204 THE MECHANICAL FUNCTIONS. 



mediately acquainted with the event, and passes quickly to 

 the spot, by the assistance of the same thread. 



Some species have the power of conveying themselves to 

 considerable distances through the air by means of threads 

 which they dart out, and which are borne onwards by the 

 wind, while the spider is clinging to the end of the thread 

 ^which is next to it. In this manner these spiders are often 

 carried up to a great height in the air: and it has been sup- 

 posed that during their flight they often seize upon gnats and 

 other flies; because the mutilated remains of these insects are 

 often seen adhering to the threads: this point, however, is 

 still open to much doubt. 



The Natural History of the spider is in many points of 

 view^, highly interesting, not only from the great extent to 

 which the organic development is carried, and the energy 

 with which all the functions of animal life are performed; 

 but also with reference to the wonderful instincts displayed 

 in the construction of its w^eb, in the surprise and destruc- 

 tion of its victims, and in the zealous guardianship of its 

 young. It would, be impossible in so brief an outline as the 

 one I am now tracing, to enlarge upon so fertile a topic, 

 without being led too far from the object I have at present 

 more particularly in view; namely, the development of or- 

 ganization with reference to the organs of progressive mo- 

 tion. 



§ 4. Crustacea. 



The plan which Nature appears to have commenced in 

 the construction of the Arachnida, is farther pursued in that 

 of the Crustacea. The portions into which the external 

 frame-work of the body was divided in the former, are still 

 farther consolidated in the latter: they are composed of 

 denser materials, and endowed with greater rigidity; thus 

 not only offering more resistance to external forces, but also 

 giving a firm.er purchase to the muscles which are the moving 

 powers. The lirnbs, as well as the whole body, are incased 

 in tubes of solid carbonate of lime: they are articulated with 



