214 THE MECHANICAL FUNCTIONS. 



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 integu- 

 ment. 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 occa- 

 sional 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 occa- 

 sionally elongated to form tubes, often of con- 

 siderable 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 locomotion. The greater number, 

 indeed, are formed for an existence as completely 

 stationary as the Zoophytes attached to a fixed 

 base. The Oyster, the Muscle, and the Limpet, 

 for example, are usually adherent to rocks at the 

 bottom of the sea, and are consequently depend- 

 ent for their nourishment on the supplies of food 

 casually brought within their reach by the waves 

 and currents of the ocean. This permanent at- 

 tachment to the solid body on which they fix 

 their abode, does not, however, take place till 

 they have arrived at a certain period of their 

 growth : for at the commencement of their sepa- 



