193 



Ch AFTER 111 

 MOLLUSC A. 



^ 1 . MoUnsca in general. 



THEseriesof animal structures, arranged according 

 to their mechanical functions, conducts us next to the 

 Mollusca ; an assemblage of beings which was first 

 recognised as constituting one of the primary divi- 

 sions of the animal kingdom by Cuvier, the greatest 

 naturalist of modern times. A vast multitude of 

 species, possessing in common many remarkable 

 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 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 for the occasional 

 protrusion of the head and the foot, when these 

 organs exist. But a large proportion of the animals 

 of this class are acephalous, 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 



VOL. L o 



