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CHAPTER III. 



MOLLUSCA. 



§ 1. Molliisca in general. 



The series of animal structures, arranged according to 

 their mechanical functions, conducts us next to the JMoUus- 

 ca; an assemblage of beings which was first recognised as 

 constituting one of the primary divisions of the animal king- 

 dom by Cuvier, the greatest naturalist of modern times. A 

 vast multitude of species, possessing in common many re- 

 markable physiological characters, are comprehended in this 

 extensive class. In all, as their name imports, the body is 

 of soft consistence; and it is enclosed more or less com- 

 pletely 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 for the occasional pro- 

 trusion of the head and the foot, when these organs exist. 

 But a large proportion of the animals of this class arc ace- 

 phalous, that is, destitute of a head, and the mantle is then 

 occasionally elongated to form tubes, often of considerable 

 length, for the purpose of conducting water into the interior 

 of the body. 



Mollusca, with the exception of a few among the higher 

 orders, are but imperfectly furnished with organs of loco- 

 motion. The greater number, indeed, are formed for an 

 existence as completely stationary as the Zoophytes attaclied 

 to a fixed base. The Oyster, the Muscle, and the Limpet, for 

 example, arc usually adherent to rocks at the bottom of the 

 sea, and are consequently dependent for their nourishment 



