M L 



protrusion of the head and the 

 foot, when these organs exist. But 

 a large proportion of animals com- 

 prised in this class are acephalous, 

 that is, destitute of a head, and the 

 mantle is then often elongated to 

 form tubes, occasionally of consi- 

 derable 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 corresponding 

 diversity in their active organs of 

 motion. The whole skeleton, the 

 solid frame-work of the body, de- 

 stined to give strength, form, and 

 support to the entire machine, dis- 

 appears in the class of mulluscous 

 animals. In the molluscous clas- 

 ses there appear much greater 

 variety, diversity, and want of 

 symmetry in the whole muscular 

 system. Many of the lower mol- 

 luscs are fixed by long peduncles 

 at the bottom of sea; some, as the 

 Km aces, 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 cephalopods 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 altthough 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 circu- 



[ 299 ] M L 



lation of the mollusca is always 

 double; that is, their pulmonary 

 circulation describes a separate and 

 distinct 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 convoluted, and all the as- 

 sistant glandular organs are de- 

 veloped on a higher plan, and are 

 more constant throughout the 

 classes. The lowest of the mol- 

 luscous classes, the tunicated 

 animals, shut up in the interior of 

 a cartilaginous, more or less elastic, 

 and biforate tunic, have no pre- 

 hensible or masticating organs con- 

 nected 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. 



In the mollusca we have the only 

 instance in creation, of a unvpede 

 structure, 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 loco- 

 motion ; 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 mollusca, the 

 ganglia have a circular arrange- 

 ment. The transition series afford 

 examples of several families and 

 many genera of mollusca, which 

 appear at that period to have been 

 universally diffused over all parts 



