CONSIDERATIONS. 12.J 



many cases, these muscles disappear. The muscles contained 

 in the femur and tibia differ but little in all the articulated 

 animals with solid integuments. In Insects, the muscles placed 

 in the joints of the tarsi disappear, and they are all moved by 

 one muscle placed in the femur or tibia; the tendon of which 

 traverses the tarsus to be inserted in the claw. Where the 

 articulation of the parts of the legs is ginglymoidal, they are 

 commonly moved by only one pair of muscles ; where the 

 articulation allows the parts to roll, they are generally furnished 

 with more. But the muscles which move the coxa and tro- 

 chanter vary much in form, number, composition, and inser- 

 tion. The flexors of the trochanters are generally simple 

 and penniform, whilst the extensors are mostly formed by 

 several heads, one at least of which has its origin in one of the 

 thoracic segments, the others generally in the coxa. 



Part. III. — Digestive Organs. 

 Organs of Manducation and Alimentary Canal. 



The digestive organs bear a constant relation to the quality 

 of the food destined for the support of the animal, or at least 

 are not incompatible therewith in form or disposition. But 

 the food being so much modified in its properties by the 

 action of the parts of the mouth, the intestinal canal is liable 

 to be acted on by so few of these, that their influence is 

 scarcely felt by it. Hence it follows, that the form of the 

 parts of the mouth must depend more particularly on the 

 quality of the food, and be more precisely in relation with 

 organs which (as the feet) concur indirectly to the function 

 of digestion. This is confirmed by observation : but the in- 

 testinal canal is found to offer a much less uniform relationship 

 with the food and with the other organs. 



That the parts of the mouth are less subordinate to the qua- 

 lity of the food in the Annulosa than in the Vertebrata has 

 already been pointed out : they differ considerably in their 

 form where the food scarcely varies, and vice versa. Nature, 

 always so admirable in all that she produces, shows us here, 

 as often elsewhere, that she is not constrained servilely to 

 confine herself to the use of one sole means ; but, on the 



