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MOLLUSCA. 



Moll DSC*. In the following general view of the different classes 

 '""Y"*' into which molluscous animals have been divided, we 

 shall follow the arrangements of M. Cuvier, modified 

 by the systematical labours of M. Lamarck. Frequent 

 reference will be made to species natives of the British 

 isles, to enable the curious reader to comprehend the 

 recent improvements which have been effected in this 

 branch of natural history. 



I. CLASS.' CEPHALOPODA. 



RwrnAi.o- THE animals w^iich are included in this class, are fa- 

 miliarly known by the name of Cuttle 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 

 appendages, while in others it exhibits fin-like expan- 

 sions. It varies considerably in its consistence, and in 

 some it is strengthened on the back internally by cor- 

 neous ribs or testaceous plates, in others protected ex- 

 ternally by spiral shells. In some species it is connect- 

 ed 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, are placed the arms or tenta- 

 cula. 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 numerous 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 

 observed. These arms and feet are capable of being 

 moved, at the will of the animal, in every direction, and 

 are the organs by which progressive motion is perform- 

 ed. In the space between the head and tunic in front 

 there is an opening or funnel with a projecting aper- 

 ture. This funnel opens into the cavity of the sac, 

 and serves to convey water to the gills and to carry off 

 the different excreted matters. 



The brain in the cephalopoda is contained in an ir- 

 regular hollow ring in the cartilaginous border of the 

 oral disk. This cartilage is thickest on the dorsal as- 

 pect, and contains the parts which have been denomi- 

 nated cerebrum and cerebellum, the remaining part of 

 the canal being occupied with the collar, which sur- 

 rounds the esophagus. The nerves, which proceed 

 directly from the brain to the parts which they are 

 destined to influence, 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, and others for supplying the 

 feet. The cerebellum, besides furnishing the col- 

 lar which encircles the gullet, contributes to the for- 

 mation of the large ganglia which supply the arms 

 the optic and auditory nerves those for the funnel, 

 the tunic, and the viscera. From the size of the ani- 

 mals, the ganglia of the nerves are very distinctly 

 displayed. The anastomosing branches of the nerves 

 of the arms are likewise conspicuous. Each nerve 



at the base of each foot sends out two filaments, one Moiiu. 

 to the nerve of the foot on each side. In this manner * ~Y~~ 

 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 perfect 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 sclerolica, 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 

 contains numerous glandular bodies, similar to the milt 

 of fishes, by which the eye is supported, and which 

 probably act as secreting organs, (although M. Cuvier 

 could not detect any excretory canals,) and likewise 

 an expansion or ganglion of the optic nerve. The 

 concave or anterior surface embraces the choroidet. 

 This membrane, after embracing the vitreous humour, 

 forms a zone or diaphragm, which may be compared 

 to the ciliary processes, 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 circular groove, by which the lens ia 

 divided into two unequal hemispheres. Its central sur- 

 face is coated, as in the higher classes of animals, with 

 the coloured mucous pigment, which has been deno- 

 minated pigmen/um nigrunt. In the cephalopoda, how- 

 ever, it is of a purplish-red colour. 



The optic nerve, after entering the sclerotica, ex- 

 pands into a large ganglion, from the peripheral sur- 

 face 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 mem- 

 brane extends to the ciliary zone, and like it appear* 

 to unite itself with the groove of the lens. 



The vitreous humour is contained in a peculiar ve- 

 sicle, having the lens seated in a concavity on its ex- 

 ternal surface. The lens divides easily into two parts, 

 the line of separation being the groove which receive* 

 the ciliary ligament. Each portion consists of a num- 

 ber of concentric layers of variable thickness, composed 

 of radiated fibres, becoming less and less distinct to- 

 wards the centre, 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 Mr. John Hun- 

 ter, 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 some is continuous 

 with the skin, but in others there are imperfect eye- 

 lids formed by its duplicature, previous to passing over 

 the lens. The skin, at the opening of the pupil, form- 

 ed by the sclerotica, in the absence of an uvea and 

 iris, is strengthened by a membrane which appears to 

 be muscular, and probably assists in the contraction or 

 enlargement of the aperture. 



The animals of the cephalodous class, besides con- 

 taining complicated eyes, are likewise furnished with 



