M O L 



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M O L 



would occupy less space than a grain of 

 sand. The ingredients of granite, and 

 of all other kinds of crystalline rocks, are 

 composed of molecules which are invisibly 

 minute, and each of these molecules is 

 made up of still smaller and more minute 

 molecules, every one of them combined in 

 fixed and definite proportions, and afford- 

 ing, at all the successive stages of their 

 analysis, presumptive proof that they 

 possess determinate geometrical figures. 

 MOLLU'SCA. (mollusca, a nut with a soft 

 shell, Lat.) According to the arrange- 

 ment of Cuvier, the second great division 

 of the animal kingdom. This he subdi- 

 vided into six classes, namely, Cephalo- 

 poda, Pteropoda, Gasteropoda, Acephala, 

 Brachiopoda, and Cirrhopoda. A vast 

 multitude of species, possessing in com- 

 mon many remarkable physiological cha- 

 racters, are comprehended in this great 

 division. In all, as their name imports, 

 the body is of soft consistence ; and it is 

 enclosed, more or less completely, in a 

 muscular envelope, called the mantle, 

 composed of a layer of contractile fibres, 

 which are interwoven with the soft and 

 elastic integument. Openings are left in 

 this mantle for the admission of the 

 external fluid to the mouth and to the 

 respiratory organs, and also protrusion 

 of the head and the foot, when these 

 organs exist. But a larger proportion of 

 this class are acephalous, that is, destitute 

 of a head, and the mantle is then often 

 elongated to form tubes, occasionally of 

 considerable length, for the purpose of 

 conducting water into the interior of the 

 body. 



The general form of the body, and the 

 kind of motions it performs, vary more in 

 the molluscous than in the articulated 

 classes of animals, and we observe a cor- 

 responding diversity in their active organs 

 of motion. In the molluscous classes 

 there appear much greater variety, diver- 

 sity, and want of symmetry in the whole 

 muscular system. Many of the lower 

 mollusca are fixed by long peduncles at 

 the bottom of the sea ; some, as the 

 limaces, creep on the surface of the dry 

 land ; the pteropods swim at the surface 

 of the ocean, where the janthinse hang 

 suspended by floats ; the naked cepha- 

 lopoda bound from the surface, and the 

 pholades are fixed deep in cavities of 

 rocks at the bottom ; the oyster is fixed 

 to the rock, while the clam skips to and 

 fro by the flapping of its shells ; the 

 pinna is anchored to the bottom by its 

 strong byssus, while the cardium swims 

 along the still surface, suspended by its 

 concave expanded foot. So that although 

 none of these animals have wings to fly 

 through the air, or jointed legs to creep 



upon the earth, or spines to oar them 

 through the sea, they possess the means 

 of almost every kind of motion, from the 

 vibratile cilia of the fixed corals to the 

 hands and feet of the finny tribe. 



The circulation of the mollusca is al- 

 ways double ; that is, their pulmonary 

 circulation describes a separate and dis- 

 tinct circle. Their alimentary canal hardly 

 ever passes straight through their body : 

 nor is the anus terminal, as in most of 

 the articulata. Their digestive cavities 

 are more numerous and capacious, the 

 intestine is more lengthened and convo- 

 luted, and all the assistant glandular 

 organs are developed on a higher plan, 

 and more constant throughout the classes. 

 The lowest of the molluscous classes, 

 the tunicated animals, shut up in the 

 interior of a cartilaginous, more or less 

 elastic, and biforate tunic, have no pre- 

 hensile or masticating organs connected 

 with their mouth. The mouth, in fact, 

 is placed at the bottom of the respiratory 

 sac, and appears to be destitute even of 

 those tentacula, appendices, or lips, which 

 are so much developed, and so various in 

 their forms, in the conchiferous animals. 

 Many of the mollusca are formed for 

 an existence as completely stationary as 

 the zoophytes attached to a fixed base. 

 This permanent attachment does not, 

 however, take place till they have arrived 

 at a certain period of their growth. 



The mollusca are the only instance of 

 a unipede structure in creation, but this 

 one foot answers every purpose of a hand 

 or leg ; it spins for the bivalves their 

 byssus, is used by others as a trowel, by 

 others as an augur, and by others for 

 other manipulations, and is generally 

 their sole organ of locomotion ; from its 

 soft and flexible substance it can adapt 

 itself to the surfaces on which it moves, 

 and by the slime that it copiously secretes 

 lubricates them to facilitate its progress. 

 It is probable that the foot may be also 

 employed by these animals as an organ of 

 touch. In the nervous system of mol- 

 lusca, the ganglia have a circular arrange- 

 ment. Cuvier. Grant. Kirly. Roget. 

 MOLLU'SCOUS. Animals belonging to the 

 division mollusca, or soft, invertebral, 

 inarticulate animals ; often protected by 

 a shell. The external skeleton of the 

 molluscous animals is consolidated by 

 carbonate of lime, without the phosphate 

 of lime which is common to the other 

 great divisions of the animal kingdom. 

 This earthy matter is secreted from the 

 skin in successive layers, mixed with a 

 glutinous coagulable animal matter, which 

 gives firmness and tenacity to the whole 

 mass, and the skeletons are not exuviable 

 as in the articulated classes. 



