GROAVTH A^'D HABITATION. 11 



from external injury ; but these require in their construction 

 a considerable quantity of the \dscous exudation already 

 noticed ; whereas a thin coatings fitted to the shape of each 

 individual, answers the purpose equally as well, and more- 

 over enables it to feel the vibrations of the aqueous element 

 in which it frequently resides. 



Thus, in subserviency to this general system of economy, 

 as well as in accordance with the instincts of the animals 

 themselves, we find that different species of mollusks are 

 enclosed in various kinds of testaceous coverings. Those 

 which defend the active family of Bonax enable them to 

 dart away on the approach of danger; while the shells of 

 their more inactive relatives, the perambulating Salens, are 

 admirably adapted to assist their movements through the 

 yielding sand. The martial Chitons walk abroad in coats- 

 of-mail, fitting like plate armour, and surrounded with horny 

 belts or margins, often covered with scales. The shields 

 of the PJwlades bristle with points resembling a file, by 

 means of which they are defended from external injury, 

 when occupied in slowly excavating the hardest substances ; 

 and the hospitable mansions of the peaceful Pinnce are suffi- 

 ciently large for the admission of their unassuming guests. 

 The conical shaped shells of the Patella , or Limpets, 



