MOLLUSCA. 



from u large nerve, n n, which is distributed to the 

 papilla of the tentaculuin, and appears to be appro- 

 priated exclusively to the sense of touch. The bulb, 

 with the eye attached to it, is represented, in this 

 figure, as half retracted within the tubular sheath of 

 the tentaculuin, gs. The cephalopoda are tile only 

 ones in which have been discovered the organ of 

 hearing, and where the brain is enveloped in a kind 

 of case of a cartilaginous consistence. 



Almost all molluscous animals have a peculiar 

 development of the skin, which covers their body, 

 like a mantle, and has received that appellation. 

 This process, however, is sometimes narrowed into a 

 simple disk, or is tubiform, or hollowed into a sac, 

 and in some cases it is divided and extended in the 

 shape of fins. There are two distinct kinds of mol- 

 luscous animals, namely, cephalous, or such as are 

 provided with a head, and acephalous, or destitute of 

 a head. These are again distinguished by such as 

 are naked mollusca and testaceous mollusca, or with 

 a shelly envelop. The naked mollusca have a 

 membranous or fleshy mantle ; which, however, has 

 frequently one or more hard laminae in its texture. 

 These are increased in dimensions and thickness, 

 by a deposit of earthy and animal matter combined ; 

 the new layers overlapping the old ones. When this 

 substance remains concealed within the tissue of 

 the mantle, the animals are called naked mollusca. 

 The common field snail, (Limax agrestis, pi. 60, f. 5.) 

 and the black snail, (L. ater, pi. 32, f. 25.) are ex- 

 amples of animals of this kind. More frequently 

 however, this shelly matter becomes so greatly 

 developed that the animal, in a state of repose, can 

 contract itself within it ; it is then termed a shell, or 

 a testaceous animal, which is exemplified by the 

 common garden snail shell. (Helix aspersa, pi. 60, 

 f. 15.) The epidermis which covers these animals 

 is very thin and sometimes desiccated, or of a woolly 

 texture. 



The variety of form, surface, colour, brilliancy, 

 and substance is almost infinite. They are nearly 

 all calcarious, although some are simply of a horny 

 consistence ; but in both cases they consist of matter 

 deposited in layers, or exuded from the skin under 

 the epidermis, in the same manner as the hardened 

 mucous covering, nails, hairs, horns, scales, and 

 even teeth. The shelly tissue differs according as 

 its transudation is deposited either in parallel lami- 

 na, or in close set vertical filaments. 



Every mode of mastication and deglutition is illus- 

 trated in the mollusca; for we find their stomachs 

 simple, complicated, and frequently provided with a 

 peculiar armature ; the alimentary canal is vari- 

 ously prolonged. Most species have salivary glands, 

 and always a liver, but neither pancreas nor me- 

 sentery. Several have peculiar secretions. 



Several of the mollusca are bisexual. Some pro- 

 duce their young alive, while others are oviparous. 

 The eggs in some are covered by a shelly envelop, 

 and others only by a simple viscosity. 



The turbinated mollusca appear to be animals in 

 a slight state of development ; they possess but little 

 industry, and are only preserved by their great 

 fecundity, and extreme tenacity of life. They are 

 all oviparous ; their reproduction consequently de- 

 pending on sexual impregnation. 



Some genera of mollusca inhabit the sea and 

 fresh waters, whilfe others are entirely terrestrial : i 

 and a few species are amphibious, as may be exem- 

 plified in the succinea, amphibia, plate 17, f. 75. 



They feed on all substances, both animal and vege- 

 table ; some will eat these even in a putrid state, 

 while others will only consume them fresh. 



The uses of this numerous class are extremely 

 varied. Many of them are taken as food by man, 



and others supply nutritious prey for birds and 

 fishes. Their shelly coverings are converted into 

 many useful articles of commerce ; and from the 

 veins of certain species of shells called purpura is 

 extracted the beautiful dye of the ancients, called 

 Tyrian purple. The P. lapillus which is common on 

 the British shores, adhering to stones, yields this dye; 

 it is represented in pi. 60, f. 79. For a description 

 and figures of the different parts of testaceous mol- 

 lusca, see Conchology, Vol. II. p. 74, and pi. 19. 



