ON THE PLACE OF MAN IN THE SCALE OF BEING. 



and with lateral jaws for the prehension and reduction of food; and their 

 movements, being principally guided by the special senses, take place in this 

 direction. The bi-lateral symmetry of the body is not confined to its exte- 

 rior; for it prevails most completely in the whole muscular apparatus; and 

 even the organs of nutrition present more distinct traces of it than are to be 

 seen elsewhere. The compact heart of the Mollusca, for instance, is here re- 

 placed by a long tube, the dorsal vessel, placed on the median line; and the 

 respiratory organs, which are usually diffused through the whole system, are 

 uniform on the two sides. Even the intestinal canal partakes of this sym- 

 metry ; in some Species it runs straight from end to end of the body ; and even 

 where it is otherwise disposed, its appendages are nearly equal on the two 

 sides. The respiration of this group is for the most part aerial; and the appa- 

 ratus for the purpose consists of a series of chambers or tubes, which are dis- 

 persed or extended through the whole body. By this means, the air, the 

 blood, and the tissue to be nourished, are all brought into contact at the same 

 points ; and a much less vigorous circulation is required, therefore, than would 

 otherwise be needed. The whole apparatus of nutrition is comprised within 

 a comparatively small part of the body ; and the bulk of the organs which 

 compose it, is never at all comparable with that which we ordinarily find in 

 the Mollusca. Thus, the liver, which in the Oyster forms a large part of the 

 whole substance, is often scarcely recognizable as such in the Insect ; and the 

 intestinal tube seldom makes many convolutions in its course from one extre- 

 mity to the other. The blood is usually white, as in the other Invertebrated 

 classes ; but it contains a larger number of corpuscles than are seen in that of 

 most of the Mollusca. The temperature varies to a certain degree with that 

 of the atmosphere ; but there are many Insects that have the power of gene- 

 rating a large amount of independent heat, which is strictly proportionable to 

 the quantity of oxygen converted by them into carbonic acid in the respiratory 

 process. All the actions of the Articulata are performed with great energy ; 

 and, at the time of the most rapid increase of the body, the demand for food 

 is so great, that a short suspension of the supply of aliment is fatal. They 

 are capable, however, of being submitted to the influence of very extreme 

 temperatures, with little permanent injury. 



30. The adjoining figure, which displays the muscular apparatus of the 



Fig. 4. 



Section of the trunk of Melolontha vulgaris (after Strauss-Durckheim), showing the complexity of the 

 muscular system. The first segment of the thorax (2) is chiefly occupied by the muscles of the head, and^ 

 by those of the first pair of legs. The second and third segments (3 and 4) contain the very larg muscles 

 of the wings, and those of the other two pairs of legs. The chief muscles of the abdomen are the long 

 dorsal and abdominal recti, which move the several segments one upon another. 



