ex creted matters.
The brain in the cephalopoda is contained in an irre gular hollow ring in the border of the oral disk. This cartilage is thickest on the dorsal aspect, and contains the parts which have been denominated ce rebrum and cerebellum, the remaining part of the canal being occupied with the collar, which surrounds the esophagus. The nerves, which proceed directly from the brain to the parts which they arc destined to influ ence, are few in number. From the cerebrum issue a few small nerves, which go to the mouth, and the base of the feet—others which go to form ganglia at the mouth, am. 0 hers for supplying the fret. The cere
bellum, besides furnishing the collar which encircles the gullet, contributes to the formation of the large ganglia which supply the arms—the optic and auditory nerve:— those for the Funnel, the tunic, and the viscera. From the size of the animals, the ganglia of the nerves are very distinctly otsplayed. The anastomosing branchci of the nerves of the arms are likewise conspicuous. Each nerve at the base of each foot sends out two filaments, one to the nerve of the foot on each side. In this manner a chain of nerves is formed round the base of the feet, probably calculated to enable them to act more readily in concert.
From the abundant distribution of nerves to the dif ferent parts, it appears probable that the sense of touch exists in a tolerably pet feet manner. There is no proof of the development of organs for the display of the senses of smell and taste.
The cephalopoda are furnished with two eyes, one on each side of the head. The external membrane on the inner side, which may be compared to the sclerotica, dif fers in many particulars from the covering of the same name in the eyes of the vertebral animals. While it surrounds the contents of the eye from the entrance of the optic nerve to the pupil, it is greatly separated from the choroides. Immediately within its cavity, there is a bag with a peculiar membraneous covering, which con tains numerous glandular bodies, similar to the milt of fishes, by which the eye is suppotted, and which pro bably act as secreting organs, (although M. Cuvier could not detect any excretory canals,) and likewise au expan sion or ganglion of the optic nerve. The concave of an terior surface embraces the choroides. This membrane, after embracing the vitreous humour, forms a zone or diaphragm, which may be compared to the ciliary pro cesses, with an aperture in the centre for the reception of the crystalline lens. The circular margin of this aperture is lodged in, and intimately united with a cir cular groove, by which the lens is divided into two un equal hemispheres. Its central surface is coated, as in the higher classes of animals, with the coloured mucous pigment, which has been denominated pigmentunz ni grum. In the cephalopoda, however, it is of a purplish red colour.