CLASS I. 



CEPHALOUS MOLLUSCA, 



OR SOFT ANIMALS WITH A HEAD. 



Lamarck divides the turbinated testaceous mol- 

 lusca into five orders, depending upon the compli- 

 cation of their internal organization. These are 

 first : 



ORDER I. HETEROPODA. 



Destitute of arms arranged around the head ; unprovided 

 with a foot under the belly or throat for locomotion, but fur- 

 nished with one or more irregularly set fins. See Conchology, 

 Vol. II. p. 376. 



The animals of this order are distinguished from all the other 

 Mollusca by the form of their foot, which, in place of a hori- 

 zontal disk, is compressed into a circular lamina, and is used as 

 a fin, and on the edge of which, in several species, is a dilata- 

 tion forming a hollow cone, representing the disk of the other 

 orders. Their branchiae are situated on the posterior part of 

 the back, and composed of plumiform lobes, directed forward ; 

 immediately behind these are the heart, a small liver, with part 

 of the viscera and internal organs of gpneration. Their bodies 

 are of a gelatinous, transparent substance, lined with a m uscu- 

 lar layer, elongated, and terminated by a compressed tail, in 

 most species. There is a muscular mass belonging to the mouth, 

 and the tongue is provided with little hooks ; the oesophagus is 

 very long; the stomach thin ; on the right side of the visceral 

 mass are two prominent tubes affording a passage for the faeces, 

 ova, &c. They generally swim on their back, with the foot up- 

 wards. They possess the property of distending their body cy 

 filling it with water, in a way which is not yet properly under- 

 stood. 



Carnaria Cymbium, pi. 17. f. 1. Body elongated, gelatinous, 

 pellucid, with a tail at its posterior termination, and provided 

 with one or more unequal fins ; heart and branchiae united into 

 one mass, which projects from the abdomen and is situated near 

 the tail, and inclosed in a shell ; head distinct, with two tenta- 

 cula; two eyes, and a contractile elongated mouth. 



ORDER II. CEPHALOPODA. 



Lower part of the body enveloped in a bag-shaped mantle, 

 with the head issuing from the sac, and having inarticulate*! 

 arms, to which are attached cups, surrounding the mouth of 

 the animal, which is furnished with two horny mandibles. 



The mantle unites under the body ; in several species the 

 sides are extended into fleshy fins ; head projecting from th <> 

 sac, is provided with two large eyes, and crowned with long er 

 or shorter fleshy arms or feet, capable of motion in all direc- 

 tions, their surface provided with cup-shaped suckers, by me ans 

 of which they can adhere firmly to othpr bodies ; these feet arc 

 used for prehension, natation, and walking. They swim with 

 the head behind, and crawl in all directions, with the hea un- 

 der the body. At the opening of the sac in front of the neck is 

 placed a fleshy funnel as a passage to the excretions. See 

 Conchology, Vol. II. p. 376. 



Division I. Cephalopoda Sepiaria. 



Lotigo vulgaris. Medium cuttle-fish, pi. 32. f. 6. With a 

 fleshy depressed body contained in a bag, which in most K|ici-if , 

 is obtuse behind and margined on both sides through their 

 whole length by a narrow fin ; within the body, and near tho 

 back is included a free, calcarious, spongy, and opaque bone ; 

 mouth terminal, surrounded with ten arms provided with cups, 

 of which two are longer than the rest, and placed on pedun- 

 cles. 



Divition II. Cephalopoda Monnthalama. 



Shell unilocular, entirely externa., and enveloping the ani- 

 mal. 



Argonauta Argo. Paper Nautilus, pi. 17. f. 2. and pi. 60. f 

 1 The arms have two rows of cups, the pair of feet nearest 

 the back are dilated at the extremity into a broad membrane. 

 The body of the animal does not penetrate to the bottom ol the 

 volutions of the shell, and there is no visible muscular attach- 

 ment, so that it appears not to be u permanent resident. The 

 two membranous arms are frequently elevated by the ani- 

 mal by way of sails, and with three other tentacula thrown 



