16 TESTACEA. 



ing the ordinary integuments of the higher orders. This extends even 

 to quadrupeds. 



We speak of such safeguards as weapons for offence or defence. Are 

 they truly designed as such ? Perhaps, in our state of absolute ignorance 

 of the fact, it might be more reasonable to say they are naturally thus 

 provided with expedients which maybe employed in injuring other crea- 

 tures, and in protecting themselves. 



Commonly their office seems for protection only ; and in progress, 

 from slighter means, it advances thi'ough various stages to great efli- 

 ciency. 



In the preceding Volume we have seen, that, by a peculiar process, 

 which it might be difficult to explain, various humid secretions are diffused 

 wholly or partially over the surface of many of the lower tribes. These 

 secretions appear either as simple exudations, escaping from the entire sur- 

 face, or distilling from the glandular concentrations, all as if to facilitate the 

 winding of the animals among the crevices of rocks or stones. If gluti- 

 nous, it combines with earth or sand, in slight temporar}^ covering, or 

 becomes a permanent dwelling, whither the tenant can retreat at will. 

 By farther inspissation, a durable silken sheath is produced, affording 

 ready and safe reception, which may be forsaken and resumed, or which 

 may itself result on repetition of this natural secretory process. After 

 numerous modifications of great interest and curiosity, a hard and resist- 

 ing substance may invest the living being, impenetrable by its enemies, 

 and sufficient to withstand the fury of the elements. 



The original source of the various secretions, the mode whereby they 

 are produced, their medium and means of application, together with their 

 final purpose, are subjects for profound investigation. They are obviously 

 derived from a certain definite organization, promiscuously distributed, or 

 specially concentrated. They seem to be dependent on the particular cir- 

 cumstances affecting the privileged being, often involuntarily, but un- 

 doubtedly sometimes from volition, spontaneously exercised, and which 

 may be often repeated. 



But Nature herself, embracing a wider scope, anticipates all these 

 latter alternatives, by providing the embryo with the rudiments of its pro- 



